Sunday, May 28, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.18

The Final Filter

I am so glad that my brain works while I sleep as that often keeps me out of trouble or at least may minimize some downstream trouble.
 
When I awoke the other morning I got a message from my brain about the effect of stray capacitance and the design of my compact board. The pads as cut are 0.5 inches by 0.5 inches and there is a pad to pad capacitance as well as a pad to ground plane capacitance. So I took my handy AADE LC meter and measured away. Boom the pad to pad capacitance is in the range of 3 PF and fairly consistent from pad to pad and the pad to ground is higher in the range of 4 PF. So those values are in parallel with what I am soldering to the board. Thus I need to re-look at the values I have chosen for the caps and this may negate the need to have trimmers installed.
 
It may take a day or two but this is a precision crystal filter and thus will need to rethink the install. There has also been traffic on the illuminati reflectors about capacitor Q and how this impacts the use of caps in various tuned circuits. I purchased precision MLCC 1% caps so I am not starting in a deep hole for those who will be quick to email me about the Q of capacitors. I have been advised accordingly so am aware of this concern. Thanks Greg.
 
 
73's
Pete N6QW

Tuesday, May 23, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture ~ 2.17


[For more info on the LM373 SSB Transceiver see http://www.n6qw.com/LM373.html)



Reworked Color Display for the Dual Conversion DifX
 

The Crystal Filter Build Out

 
5/25/2017 ~ Where "Noodling" pays big dividends.
 
Yesterday I made a "linear layout" of the six pole crystal filter and the result was a series of 11 pads where the crystals and caps would be installed and that assembly was about 6 inches long. This is an unwieldy dimension to handle inside of a compact radio. Then I spotted a photo of an 8 pole filter as used in the Elecraft rigs and saw how they had compacted the build. So that led me to think a smaller footprint. I now have come up with a compact version that get the footprint down to something like 2 (or 3)" X 4" inches --much more accommodating. I also had concerns about signal leakage between the sections of the compact layout and think that can be fixed by the addition of a shield between the sections. See my "noodling" sketch below. (I are not an artist and barely an engineer!)
 

 
I have already created the design for the compact board in G Simple and have the dxf file ready to cut a board on my CNC Mill. I will add a photo of the cut board when I am done.
 

 
Noteworthy I have received an email about my capacitor selection process and a link to a video about polarity of capacitors and the stability of capacitors. Hopefully the extra $$$ I spent for the 1% caps and the NPO criteria will abate some of these well founded concerns. I will also exercise caution in NOT applying too much heat to the caps during the soldering process.
 
73's
Pete N6QW
 
 
5/24/2017
 
The caps arrived today and I also measured the 150 PF NPO caps I had in stock. One of values needed (4 places) was 151.2 PF. Using my AADE LC meter, I measured 13 caps and here are the results.
 

The results of this test shows that of the 9 caps we have nailed down 5 of them. The two 115.4 PF I think will be asy to achieve with the 100 PF NPO I bought and the 15 PF trimmer. The 430 PF that were to be the basis of the 437.1 PF are so small I can hardly see them so I may have to invoke a work around.
 
I also designed the PC board and it is about 5 inches long and 3 inches wide. There may be a way to compact this but for this 1st run it will be linear. Hope to cut the board tomorrow and start soldering in crystals and caps.
 
73's
Pete N6QW
 
I have been awaiting the delivery of some 1% caps and they will be arriving in the next couple of days so in the meantime I will flesh out the circuit board for the crystal filter. So let us examine what is required. My Dishal filter for the 6 Crystal Filter has a total of nine capacitors and they are as follows: 
 
  • 2 X 151.2 PF
  • 2 X 437.1 PF
  • 2 X 115.4 PF
  • 2 X 151.2 PF
  • 1 X 156.8 PF
Noteworthy is that we really only have 4 values of capacitors including 4 caps of the same value. The precision caps I ordered are 100 PF, and 430 PF and these are 1% and NPO AND Surface Mount. I have a bag of 150 PF NPO caps which I will measure to find values all below 150 PF. (Hopefully finding four that are all below 150 PF but close in value. I have an AADE LC meter and that is my measurement tool.
 
The PC Board will require a total of 11 Pads to accommodate the crystals and the caps. To get me exactly on the values needed I will make the pads large so that there room for the 15PF  air variable trimmer caps in addition to the fixed value ones and the crystals. Playing with these parts before I cut any pads is critical to the final assembly. Thus 437.1PF = 430 PF + 0-15 PF Trimmer
 
I will add to this posting with the layout of the crystal board after I get it laid out.
 
73's
Pete N6QW


Tuesday, May 16, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.16

5/21/2017
I am now documenting the LM373 build on my website so see the
following LM373 Link So for those tired of seeing me gush over my vintage 1970's designed, new rig you will have some relief. Have made about 2 dozen contacts including some at 600 watts. You got to have power to make lots of contacts so get over that QRP stuff! Soon we will be back to the crystal filter build on the DifX.
 
73's
Pete N6QW

5/19/2017 ~ QSL Card from the 1st contact with the LM373 Transceiver. Here is proof! Also noteworthy I was running only 15 Watts and the Mosley 2 Element Beam. What fun! I am now up to 10 Contacts with the LM373 Rig.

Pete N6QW
 


5/17/2016 Some added photos of the innards of the LM373 rig. We have now made six contacts with the new rig. I still need to tidy up the wiring. Yes, I succumbed to an IRF510 in the output which is not my favorite RF device --but it was an expedient. I am redeploying the K5BCQ 5 Watt final to the DifX dual conversion rig. With the outboard amp this will now do in excess of 100 Watts -- more than QRP!!!!! In the last photo you can see the LM373 IC square in the middle of the photo. All that work being done with one IC. To the aide and aft of the 9.0 MHz crystal filter is the 11 VDC power supply and the N6QW solid state power switch. The relay on the back panel only switches the antenna between T and R and one extra set of contacts switches in the outboard amp. In addition to the final wiring clean up I will add some aluminum support structure so that the front/back panels are rigid. This rig has taken about six weeks to build with some of that time awaiting the new Rigol scope.
 
Pete N6QW




5/16/2017

While I gather up some high quality capacitors to build the six pole 11.5 MHz Crystal Filter, I wanted to share more exciting news about the new LM373 20 Meter SSB Transceiver. Yes that is a photo on the masthead --Pretty Cool Blue isn't it?
 
This transceiver is more than a one hit wonder as I now have made three contacts with the new rig and I am please at how it works. It is also important to note that it is not a bilateral approach and thus truly not a Bitx! By that I mean that signals pass through the IF stage (which is a single IC -- the LM373) and steering through the single direction stage is done with 1N914 signal diodes. On receive the receiver mixer stage (SBL-1) is connected to the front end and the BFO is injected into one port of the LM373. On transmit the output from another SBL-1 acting as the balanced modulator is fed into the front end and now the LO is introduced into the port where formerly we injected the BFO. The port where the audio is recovered on receive is now the transmit output stage. Thus three ports on the LM373 are switched in going from Receive to Transmit --all in a single 14 Pin Dip package!
 
As I often saw with analog VFO's and BFO's the transition from R to T frequently caused a shift in frequency because of the loading effect as the LO and BFO were shifted to other parts of the circuits. That is not the case with the Si5351 --there is no shift. This single pass through an IF stage in older designs employing Analog LO/BFO's was problematic in this aspect. I wrestled with this issue and had some elaborate schemes to mitigate the problem. The bilateral approach where the LO and BFO were not shifted was a solution. Now with the Si5351 this is a solution with a single pass approach.
 
I have challenged myself to use as much stuff as I have in the junk box without resorting to placing orders for parts and that has been very rewarding in that "you use what you have!"
 
Beyond the LM373 I am using my standard Audio Amp stage as well as the bidirectional 2N3904 amp stage and the driver with the 2N2222 and BD139 produces a whopping 400 MW. I now will add the 5 watt K5BCQ RF Power MOSFET board that uses the Mitsubishi RDD series of transistors. Right now the 400 MW is driving an outboard SS amp all the way to about 15 Watts output.
 
 
 
Touring the knobs, the left most smaller knob is the audio gain and the right one the microphone gain. The three switches on the bottom center are for USB/LSB, Tune (Red PB) and the right switch is the MOX. The right hand large knob (ala Drake R4B) is the main tuning knob. There was some careful thought (about 1 minute) to the panel layout relative to ergonomics. Since I am right handed the main tuning and step rate change is the most frequently used so it is to the right not unlike the Drake rigs!

The new Rigol DSO has been very useful as I worked on peaking a tweaking this new rig.

73's
Pete N6QW

Thursday, May 11, 2017

A New Line Of Transceivers ~ DifX

Transceiver Architecture 2.15


News Flash --- The LM373 SSB transceiver is on the air and the 1st contact was with W4DNQ, Ron who is in Florida. The contact took place at 1525 PDST on May 11, 2017 on 14.220 MHz. Here it is "al fresco"




Lets Build the 11.5 MHz Crystal Filter ~ Part III

 
I re-measured the 9 candidate crystals using the G3URR oscillator only this time using the 1 Hz position for fine tuning on my SDR transceiver. The field is now down to 6 crystals with five of those being exactly the same frequency in the loaded state and the 6th is only off by 5 Hz. As we used to say when I worked in Aerospace --Close Enough for Government Work!
 
So now I was ready to go back into the Dishal Software and using the new data, I  re-plotted the curve for 6 crystals. Again the setting of the Band Pass ripple materially affects the Z in/out and the capacitor values. Less of a ripple translates into a Lower Z in/out. So there has to be some reasonability (trade-offs) between Ripple and Impedance. Below is the new plot and we will now highlight some of the factors.
 


First and foremost is that it is a 6 pole filter that is characterized by steeper slopes and a 3dB bandwidth of 2.35 KHz. The band pass ripple is 0.13 dB (pretty flat) and the Impedance is 127.4 Ohms. 127.4/50 = 2.55 and a 8 turn to 5 turn broad band transformer is 64/25 = 2.56 ==so really close.

There are 9 capacitors needed for the 6 pole filter and in looking at the values these are easily achieved using high quality (NPO) caps in parallel with very small air trimmers (0-15 PF which were provided to me by a kind ham located on the east coast --Thanks Bob!)

An old time filter measure was to compare the 6 to 60 dB "shape factor". The BW at 60 dB is only 5.43 KHz --so that should do well with those California Kilowatts located just down the street from me. BTW the math shows 1: 2.225.

My next steps will involve acquiring some capacitors and building the actual filter. Stay tuned.

73's
Pete N6QW

Wednesday, May 10, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.14


Back to the LM373 SSB Transceiver.


While I take the time to re-measure the 9 crystals for the Dishal Filter, I thought I would update the LM373 SSB Transceiver project. This rig qualifies as a DifX as it is not a Bitx and the subject matter that will be covered has direct applicability to the DifX Dual Conversion project.
 
W5BAA's LM373 SSB Transceiver (Ham Radio November 1973) used 11.0 VDC as the supply voltage for his rig. If you look a the photo of his rig in the article you will see it connected to a 12 VDC Gel Cell. So connecting the dots, this gave him some lee way as the Gel Cell discharged. My evaluations of his approach actually demonstrated that his design did indeed "love" 11 VDC and so I followed his direction.
 
But now you have to have a source for 11 VDC (at least for the lower level stages (which is most of the rig) and you also needed a means of efficiently switching 11 VDC to the steering diodes to change the application of the circuit elements.
 
So task #1  was to come up with a 11 VDC regulator circuit. For this I went to an LM317 TO-220 style that is good for 1.5 amps. My circuit has the element of being adjustable which you will shortly see the why. The Internet abounds with regulator calculators and favorite circuits. I simply lifted one of those circuits and used values from one of the voltage calculators.
 
Essentially the LM317 samples the output voltage and using a voltage divider feeds that back into the V adjust pin. The ratio of the feedback resistor and the resistor from V adjust to ground sets the final output level. The LM317 can handle input voltage up to 40 VDC in and will regulate voltages up to 37 VDC out. The delta difference is needed for the regulator to work properly.
 
In my design the two resistors were 220 Ohms and 1715 Ohms. The 220 Ohm is a stock value but 1715 is not! So I homebrewed a 1715 resistor by placing in series a 1K connected to a 200 Ohm pot wired as a variable  resistor and that in turn is connected to a 680 Ohm resistor to ground. Thus with the pot a "0" we have 1680 Ohms and with the pot at 200 Ohms we have 1880 ohms. The pot also compensates for the tolerances of the 1K and 680 Ohm. The circuit works very well and this is shown below.
 
 
 
So now we have a 11 VDC source and as was mentioned when I first covered the LM373 Transceiver, W5BAA used steering diodes to route signals through the single direction IF block and it is through the application of the steering voltages that the LM373 rig goes from Receive to Transmit. Long ago I designed a circuit to do this. It was out of necessity as I was using an analog VFO (Yes Bill one of those old technology devices) and a power relay to switch voltages. Well the back EMF was so large (even with a snubber) that the VFO would jump frequency. This resulted in my solid  state switch design shown below to replace that pesky relay and has been updated for the 11 VDC. This same switch will be used in the DifX Dual Conversion rig although at +12 VDC.
 
Stop! I can see all of those who are into power FETs that will quickly make a posting about doing this with power FETs --you know will supply the arguments  that no current is drawn and how when you take this to the field battery life is precious and how your design is so much better. Well good for you. I am not taking this to the field, I have a stock of parts and it works just fine.
 
So how does this work. First I thought it would be cool to use an optoisolator like the 4N35. That device is also important as the next element in the circuit is quad NAND Gate and a bouncing input signal will give a lot of false triggering --the 4N35 acts as a buffer between your mechanical PTT switch and the 7400 IC. Two of the NAND Gates are wired as inverters with one Gate triggering the second Gate. With the PTT in an OFF state the upper 2N3904 has a signal applied to the base which powers on the TIP32C and we have juice for the receive condition (R). When the PTT is engaged the 7400 is toggled so that the HIGH output is not on Pin 6 but Pins 3,4,5 and thus the upper 2N3904 is now OFF and the lower 2N3904 is ON and we have juice for the Transmit stages.
 
You can build your self a truth table to verify what I said. Since there is a pull up resistor on Pins 1 and 2 that means it is HIGH (with the PTT disengaged) which results in Pin 3 being Low. But Pin 3 is bussed with Pins 4 &5 which make them low and now Pin 6 is High --driving the upper 2N3904 to ON. Now when you trigger the 4N35 with the PTT the output goes low which means the 10K is to ground and that drives Pins 1 & 2 to a Low state. Now Pins.3.4.and 5 are high and Pin 6 is Low. But since we are tapping off of Pins 3,4,and 5 and in a  HIGH condition we now see a signal on the lower 2N3904 and we have a transition from R voltage to T voltage. No relays are involved for this part of the circuit. Below is the build.
 
 
Yes there is a combo of leaded and SMD parts and the layout is close to the schematic. This is a keeper and no power FETs were involved in the build! When I connected the regulator and solid state switch to my LM373 I measured the voltage at the R and T ports and then adjusted the 200 Ohm pot so that they read 11 VDC at the R and T pins (collectors).
 
After some additional tweaking I now find that with my single 2N3904 pre-driver stage it will produce 10 Millwatts into a 50 Ohm load - (2 Volts Peak to Peak) . Now we will proceed with the driver and final stages. This is really moving along.

Tuesday, May 9, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.13

Lets Build that 11.5 MHz Filter Part II

 
In Part 1, (2.12) we shared how we measured all 25 crystals and found a grouping of 9 that were essentially "carbon copies" of each other. I never play the lottery because of that exact situation. I suspect all lotteries are rigged and the likelihood of having nine exactly the same crystals is somehow rigged. So the first thing I will do is to re-measure all of those nine in hopes of finding 6 out of that batch that are exactly the same. That may happen yet today.
 
So once you have the crystals and the two frequencies that were measured on the G3URR oscillator proceed to the WA5BDU tutorial on how to calculate Lm and Cm. I made an assumption that the crystal holder capacitance was 3 PF --probably not good or bad. Regardless, using the equations provided by Nick Kennedy I found the Cm to be 16.356 Femptofarads (10 to the -15) and using that value you can calculate the Lm which is 11.7 MHy.
 
Now for the actual crystal filter calculator go to the ARRL QEX download page and for the year 2009 find the Zip file 11/09_Steder_Hardcastle and download to your computer. Do NOT place it in the Program Files or Application Files!!!!!!!! I have a directory called N6QW and in that directory I have a folder called Dishal --extract it to that folder. Milton Dishal wrote a piece of software to simulate crystal filters. Steder and Hardcastle take that software and turn it into "ham speak" so it can be used by people like me.
 
Now I had a problem with the QEX download --my computer kept telling me it contained a virus and promptly removed it from my computer no less than 3 times. The fourth time on the Zip file I used a tool called compatibility --it seems when I now use the Dishal software my computer thinks it is a windows XP machine and all works good.
 
Here is where the dishal software can save you lots of effort --if you enter the shifted frequencies it will actually calculate the Lm and Cm --and they match my hand calculation. After playing with it a bit I can see that it would be ideal to have six crystals in the filter and here is why. See below.
 
 
 
 
The first plus is the 3 dB bandwidth which is about 2.2 KHz and at 30 dB down is probably less than 3.0 KHz. The 20 dB numbers and 40 dB numbers look OK. It appears to have a more symmetrical shape ( a factor that improves by having more crystal in the filter), thus getting away from the idea of strictly an LSB filter.
 
I found out the pass band ripple is linked to the input impedance. In this case it is 102 ohms. 102/50 = 2.04 and if we did a 7 turn and 10 turn winding on a FT37-43 core we would have 100/49 = 2.04 --so a perfect match. But if you drop the ripple to 0.2 dB then the input impedance raises to 127 ohms and this is a ratio of 2.54 when transformed to 50 Ohms. So if we use an 8 turn and 5 turn winding on a FT37-43 core that transforms to 64/25 = 2.56 ---really close.
 
The software has a built in  LC matching network calculator so that you can essentially design "L Networks" for the matching. I prefer the broadband match.
 
The 0.2 dB pass band ripple also "twizzles" the caps so that the values are much nearer to close tolerance standard value caps. I have some low value air trimmer caps (0-15 PF) that could be paralleled with the standard values so that the filter will be dead on.
 
I also believe this filter will have to be built in such a way where the crystal cases are flat to a ground plane and the crystal can soldered to that plane. I will use SMA connectors to the filter and it will be totally enclosed in a metal box. This will be bullet proof.
 
My plan is to update this posting with the re-measuring of the 9 crystals --to see if I should play the lottery today.
 
73's
Pete N6QW




Friday, May 5, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.12

Let's Build That 11.5 MHz Crystal Filter!

 
 

Sunday May 7, 2017

 I have completed the first round of crystal measurements and have found several groupings of crystals that have potential for a filter. One group has nine crystals and my next step is to re-measure all 9 to verify that indeed that all have the same specifications. I used different colors to highlight the crystals with the same specs. See the table below:
 
 
 
 
Stay tuned for the next round of measurements.
 
73's
Pete N6QW
 
___________________________________________________________________________________

 
 
See video at end added today on measuring the loaded crystal frequency --yes being done with a Raspberry Pi3 and a SoftRock V6.3 SDR radio.
 
My bag of 25 crystals arrived (May 4th) and in case you are wondering what I bought here is the Mouser Part Number (See below) The one thing nice about the manufacturers specifications is that the ESR (Equivalent Series Resistance) is provided and that number is 35 Ohms.(That could be good or bad but at least I know what they say it is,)

449-LFXTAL026687BULK

 

 

It is amazing how crystal filters are suddenly at the top of everyone's list. The man himself, W7ZOI, Wes Hayward on May 1, 2017 just released an exciting document that details the "innards" of our beloved crystals as used in filters. You can find that document here  Essentially Wes describes the crystal model and how today's crystals present some issues when we try to build crystal filters. Download this and keep it under your pillow at night.
 
In Transceiver Architecture 2.10, I mentioned that there are some excellent you tube videos on the subject of homebrewing crystal filters and w0qe, Larry Benko, in his part 13 and 14 has taken up that task. Part 13 deals with some of the front end analytical work and Part 14 gets to the nuts and bolts.

One tool used in Part 13 was the use of Elsie (free Student Version from Tonne Software) to simulate a crystal filter. The "free version" will only let you simulate a 4 crystal filter but that is good enough for our purposes. The professional version lets you add many more crystals.  Elsie will not design a filter for you --BUT it will let you "twizzle" the parameters so that you can visualize how changing parameters impact the shape of the filter curve and the SWR. Interestingly the Elsie parameters match closely what you will finally need for a good filter. The Zin/out looks like less than 200 Ohms and the coupling caps are in the range of around 100 PF.
 
Using the Elsie Software (with help from Jim Tonne, the developer) I was able to simulate what a crystal filter might look like using my 11.5 MHz filter frequency. If I could match this model in practice, we would have a pretty good second filter for our DifX Dual Conversion Transceiver. It is really interesting to see how changing the bandwidth and Zin/out materially affects the shape of the curve. My latest run at this suggests that the Zin/out should be about 100 Ohms and the bandwidth 2.8 KHz which of course will place me in the league of the enhanced SSB guys. But those numbers are not the final values --just data.The combined plot below shows the filter shape and the SWR. Don't overlook the SWR as that impacts the filter capability to pass signals without distortion!
 

 
 
In Part 14 Larry, presents a plan on how to go about actually building a filter and it starts out with "get a bag of lots of crystals" and then "measure them and record the results in an Excel Spreadsheet" -- hmmm sound familiar?

The Elsie Software presents an idealized filter with crystals that are exactly alike (you'd like to see to 10 decimal places).  But that is not the real world as the Elsie software only captures just a few decimal places-- like out of the box maybe two or three places. As filter builders we in essence TUNE each crystal so that it will match the Elsie model. This is not a five minute job! But it can be done. In Part 14 Larry, show how he kept changing parameters and how that fine tuned the filter. I suspect the first time through the process will be like being 7 years old and waiting for Christmas and it is only June.

My first step is to identify and then  measure each crystal and finally record the results. Fortunately long ago I built a G3URR oscillator and will use my Raspberry Pi SDR radio (with Quisk) as the measurement tool or I might use my Omnia with HDSDR --so I have some choice here. Or my new Rigol Scope that has a built in frequency counter.

But having just run some tests using the Rigol Scope  "counter function option" has shown that will not work. The reason is that the scope (or my lack of knowledge on how to have this do something different) only takes you to 4 decimal places. I ran a few crystal into to the G3URRR test oscillator and the scope counter read 11.4998. This is not enough resolution! They tutorials all sing the same song. If you use an external oscillator method to excite the crystal for measuring Lm and Cm then you need to have a generator with a 1 Hz resolution. So you will need to have something as good to measure the crystal response.

My friend KV4QB, DuWayne was so kind to send me an SNA Jr. board and it will produce the 1 Hz resolution for the DDS generator. I have an Omnia SDR and with HRSDR can read the signals to 1 Hz. Keep in mind we want no more than a 50 Hz spread for a SSB filter and as I found out no more than 10 Hz for a CW filter -- the Rigol scope will work to get you into the ball park but will not tell you the seat number.

w0qe suggested several actual crystal filter design programs (including the AADE one I mentioned). But did indicate several that were more user friendly and so I will look to using one of the ones suggested.

For now I will have to take a short respite while I measure the crystals and record the results. Stay tuned. Yesterday using my handy Brother tape label machine I punched out 25 numbers and will start first by affixing the numbers to each of the crystals. Then I will need to clear a spot off of my work bench so I can do the measuring. I may get a chance to video the crystal measuring process as that may be helpful t those wanting to build the 11.5 MHz filter.



Too many projects and not enough time --but I will continue to work on the LM373 transceiver and keep you posted on its progress.

73's
Pete N6QW

 

Monday, May 1, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.11

5~04~2017 1St Transmitter Tests of the LM373 Transceiver. Pure Joy.



A Way Forward is to Look Back.
Soon it will be time for another Solder Smoke Podcast (#196) and Bill, N2CQR and I hope to spend some time on "Old Time Homebrew Transceivers". Not necessarily that the transceivers themselves are old for they are both new; but that they involve older technology. BTW this is the 3rd year that I have been doing the podcasts which started with #161 in May of 2014.
 
[I am taking a short detour from the dual conversion DifX as I await my bag of twenty five 11.5 MHz crystal to arrive and then I will share (using Method #4) how I built the filter.]
 
In SS #195 I casually mentioned that my first solid state SSB transceiver used an amazing single IC from National Semiconductor (now a part of Texas Instruments) called the LM373 and that was in the early 1970's. My rig was on 20 Meters. My first introduction to this device was from an article in a 1970's QST by Tom Sowden, W6KAN and today a friend of mine. His project used a 9.0 MHz crystal filter and covered two bands with a single VFO -- the old 20/80 Meter trick. If you take the sum frequency of a 5.0 MHz VFO and the IF you get 14 MHz, and of course the difference 9 - 5 = 4 which is 75 Meters. In one case the VFO tunes forward and in the latter case backwards -- but hey two bands. That receiver circuit formed the basis of my transceiver project. My transceiver worked as did one built by my then next door neighbor, Bill K6ACJ. I think Bill still has his rig.
 
 
 
Below is the LM373 datasheet. It was good through 30 MHz which of course is 10 Meters.
 
 
 
But a real uptown version of an LM373, 20 Meter SSB Transceiver was built by Charles Hill W5BAA as detailed in a November 1973 Ham Radio Article.
 
 
 
It should be mentioned that I contacted Charley, W5BAA at the time this article came out and he sent me some additional data not in the article. Today Charley and I are still in regular contact and I even bought my first SDR kit from him which was the subject of a QRP Quarterly Article.
 
Charley's transceiver was absolutely amazing. His rig used three LM373's as this versatile IC had many applications. The primary use was as the main circuit element of the transceiver where it was used as the IF amplifier wrapped around a crystal filter and as the product detector on receive and as the transmit mixer on transmit. A second LM373 was wired as the Receiver Mixer stage. In W6KAN's receiver it used a 40673 dual gate MOSFET for the mixer stage. The third LM373, in W5BAA'a rig was used as the Balanced Modulator.
 
In most of my rigs I use double balanced mixers such as the SBL-1 or, TUF-1 or even the ADE-1L for the front end Rx Tx mixer stage and a second DBM is used as the Product Detector on receive and as the Balanced Modulator on transmit. This approach facilitates making a singular connection of the LO to one DBM and the BFO to the second DBM. So there is no switching of these signals as you go from receive to transmit. This saves some wiring and the need to do switching of signals. What we have is the bilateral approach where the signals are sent left to right in the IF for receive and right to left on transmit.
 
But another non-bilateral method is to send signals a single way through the rig and at either end you change the elements. On receive the input side has the receiver mixer and on the back end is the product detector. In transmit the front end has the balanced modulator and the back end the transmit mixer and RF chain. In this case circuit elements  must be steered to the front and back ends of the IF block depending whether you are in receive or transmit.
 
In W5BAA'a rig his signal steering involved the use of a property of diodes to act as switches. When diodes are biased properly they appear as a closed switch. Change that bias and it is like an open circuit. In the receive mode the VFO is "steered" to the LM373 acting as the Receiver Mixer and the BFO is steered to the product detector port on the main LM373. In the transmit mode the BFO is now steered to the 3rd LM373 that acts as the Balanced Modulator and the VFO is now steered to the port where the BFO was connected. Interestingly the output pin that connected to the audio chain on receive is now connected to the transmit circuitry in the transmit mode. [An Italian ham built a vacuum tube transceiver ( I think it had five tubes) and the audio output tube became the final RF amp stage on transmit]. Charley's rig put out a whopping 5 watts --so certainly QRP.
 
So based on my utterance on SS#195 I decided to recreate my first solid state transceiver circa 1970 using the LM373. I thought this would be blast although my first easy decision: NO Analog VFO and No Crystal BFO. Going back in time is fun; but using an Analog VFO is like doing brain surgery with a rusty spoon!
 
I also decided that I would use SBL-1's for the receiver mixer and the balanced modulator. It wasn't until I had finished cutting a PC Board and began installing parts that I realized that I really only needed one SBL-1 as it could have functioned as the Receiver Mixer on receive and as the Balanced Modulator on transmit. Should have noodled more on this one. But true to Charley's design I have used diode steering. Well if I ever build a version #2 that would be a change.
 
I also decide to use two separate Band Pass Filters as this simplified switching. My board at this time has only the receive one installed but the second unit is on today's work list. The Rx RF amp is a single 2N3904 as will be the transmit pre-driver stage.
 
So where is my project now? Well after wiring it up I discovered it didn't work and then right in the middle of doing some trouble shooting my Hantek 200 MHz Digital Storage Scope shot craps and it was toast! I always thought it acted flaky. So I had to get a new scope which I settled on a Rigol 100 MHz DSO --good move and works much better (Model #DS1102E).
 
Initially I had chosen to build the LM373 rig using a mix of mostly surface mount parts and a few leaded parts and my board layout was designed for that purpose. I should mention that I had a small stash of LM373's left over from 1970 and some were the 10 Pin TO-5 style while others were the 14 Pin DIP. 10 is less than 14 and so the board was designed using the 10 Pin can style. When the rig didn't work, my first thoughts were bad devices and so I made an adapter board so that the 14 Pin Dip style (using sockets) was retrofitted to the board. Still didn't work --thus had to wait on the new scope arrival.
 
Well 5 minutes with the new scope convinced me the problem was N6QW. What I hadn't mentioned was this was the first time I used a new soldering iron and what I had done is applied too much heat to many of the SMD parts and they were burnt "open". If you put the scope probe on one side of a cap, say on the LO side, you get a nice signal and plenty of juice. Put the Scope on the other side and you get almost nothing. So step one: change out all of the SMD coupling caps (where I found several others that were open) to leaded versions and boom it worked. At this point just the receiver is working as I am awaiting some parts (coming with the bag of crystals) to finish up the transmitter stages and then we can have another go with my 1970's SSB transceiver. Maybe before the next podcast
 
Here for your listening pleasure is the LM373 SSB transceiver vintage 1970's
 
video
 
 
 

 
 
 
As was stated the way forward is to look back -- signal steering would not be my 1st choice today; but in 1970 it was the right thing to do. The LM373 has a built AGC circuit and it can even be made adjustable --that certainly is a desirable feature in today's rigs. I have no good explanation why there was not more widespread use of the LM373. But for a low part count rig --it is perfect.
 
Oh you will have a terrible time finding LM373's (I happen to have some in the bin's) so for those who want to replicate this rig -- just buy a Bitx40 kit and move on.
 
When I get the transmit side working I will share that with you. Lets have a cheer for W6KAN and  W5BAA and the amazing 1970's --remember the  even / odd gas days.
 
73's
Pete N6QW
 
 
 
 

Saturday, April 29, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.10


How to Build a 11.5 MHz Crystal Filter

Method #1:

Simply Purchase four 11.5 MHz Crystals at the cheapest price you can. Next build a Ladder Filter using five coupling caps of the same value. For a SSB Filter use 68PF and for a CW Filter use 470 PF. A guess at the in/out impedance would be in the neighborhood of 150 to 200 Ohms. Use 200 Ohms, as a 50:200 match is just a 4:1 transformer.
 
With this method you rely mostly on luck. It will probably not work too well. However if it does then you should immediately go out a buy a single lottery ticket as you are on a path to striking it rich.
 
Here are the shortcomings of Method #1. With only four crystals and making no measurement of their actual frequency you will never know: 1) how well matched they are in frequency 2) what is the filter center frequency and 3) the actual Zin/out. Did I also mention that if you don't test the crystals in an oscillator circuit prior to just installing them in a filter you may not know that one or several are inoperative (you did buy bargain crystals). But many "good enough" filters have been made this way. That said very likely there are substantially more poor filters than good ones that were built using this approach. But hey you built a crystal filter.
 
 

Method #2:

You purchase four crystals at the cheapest possible price and you make a measurement of the crystal frequencies using the G3URR test oscillator. You dutifully note the "loaded" frequency and the spread of each crystal as related to all of the crystals. (A goal is no more than about a 50 Hz spread across all of the crystals). After obtaining this data you simply ignore the information and follow the Method #1 approach. Again if it is perfect, then buy more lottery tickets. But more than likely it will not be. Oh by the way --you will probably need the center frequency info in your Arduino Sketch so you know how much to shift the USB LSB BFO frequencies -- but hey close is close enough. So you get a few dings from the SDR police on 40 Meters --who cares?
 

Method #3:

This is where you find some ham who really knows what they are doing and after an enticement of the standard B^3 (Booze, Bucks and Babes) have them build you the filter. Just sit back and relax and wait for the unit to arrive. This is a lot less stressful and all you need to do is install it in your rig. Now wasn't that easy? Never let the XYL find out you spent $250 for a crystal filter is the real issue.
 

Method #4:

This is where you get serious about homebrewing a crystal filter. The process involves the following:
  • Collecting information on how to actually build a crystal filter. There are several really good sources. First do an Internet Search on Nick Kennedy WA5BDU, as he has prepared an exhaustive tutorial on the steps needed. Also search on Almost All Digital Electronics as they have a computer program that is very handy to design a filter. I also believe that EMRFD has a program on the DVD that is located in the back jacket. Do not overlook You Tube Videos on how it is done. Bottom line you need a disciplined process and resource information.
  • You will have to build some test hardware including the G3URR test oscillator. Basically this oscillator enables you to measure the frequency of a crystal and then by loading that crystal with a small capacitance shifts the crystal frequency. That amount of shift is an important parameter in the final calculations. (It has something to do with pole zero spacing) This is where you need to buy one of those $13 TV SDR Dongles! Get one and modify it so it will work on HF. You should also download the free software program HDSDR. This $13 device will let you precisely measure the crystal frequencies with and without the load. Almost better than a frequency counter. All you do is power up the oscillator and with a short "antenna lead" bring the oscillator near the Dongle and look for the output.
  • Purchase twenty five 11.5 MHz crystal (about $0.30 each at this quantity) from Mouser. When they arrive use a Brother tape label machine on the smallest print size to label every crystal from 1 through 25. Open up a Excel Spreadsheet on your computer and record the loaded and unloaded frequencies for the crystals marked 1 - 25. The first thing that should amaze you is that the crystals while nominally 11.5 MHz are all over the map. Now you can either do it by visual inspection or have Excel do it but rank order the crystals from high to low frequency. Once you do that you should now look for groupings of crystals that are within 50 Hz total spread from low to high. You might get lucky and find five or six that meet this criteria but you need a minimum of four. You will most likely find out of the batch of 25 that you will have several groupings of at least four crystals that are close in frequency. Once you have at least four then you need to follow the process outlined by Kennedy. Look to the manufacturers specifications as you might be lucky to find the "Average" series resistance as this number is needed for the calculation.
  • Most likely the final filter (for SSB) will have low value coupling capacitors (around 100PF not all the same) and the Zin/out would be in the 170 Ohm range. But unlike Method #1, you are being precise in the measuring process and WILL have four crystals that are close in frequency.
  • Method #4 is not a 1 or 2 hour process -- it might take several sessions to complete the data analysis and calculations before you start soldering crystals to a circuit board. Be sure to connect all of the cans together and ground that connection. Build the filter over a large ground plane area.
  • At this point Method #3 has a lot of appeal.
Have fun.
 
73's
Pete N6QW



Friday, April 28, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.09

 

Dual Conversion Band Switching


In this posting I want to talk about some of the specifics of the band switching and how to cause the proper Band Pass Filter and Low Pass Filters to be put in line for the band in use.

The Arduino Mega 2560

The first realization I had with the dual conversion multi-band approach was that you needed a lot more pins. There will be those who immediately jump up and say but you can add a pin expander to the standard UNO, Nano and/or Pro-Mini and no need to move up to the larger footprint and more costly microcontroller. There is another requirement that is answered by the Mega 2560 and that is the 10X increase in program size. That perhaps is the bigger driver for the Mega 2560. Undoubtedly there will be more things you want to add to your homebrew rig and then pins is not the issue but programming space will be. The Mega 2560 has 54 digital pins and 16 Analog pins and thus you have many more options available to you. Now as I discovered there are differences from the  Uno, Nano and Pro-Mini so you will need to think about pin assignments and it is not a straight pin for pin compatibility. I will highlight those differences.
 
 

The Band Switching Scheme

My band switch scheme involves two band switches and has the capability for 17 band positions. One band switch has 12 positions and the second band switch has 6 position. So OK you have a blank look on your face. Hey guys Ten Tec did this with some of their transceivers, where you placed the main band switch on 10 Meters and the auxiliary band switch then selected four sub bands within the 10 Meter band.
 
For my Dual Conversion DifX when the main band switch is placed on 60 Meters  that merely connects to the auxiliary band switch where you can select the five channels on the 60M band including one channel that will be tunable as I did in my 60M DifX rig. Moving away from the 60M position in effect disconnects the second band switch.  The process of selecting the operating band with the band switch will automatically trigger a comparable Mega 2560 Pin that has 5 Volts on that Pin to control either relays or Pin Diodes in the Low Pass and Band Pass filters. One switch position changes frequency and selects the proper filters.
 

Mega 2560 Pin Assignments

 
 
 
 
 
The logic of the code is that the digital pins are read, as an example Pin 30, which is designated for 20 Meters when that pin is sensed LOW, then Pin 39 will go HIGH and that switches in the proper Band Pass Filter and Low Pass Filters. On 60 Meters Pin 49 is made HIGH for all of the 60 Meter Pins.
 
Pin A10 and A11 on the Mega 2560 are used for Encoder A and Encoder B. (Thanks Rob!)
 
Keep good notes as we are moving forward with the design.
 
73's
Pete N6QW



Wednesday, April 26, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture Part 2.08

The "Math" of the Frequency Display!


Don't you just hate it when you see information without detail and much like the guy at 1600 Pennsylvania Avenue you hear 'Folks You Will Love This". But it still leaves a lingering question --just how did we get here and how do you insure this is not Fake News!
 
In our last posting we advanced the idea that with dual conversion this presents some unique problems because of the frequencies involved and how to accurately display the true transmitted frequency. Essentially we have about five frequencies we must deal with in our display process --and that is just for SSB. By way of recap we have the incoming frequency, then the 1st mixer which is the tunable Local Oscillator (LO) followed by the fixed frequency 2nd mixer and then the two BFO frequencies that address either USB or LSB.
 
Mind you this is Pete's scheme -- there are obviously others and better ones. BUT I understand this and I can be assured that I will not be transmitting out of band. In the final analysis there are many roads to San Diego --the objective is to arrive in San Diego.
 
In my scheme when you shift from USB to LSB the dial will change by 3 KHz without moving the encoder. In going from USB to LSB you will have to move the LO down by 3 KHz to transmit on the same frequency using the opposite sideband. Inconvenient for some -- well not for me. So if this is objectionable to you, then stop here and go write your own code!
 
There are several actions taking place in the background as you change bands and change from USB to LSB which I will now detail. [Because we are using two conversions and the conversion frequency is above the incoming -- the net effect: There is no sideband inversion. Thus the lower BFO frequency will be used for USB and the higher frequency BFO for LSB.]
 
  • Let us say you will start by tuning in 14.2 MHz on 20 Meter Upper Sideband --a favorite spot of mine. As you put the mode switch into the USB mode and turn the encoder to 14.2 there are some behind the scenes steps taking place. The first is that 1500 Hz will be subtracted from the display formula and secondly the LO is preloaded with a frequency that will show 14.2 MHz on the display. But the display is actually the subtraction of 45 MHz and the subtraction of 1500 Hz from the start up frequency. The 1500 Hz is the nominal offset from the center frequency of the SSB crystal filter. [Typically it is +/- 1500 Hz depending on the sideband. ] For LSB on 14.2 MHz we will be adding 1500 Hz.
  • The LO uses a subtractive mix process so the LO - the incoming signal = 45 MHz. Our pre-loaded LO frequency already contained a 1500 Hz add. So our subtractive mix actually resulted in a frequency of 45001500 MHz plus the Voice signal. Since the Bandwidth of the ESC 45 MHz Filter is 7.5 KHz --this is not an issue.
  • The second mixer is at a fixed frequency of 56.5 MHz and the second down mix results in an output of 11.498500 MHz + voice.
  • Feeding this signal into the Product Detector with the USB BFO of 11.498500 leaves only the audio voice signal which is upper sideband.
  • For a LSB signal the preloaded LO will have to be tuned down by 3 KHz to put you on 14.2 MHz LSB. So now the LO is at 59198500 MHz and when we subtract 45000000 we must add in 1500 Hz to make the display read 14.2 MHz. So putting the USB/LSB switch into LSB causes the addition of 1500 Hz to the formula so that the display will read the true transmit frequency.
  • Thus the first mix has the LO at 59198500 - 14200000 + Voice = 44998500 + Voice and this mixed with 56.5 MHz 2nd Mixer signal results in a frequency of 11501500 + Voice.
  • Sending that signal on to the product detector with a LSB BFO of 11501500 results in LSB Voice.
 
 
 
Thus you have the decode on how to use the dual conversion scheme yet have the frequency display read properly for the true transmitted frequency. CW can be done the same way --but involves a lot more code and since I am not a CW person have not done any more with it. But shifting one of the BFO frequencies for CW transmit will get you there as was done in the KWM-4 and use USB for receive.
 
Time to start building your DifX. In a couple of days I will post a link to the Arduino Mega 2560 code and that will be on my website at http://www.n6qw.com
 
73's
Pete N6QW

Sunday, April 23, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.07

So How to Get the Display To Read Right!

4-26-2017 Update

Thanks to Addi dc0dw , here is a link that explains about interrupts. This actually makes sense. LINK

There is a caution here in that the 328 (internal structure and wiring) is different from the 2560 and that is why I was having problems. But a really good treatise on interrupts.

73's
Pete N6QW

4-25-2017 Update

Get Your Heart Racing! --Here are some screen shots of the DifX Dual Conversion display. When you shift from USB to LSB the Display changes by 3 kHz.






The current processor is an Uno or Nano but because of the pin limitations I can only support 7 bands. I have loaded the code on a Mega 2560 and am able to have 15 bands.

But I am asking for some help with the Mega 2560. While I can get the bands to change and USB/LSB to change and the TUNE function and S Meter to work. The Encoder is Dead In The Water. Evidently there is a different code set to make the Mega recognize the Interrupt on Pins 2 & 3. There is some goofy function called attachInterrupt --well I tried the sample code and it just stares at me or nothing happens. So if any kind soul knows how to get the interrupt function to work with digital Pins 2 & 3 so that I can tune the encoder --please let me know.
 
73's
Pete N6QW

I have very much admired Ten Tec and the products they built. Sadly I am not so sure about the future of that company given the number of recent owners and the state of the technology coming from offshore. I have a Ten Tec Triton Model 544 and always thought it one of their best rigs. Some will argue with me about later rigs and how much better crystal mixing was/is over a synthesized rig. But the fact remains they did build some great gear.
 
One anomaly I found in some Ten Tec designs (I think was the Corsair series), where the display would read very properly on the normal emissions for the ham bands id est [that is Latin for i. e.] LSB on 40 Meters but if you switched to Opposite (Ten Tec didn't use LSB or USB but Normal or Opposite), the display would not show the correct frequency. In the Ten Tec documentation there was a small obscure note to this effect.
 
In my DifX designs I have paid particular attention to this issue in my Arduino Code. I always place the LO above the incoming frequency and there is math in the code that for the display what is read is the LO - BFO. So If I switch from LSB to USB on 40 Meters (say 7.2 MHz), and since there is a sideband inversion taking place, the higher BFO frequency is used for USB. Thus the display while reading the true transmit frequency will now display a lower frequency on the LCD. When you shift back to LSB, the lower BFO frequency is used and so now the LCD will read higher. So I do have to move the encoder to 7.2 MHz when shifting from LSB to USB but the true transmit frequency is always displayed.
 
Some would like it so that in switching from USB to LSB you would not have to move the dial --and that is just more code. So I  move the dial and not add the code. But what ever the display and whatever the mode when I read the LCD --that is the true transmit frequency.
 
I addressed this problem in the KWM-4 by using an LO that changed based on the mode. The 1st conversion frequency was 10.7 MHz. Because I simply shifted channels for USB/LSB such that the difference of the LO and incoming would not be 10.7 MHz but either slightly higher or slightly lower than 10.7 MHz and this would easily pass through the roofing filter. The slight difference was then the nominal +/- 1.5 kHz. So then the BFO selected for the Mode would make up that difference. Thus 7.2 MHz for either USB or LSB would read correctly on the LCD and that was the transmitted frequency.
 
In Dual Conversion designs the true reading of the display becomes more complex because of the multiple conversions and the Dual Conversion DifX is no exception. Lets us look at what might be required to get the display to read correctly. We will use 40 Meters as the example.
 
The 1st IF is at 45 MHz and thus for 7.2 MHz the LO injection would be 52.2 MHz --thus all incoming signals are subtracted from the LO and converted to 45 MHz and in the case of the 2nd mixer which is operating at a fixed frequency of 56.5 MHz the second conversion is to 11.5 Mhz. There is no sideband inversion (we did it twice) and thus to receive LSB our BFO must operate at 11.5015 MHz. So now the problem of how to get the display to read 7.2 MHz LSB. Let us not forget if we shift to USB the BFO is operating at 11.49985 MHz and again how do we insure that the LCD and transmitted frequency is in fact 7.2 MHz USB.

In earlier display designs (1970/80's) there were summing circuits that read the HFO (High Frequency Oscillator), BFO and VFO and the results were then displayed. The DFD-2 (from AADE) did exactly that process. We will do something similar in the DifX. My first encounter with this issue was in 2009 when I built the Tri-Band SSB transceiver that used the frequency scheme AND components from an HW-100. You can read about it here http://www.jessystems.com/2009_xcvr.html

In the description I speak about the summing circuits so that you combine the HFO, BFO and VFO so that the display reads correctly. The innovation from N6QW was to mix two of the components in an SBL-1 and then to use  tuned band pass filters so only the correct mix was used and then that was finally mixed with the VFO signal. Using an EI9GQ huff and puff stabilizer it was possible to shift the display depending on the sideband and thus what was displayed was the true transmit frequency. It was a lot more hardware but 8 years ago I was on frequency!
 
The lazy approach would be to simply display the LO - 45 MHz [Display = LO - 45 MHz] and that would get you close (within KHz) and so in shifting from LSB to USB the display would not change with the mode. But in either case you would be off the true transmit frequency--the 40 Meter SDR Police will have a field day with you -- keep in mind they get gnarly when you are 20 Hz off --imagine being off by kHz. So that is not a good solution -- but a start.
 
Now what if there were one more factor in the math that changed depending on the mode. In case you got lost in the last paragraph we are up-converting the incoming signal to 45 MHz and thus to get the actual signal value we subtract 45 MHz. So now if we rewrite the equation to say the following to account for the  total of 3 KHz in the shift in the spread then we would have Display = LO - 45000000 + Offset. Now we can sign the offset as +/- depending on the mode. So if we were on LSB the Offset would be one value and for USB the Offset would be another. The display would now change by a total of 3 KHz depending on the mode. The second conversion frequencies can be simply ignored as all that is doing is getting the signal to the IF frequency. This is not unlike our single conversion code. Yes you will have to reset the dial when shifting USB/LSB but you will be displaying the correct transmit frequency.

In the next installment I will detail how I did it and the math involved. Maybe if I am lucky I will have code I can share. But the bottom line is that when shifting mode (USB/LSB) the display must account for the BFO shift and display the true transmit frequency. In the old analog VFO days many manufacturers fudged this problem by putting two (yes two) scribe lines on the dial window. One you used for USB and the other for LSB. Amazing solution --like using a rusty spoon to do brain surgery.

73's
Pete N6QW
 
 

Tuesday, April 11, 2017

A New Line of Transceivers - Difx (On Steroids)

4-13-2017 ~ A small Radio story.

Last evening (4/12) my XYL and I went to our favorite Chinese restaurant. Great food and really reasonable prices. As we walked in the door we saw a large party of about 25 people who were celebrating a birthday. I noted an older gentleman seated at the head of the table and our booth must have been only about 5 feet away. I leaned over to another gentleman sitting closely to me and inquired if it was a birthday celebration. He responded back that it was his father-in-law who turned 97 that very day.
 
I wished the celebrant a Happy Birthday and then commented to the other gentleman that his father-in-law experienced the evolution of radio broadcasting, the stock market crash, the end of prohibition, the New Deal, completion of Hoover Dam and the SF Bay Bridge, WWII, TV, space travel and the computer revolution. With that there was a bit of a buzz at the table as the other well wishers suddenly grasped what this man had seen.
 
Then the Birthday Boy spoke up and said I built my first crystal set when I was age 9! Boom that was amazing. I then asked if he wound the coil on an Quaker Oats box and used a piece of aluminum as a slider tuner -- and of course finding the hot spot on the galena crystal. He then said yes and asked how I knew that. I explained that about 18 years after him I built my 1st crystal set. What an experience. Just think anyone in their 90's has seen a dramatic change in our world. Of the 16M who served in WWII only 600,000 are still around. Not many left.
 
73's
Pete N6QW

 

DifX A Dual Conversion Transceiver!


In 2017, VU2ESE announced his uBitx (Micro-Bitx) transceiver which uses an up-conversion technique to a 45 MHz 1st IF and then a lower 12 MHz second IF, which handles the normal transceiver functions. This is a well known Gain and Selectivity approach to minimize "birdies" and to better process a lot of crud showing up on our beloved ham bands.

In theory the higher IF provides the gain and the lower IF the selectivity. But there may be more subtleties with actually having the higher IF to also provide a degree of selectivity much like a "roofing filter". So the first IF must be designed as such as to have  gain and bandwidth parameters in keeping with up conversion and crud prevention.

The magic key in a successful transceiver design is a reasonable gain distribution with attendant selectivity over the entire rig topology. Super high gain front end amplifiers that overload a receiver can and should be eliminated and the gain made up in later stages. When you hop up the signal gain at the front end --you are doing the same to the noise coming into the receiver chain! So what have you gained (that was a pun)?

A dual conversion has other desirable attributes such as the second filter selection. My KWM-4 has a 455 KHz "primo" Collins  mechanical filter embedded as the second filter; but the choice is mostly open to the many of the popular, 8, 9 ,10, 11 and 12 MHz filters. I say mostly as you really have to perform a frequency analysis to determine unwanted mixing products and ones where harmonics of BFO's or LO's end up in the middle of some conversion process. Later you will see such an analysis for this DifX rig. Don't overlook the 4.9152 MHz IF frequency as used in the Elecraft K2.

In fact there is some very strong support for keeping crystal filters in the range of 4 to 10 MHz and avoid those above and below that threshold. The reasons are many especially with the higher frequency filters where  stability is a very major factor. Yes stability -- usually crystal stability ratings are in PPM (parts per million). The inexpensive units (C^3 = Cheapo Chinese Culls) may be rated at 50 PPM or worse. So lets run the numbers at 4 MHz and 50 PPM that is as much as 200 Hz and at 12 MHz (like in the Bitx) = 600 Hz. The better units are 30 PPM (real crystals and more expensive) so our 4 MHz example is now 120 Hz and the 12 MHz versions are 360 Hz. When the 40M SDR Police report you for being 30 Hz low, you can see where this is headed. By the way the lower end filters like say 1600 kHz units (that notably were used in the hallicrafters SR-150) present some issues with regard to image rejection.

Later mention is made of commercial monolithic crystal filters -- the superior characteristics of these filters comes from the use of a common base crystal structure and tight control of the manufacturing processes. That is hard to do with 6 or 8 discreet crystals tack soldered to a PCB board that may be varying in frequency all over the place.
 
Speaking of crud, as I earlier mentioned, I am suspicious that someone in my neighborhood (Southern California) is growing "pot" in their garage, as I hear what appears to be the cycling of "grow lights" especially on 40 Meters. Thus some receiver architectures are better than others in handling such man made interferences. My only hope is that with the recent legalization of pot, my neighborhood grower will soon be out of business. I walk in my neighborhood on a daily basis. Up to now I have been looking for the culprit from among my neighbors who seem very happy (all of the time)  and sporting beer bellies from consuming too many munchies (Overly happy YL's and one's with the onset of mid-drift bulge are included). But I need to be more get scientific and use my compact 20M Transceiver (DifX II) with a battery pack and do some serious direction detection. You all saw that rig on the cover of a 2015 QRP Quarterly --right?

 
 
In 2012 I built a dual conversion SSB transceiver known as the KWM-4 which of course, was a DifX (Different than a Bitx.) Now we are moving to another DifX (on steroids) which uses that earlier topology but takes advantage of newer technology not readily available just a short five years ago. Essentially I will be strapping on a mixer conversion stage ahead of the basic single conversion DifX stages that I have been building for some time and most recently in the 60M Rig, the Big Kahuna and the DifX II.

I guess my 1st use of a DifX topology was in  the Spring of 2010, when I built the 20M MMIC Based SSB Transceiver which graced the cover of QRP Quarterly. Now that was a bit of innovation --MMIC IC's used in bilateral amplifier stages. For my friend N2CQR, that rig started with an analog VFO. But as you can see, soon morphed to the "Digi" stuff --- 7 years ago.

 
 
For my newest rig, the directed approach is to take advantage of a packaged 45 MHz, 7.5 kHz wide commercial monolithic crystal filter available from ECS (Digikey). While the uBitx employs a 45 MHz homebrew multi-pole filter, unless one is extremely lucky it will be difficult to match the performance of the ECS unit. Yes I know, there are the purists who want to homebrew everything and certainly that is commendable. But when I have an opportunity to utilize a device with known specifications at the outset, then it is a simple engineering decision.

I liken this to building homebrew double balanced mixers. Was I successful building DBM's  and did they work --yes? BUT an SBL-1 beat the pants off of the six that I homebrewed. It is hard to match components built on a manufacturing line using precision parts and tight process control while attempting to do the same with non-precision stock components, in a cold garage with an 80 Watt Radio Shack Soldering iron with a "Fat Tip"!
 
At the end of the day I want a top notch transceiver not just one that works. The packaged ECS filter is less than $17 USD and the cost differential of what would be spent building a multi-pole filter is not so great as to warrant a non-consideration of the commercial "black box". Keep in mind you might need to buy two dozen 45 MHz crystals just to find six close enough in frequency. With a matching circuit provided by the manufacturer the 45 MHz four pole filter with a 7.5 KHz bandwidth has a Z in/out of 50 Ohms which is perfect for insertion into many of the receiver topologies. The Si5351 third clock provides the 2nd mixer frequency. The matching circuit is shown below and L1 and L2 can be 11 Turns of #24 enameled wire on a T-30-6 core. (T-30-6 ~ Al = 36)

The first task I tackled was coming up with a PCB layout for the filter (really small) and the matching network. I simply took the manufacturer's pad layout which was in mm and I made a drawing at 10X in the normal inches (forget that metric BS). Once I had that done I used the design program (G Simple) to scale everything by 1/10 and then using the fact that 1 meter = 39.37007 inches (1 inch=2.54 CM or 25.4 mm) I had the design program further scale the design down to mm. I made a test cut on my CNC and was very close. I then had the design program scale everything by 1.10 (10% Increase) and it was dead nuts on. The alternative for those without a $250K CNC machine --you can just turn over the filter and wire it dead bug.

Below is a quick and dirty first cut prototype after the 10% upward adjustment; and the overall width is less than 5/8" so you can see fairly small. I have dxf files for this pad layout and the  ADE-1L DBM which I will be happy to share. I used a dull engraving bit for the prototype and when I make the final unit the lines will be crisp and sharp. It is envisioned that the conversion board will be about 2X3 inches and contain the 1st and 2nd Mixers (ADE-1L's) the matching network and the ECS filter. In discussions with the manufacturer the whole assembly will be shielded.



 
Just think the space you'll conserve by not building the VU2ESE multi-pole filter. Four poles of filtering having a 7.5 KHz bandwidth at 45 MHz is very in keeping with the gain and bandwidth  parameters. The specifications for this filter are 30 dB down at the half power points ( 3dB) and an ultimate  stop band   of 80 dB. The stability will exceed the homebrew filter!
 
 
 
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Lets us start with the premise that the 1st LO (tunable) would up convert everything to 45 MHz. Thus in going from 1.8 MHz to 30 MHz the table below shows the conversion frequencies. My first thought was to use some packaged crystal filters for the 2nd IF such as those from the GQRP Club or INRAD. These filters are at 9 MHz. If you do the math the 5th harmonic of the BFO is right in the middle of the ECS filter pass band (5X9 = 45). So that choice is not a good one. If you use the uBitx approach, you avoid that problem. But this being a DifX I picked 11.5 MHz for the second filter frequency.
 
My frequency analysis shows you avoid the problem of having harmonics of the BFO end up in other parts of the rig. In checking the current price of the 11.5 MHz crystals in the large can form factor, 25 pierces can be had for 30 cents a piece form Mouser. I suggest getting the 25 pieces so that you have enough stock to find at least four that have no more than a 50 Hz spread across all four crystals. Most likely you will end up with several filters so not all bad.  If you can find five or six within that frequency constraint then you have the makings of a superior second IF crystal filter
 
 


 

The above table looks like it will avoid the problem of a tuning range sitting on the same frequency as the second mixer so we should be good.  I think we can now move forward


We now can take a peek at a first cut of a block diagram shown below. In a more detailed view all interfaces and matching are to 50 Ohms. So if you don't know --time to get smart on broad band matching and turns ratio squared! The block diagram below was based on the original thought of using 9.0 MHz filter; but is essentially the same with the 11,5 MHz homebrew filter. So where it says 9 substitute the 11.5 and the appropriate USB/LSB frequencies. Since we are using broad band amps for the bilateral amps no other frequency adjustments are necessary. CLK1 will now be 56.5 MHz and CLK2 will be either 11.4985 MHz or 11.5015 MHz. There are no changes to CLK0.

 
 
So lets examine closely what is happening with our frequency scheme and why does this and how does this arrangement work. Keep in mind ahead of this is a broad band amp stage (2N3904) that is adjustable and can provide up to 10 dB of gain. Typically I run these at about 1/2 the gain --enough to perk up the signal but not overload the downstream stages. That amp feeds a bank of relay switched band pass filters.
 
When the 42IF123 transformers were available on the market this would have been the tool of choice --so now you are stuck with buying a stock of TOKO transformers or building the BPF's from discrete components. If you are lucky enough to own a copy of the SSDRA (Solid State Design for the Radio Amateur) you can hand calculate the BPF's.
 
There may be a computer program in the EMRFD that will let you do the same; but I have never used the CD that came with my EMRFD. As you can tell, even though I have a copy, EMRFD is not my first choice for a reference document. Right now my EMRFD is a pretty expensive book end. After hand calculation, using LT Spice you can run your BPF's and fine tune them. The hand calculation process coupled with LT spice let's me really get inside my BPF's and when it is necessary to make a new one, I know how to do it!
 
So now the signal entering the block diagram will be relay selected and covers just the ham band we picked. So already there is some signal clean up in play. The 1st LO from CLK0 in the Si5351 up-converts the ham band signals using the difference frequency (59.0 MHz -14.0 MHz = 45 MHz or 59.2 MHz - 14.2 MHz = 45 MHz) The sum frequency would be (59 + 14 = 73 MHz ) way out of the pass band. Now for the first bonus -- the ECS filter has a 7.5 KHz bandwidth --so anything 3.75 KHz +/- away from the desired frequency is also filtered out! You are screening out (attenuating is a better word choice) the California KW station 10 KHz away from your rig!
 
The signal is now passed on to the second mixer stage operating at 56.5 MHz and so the down mix is at 11.5 MHz. In both of our mixing processes the mixing frequency was above the signal (most desirable) and in effect we have two sideband inversions -- so the lower BFO frequency will yield USB and the higher BFO frequency LSB. Again all interface matching is at 50 Ohms and three ADE-1L's are used as the double balanced mixing devices. The ADE-1L's are 3 dBM devices so low drive requirements -- set the drive level in the Si5351 to 2 Ma.
 
I will now outline some of the performance specification for the new DifX rig.
  • Dual Conversion ~ 1st IF @ 45 MHz and 2nd IF at 11.5 MHz
  • 10 Band Operation: 160, 80, 60, 40, 30, 20, 17, 15, 12, and 10 Meters 
  • USB/LSB Operation
  • 15 Watts Output
  • Color TFT Display
  • PIN Diode and Some Relay Switching
  • Si5351 PLL Clock Generator
Some words here about the VFO. Forget building a quality rig like the DifX with an Analog VFO as you are really limiting yourself. Time to move up the Big Boy's sandbox.  The use of the Arduino in addition to providing the digital VFO capability also provides the color display, Tune function, USB/LSB, band selection and on and on. Analog VFO's can be fun in some rigs, but not this one! This is a DifX so move on up.

When I build a transceiver I always start at the back end and as I progress forward each part of the completed build is now part of the test system that will be used to test new additions. If I build a circuit and install it into the test systems 99% of the time it is what was just installed is where the problem lies as everything up to that point works! Thank you Heathkit
 
Thus starting at the back end is the audio amplifier. The audio stage deserves some respect so use the low noise version of the NE5534 driving an LM-380. Forget that crap about building a discrete component amplifier with 2N3904's/2N3906's --yes building one using that approach is like earning a Boy Scout Merit badge. It is also like doing brain surgery with a rusty spoon; thus it is time to move on to something more robust. Another NE5534 can be used as the microphone amplifier. This is an uptown rig and so move on  past the low rent district.
 
 
This is enough to start your heart pumping and the insatiable to desire to heat up the soldering iron. Welcome to the world of the DifX.
 
73's
Pete N6QW