A New Line of Transceivers ~ DifX

Transceiver Architecture 2.03

The Dual Conversion Scheme an evaluation of Frequencies.

In the KWM-4 I was presented with a real engineering problem that being how to take advantage of the Collins Mechanical Filter capabilities yet deal with its low frequency of 455 kHz. The path quickly leads to a dual conversion scheme, where you can manage the gain at the higher IF and manage the selectivity at the lower IF.

 

This is a clue in that at the higher IF the filter does not have to be 2.1 KHz wide but something on the order of 7.5 KHz will certainly keep down the crud ending up in your receiver band pass. The lower IF (Collins Filter) can do all the heavy lifting as now signals in the pass band coming into the filter are very narrow and the mechanical filter is essentially slicing that down to about 25% of the bandwidth -- 2.1 KHz. Below is the frequency scheme for the KWM-4.

 

 

 

 

 

 

Five years ago this is how the frequency mixing was handled. This exemplifies the concept of gain at the higher IF and selectivity at the lower IF. A filter at 10.7 MHz was chosen as the higher IF because given what I had at hand five years ago this worked with the COTS (Commercial Off The Shelf) components that could be easily and readily purchased. This design has a shortfall in that because of the high IF chosen, operation on 30 Meters was questionable. For some this is a deal breaker.

 

But today were I building this I would have chosen another IF frequency as frequency translation with an Arduino + Si5351 is a far easier task. The uBitx uses a homebrew multi-pole filter at 45 MHz  MHz and that is a good choice of frequency as it avoids the ham bands and does provide for 30M operation. However I would stick with a packaged 45 MHz crystal filter from ECS that does cost about $17 (Digi-Key) but has some really desirable and predictable parameters. It is good for 30 dB of attenuation for +/ 3.75 KHz and the stop band is 80 dB. The Z in/out is 350 Ohms and that is easily matched to 50 Ohms with a 3 Turn to 8 Turn broadband transformer (350:50 = 7:1, 3^2 = 9 and 8^2 = 64, 64/9 = 7.111 --close.). The injection frequency into the 2nd mixer is a matter of what you have and also close examination of the mixing by products.

 

In the KWM-4 the 10.7 MHz filter with a 7.5 KHz bandwidth resolved the subtractive mixing issue where 10.245 - .455 was out of the filter pass band. So lets say you wanted to use a commercial crystal filter such as the INRAD Model #351 which has a center frequency of 9.0 MHz. Thus the injection frequency would be 45 + 9 = 54 MHz which still puts it outside the ham bands --a bonus. Then the third clock would provide the normal 8.9985 and 9.0015 MHz BFO frequencies --OR a third BFO at 9.000 MHz for CW. The KWM-4 CW scheme would work perfectly here.

 

The Arduino and Si5351 would provide a far easier approach in a today build of a KWM-4 --but with some noodling I worked with what I had five years ago.

 

In a recap of a DifX version of a Dual Conversion Transceiver I would use the following:

 

• 1st IF at 45 MHz using the ECS packaged crystal filter ~ 7.5 KHz wide
• CLK0 on the Si5351 would provide injection frequencies above the 1st IF ranging from 46.8 to 75 MHz --all with in the capabilities of the device to give 160-10 Meter all band coverage. A quick math analysis shows that the injection frequencies required for all of the ham band avoids the second mixing frequency. The LO injection frequency for 7 MHz would be 52 to 52.3 MHz and the LO frequencies for 10 MHz would be in the 55 MHz range
• CLK1 on the Si5351 would provide a 54 MHz fixed inject frequency to convert the signals to 9.0 MHz (54 - 45 = 9 MHz)
• CLK2 would provide the USB/LSB and CW Carrier Oscillator frequencies
• The INRAD Model #351 is a 4 pole 2.3 KHz wide filter with a Z in/out of 200 Ohms an easy 4:1 match to 50 Ohms. A word here about homebrew crystal filters. It is not a simple matter to build a good quality crystal filter especially for someone who has never done one. You do need test equipment and you do need to understand what you are doing. There are some excellent tutorials on how to do it --but excellent  results for the neophyte with no test equipment would be like winning the Mega Millions having only played once and having only bought one ticket. The two filters suggested will cost you about $50  and one of them is a surface mount. But their specifications are well known as is their performance. Let me not discourage you from homebrewing a filter --but I am for a sophisticated project like this. A six pole 45 MHz filter may require the purchase of 20 to 30 individual  crystals before you find six that are close in frequency. You can't simply buy 6 crystals and think you are there. If by happenstance you are after buying only six, then I suggest you go out and buy one lottery ticket --for you are indeed a lucky person.
• For the Bi-Lateral Amps you have a choice of the TIA or the Plessey Amps.
• For DBM's I would use the ADE-1L (low drive requirements at 3 dBM).

Thus the Dual Conversion scheme offers some real advantages but also brings about the need to be careful in frequency selections.

 

73's

Pete N6QW

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.02

In our last post we hit the highlights of the KWM-4 and in the closing paragraphs I mentioned the band switch decoder and how using the three digit code provided by the K5BCQ digital VFO that automatic band switching was possible. Today we have with the Arduino far more efficient means of doing this; but this was how it was done before the uBitx.

Basically the circuit detects the BCD code and translates it into decimal outputs from 1 to 6 (Six Bands). With each output is a PFET that is "switched on" to provide power to the appropriate Band Pass and Low Pass Filter banks. An Arduino Mega 2560 having lots of pins could provide the 3 digit code or if you wanted to waste 10 digital pins you could do it directly. Also shown is how you could switch the bands using 3 toggle switches.

 

 

Another innovation was how the "Push To Talk" was handled including how to key the transmitter for CW. This was a total "in house design" and I think one of the special features that are found in the KWM-4 again pre-dating the uBitx.

 

Shown below is the schematic for the control board. Noteworthy some of this same circuitry is found in the Big Kahuna. On SSB the PTT trips the 4N35 opto-isolator and that toggles the SN74LS000 so that the trigger signals shift from the receive side to the transmit side. It is all DC switching, so no big clunky relays with back emf. There is a separate solid state switch to key an external linear amplifier. [The 4N35, SN7400, 2N3904's and the TIP32C's form the basic control functionality in the Big Kahuna.  If the CW capability were added to the Big Kahuna, then virtually the entire same circuitry shown below would be required.]

 

CW on the other hand is a more complex process and this involves the use of an NE555 timer --not for timing per se but to supply a voltage for a fixed duration. The 4.7 Ufd cap is part of the timing circuit so that the CW oscillator is held on for a period of time determined principally by the value of this cap. Most high speed CW ops prefer a shorter cycle and so values down to 2.2 or 1 Ufd would be used.

 

Here is the CW sequence. You tap the key and two actions take place the first of which is to start a timing cycle by placing a "high voltage" on the NE555 pin #3. This closes two relays the first of which is a relay whose contacts are in parallel with the PTT switch and the second is the relay that takes the 1st Bi-lateral amp which is normally connected to the Collins filter for receive and SSB transmit but now is connected in the transmit mode to the output of the buffer amplifier. You will recall that the CW signal on transmit does not go through the Collins filter. Tapping the key also keys the buffer amp and keeps the NE555 in the On state. Let up on the key and let the timing cycle complete and the rig is back in the receive mode. Voltage off of Pin #3 also is the source voltage for the CW oscillator. This is quite a complex switching and control system and distinctly feature rich and perhaps far different than other rigs.

 

It is a N6QW design! Also keep in mind this was designed  many years ago and while the Arduino capabilities today would render some of these design elements as being antiquated --it was and is a successful control system. Certainly not QSK; but that was not the design intent. The main design problem was CW with offset and how to simply tap the key and transmit CW all automatically. As with the Collins KWM-2, the panel mounted Mode Switch had to be set to CW -- this also accomplished setting the CW receive mode to USB. The elegance of the KWM-4 was not just casual happenstance but careful and reasoned thought -- five years ago!

 

 

73's Pete, N6QW.

 



 



A New Line of Transceivers --- DifX

Transceiver Architecture 2.01

Please note the DifX is not a singular transceiver (like the Big Kahuna) but instead is a concept to demonstrate that successful transceiver projects can be achieved with something other than the Bitx20 footprint. The Bitx is a long standing successful design; but is not the only approach to homebrewing a rig. We are now at a point where with the aid of low cost technology we can build in many new features from the outset. The DifX series of radios will provide some insight into the "how to do it."

 

As promised in the previous post, I will explore homebrew transceiver architectures that are different than a Bitx. I can think of no better place to start than with my KWM-4 design which began in late 2012 and resulted in a completed transceiver in early 2013. This project was published in an 2013 article in QRP Quarterly.

 

What is significant about this project was that it is a dual conversion transceiver and covered six amateur bands and for my good friend N2CQR had a digital VFO. Just announced and mentioned in the SolderSmoke Podcast 194 and just appeared in Hackaday is the Micro-Bitx or uBitx from VU2ESE. The uBitx is a dual conversion transceiver and has created quite a stir in the homebrew community. While I can't comment on the design details of the uBitx as I simply don't know, I can share with you my concerns about a dual conversion architecture and what I had to "noodle my way through" on the KWM-4 rig.

 

Let us start with a high level block diagram of the KWM-4 and identify some of the key areas of noodling. This is shown below and highlighted in green shading.

 

 

 

 

Let us first start with the two blocks identified as BLA --yes Bilateral Amplifiers. But these amps are based on a design by Ron Taylor, G4GXO and appeared in the GQRP SPRAT #128. Essentially the amplifying devices can be either a Dual Gate MOSFET such as a BF991 or two J310's connected in a cascode circuit (source of one connected to drain of the second). The stage gain is 17 dB and the signals are diode steered. Z in/out is matched to 50 Ohms. This BLA is not the Bitx BLA! So Ok you want to see it. Noteworthy I first used this BLA in a 2007, 17M/40M Transceiver and later in the 2009 Tri-Band transceiver that used the HW-101 frequency scheme.

 

 

 



 

 



 

 

This circuit was selected as the key element over the Bitx or Plessey BLA for one specific reason, that being Gain Control. This circuit has the capability for either manual or Automatic Gain Control at the IF stage. Yes the W7ZOI Hycas AGC circuit provided the gain control in the KWM-4. The Bitx does not have that as a direct capability. There was a second reason for this approach which is shown later in a detailed block diagram and that is CW. In the KWM-4 the output/input of the 1st BLA ahead of the Collins Mechanical filter is relay switched so that in SSB it is always connected to the Collins Filter. But on CW transmit that connection is switched over to a keyed buffer amp that is supplied a signal from a third BFO frequency centered on 455 KHz.

 

Thus CW does not go through the Collins Filter on Transmit. For CW receive USB is used. But had I used an additional panel mounted switch the operator could have chosen either LSB or USB for receive --CW reverse. This is not found in the Bitx. Optionally if you had a Collins CW filter --then the ne plus ultra for the CW enthusiast -- selectable receiver filter bandwidths. W7ZOI in the SSDRA shows how to diode steer two filters and even provided an extra gain stage for the CW filter.

 

Now to the dual conversion part of the KWM-4. This project started innocently with my acquiring a 455 KHz Mechanical Filter as used in the KWM-2. I also had managed to scrounge up the USB and LSB crystals as well as a crystal smack on 455 KHz. There was a siren's call to "use me in a transceiver". But if used only with a single conversion (and dual conversion too as you will see) becomes problematic because of image and frequency mixing issues as you go higher in frequency.

 

Here is an example. I generate a SSB signal (we'll use 455 kHz as it will make the math simple) at 455 KHz and I mix that with 20.845 MHz LO. Normally the sum frequency would be 21.300 -- a really great place to be on 15 Meters! But the difference frequency is 20.845 - .455 = 20.390 -- That is really close to the 15 Meter band and unless you had some really brick wall Band Pass Filter --difficult to knock down. As you go even higher in frequency the spread becomes even less. So LOW IF frequencies tend to need some way of distancing themselves. Thus dual conversion.

 

In the three shaded blocks that show the LO, and 1st and 2nd mixers, I will now detail how I resolved that matter. Touring through the Mouser catalog, I found that they carried crystal filters at 10.7 MHz and these came in two bandwidths --15 KHz and 7.5 KHz. I also discovered that one of the stock computer crystals was 10.245 MHz and the light bulb went on. Initially I bought the 15 KHz filter but then changed that out for the 7.5 KHz filter and one crystal at 10.245 MHz. So here is why the light went on. If you add .455 MHz to 10.245 MHz, the sum is 10.7 MHz (hold that number) Now if you subtract the 0.455 MHz from 10.245 MHz the sum is 9.79 MHz. The 7.5 kHz bandwidth of the 10.7 MHz crystal filter will reject the difference frequency. Thus anything coming through the 10.7 MHz filter would only be he SUM frequency. That eliminated the undesired frequency product.

 

The K5BCQ frequency generator thus converted the 10.7 MHz SSB/CW signals to the appropriate ham bands. The only negative to this scheme is the 30 Meter band is too close to this 10.7 MHz conversion frequency and thus was not included -- not a problem for me but a problem for some I am sure.

 

The K5BCQ frequency generator has some 900 channels (900 VFO's) and I used that for assuring that what the dial says is the real transmit frequency. Thus the LO frequency is offset (switched to a different channel and different offset frequency) depending on the mode. There is about a 3 kHz spread in BFO frequencies between USB/LSB, thus by changing the LO injection frequency by that amount accounts for the mode. In the old days of analog VFO's this was done with a varactor diode somewhere in the VFO box that would shift the VFO frequency for the same analog dial setting.

 

Today using an Arduino with a Si5351 would eliminate the three BFO frequencies and the need to switch channels -- all done in software --so 4+ years does make a difference in the technology.

 

Now for the detailed block diagram of the KWM-4.

 

 



 

 



 

One other noodle problem is how to change the band pass and low pass filters at the same time you change bands? Luckily the K5BCQ frequency generator also had built in a 3 digit binary code that could be linked to the band change. I simply decoded that information and that provided the appropriate set of Band Pass and Low Pass filters for the band in use. I had to learn how to decode the information and steer the signals --more on that in a subsequent posting. This is a DifX!

 

73's

Pete N6QW



 



 







A New Line of Transceivers

Transceiver Architecture 2.0

Several posts back I covered a few of the current crop of approaches to transceiver architecture. Judging by the number of visits to this post, it seems to have struck a chord with those who frequent this blog.

 

Over the next few months I intend to cover in detail a line of N6QW designed and developed transceivers that I call DifX. The acronym DifX is a simple contraction of the words Different than a Bitx. While due respect must be given to VU2ESE and the solid Bitx design --there are other creditable designs and I think the DifX series satisfies that criteria.

 

Hopefully you all will enjoy the journey. Stay tuned. The Big Kahuna (photo on the masthead) is a DifX.

 

73's

Pete N6QW

 

 

A Solid State QRP Rig from 1955!

A 1955 Solid State QRP Transmitter using the Philco SB-100.

3/14/2017 ~ Late breaking -- I put a 4:1 transformer in the collector lead and as measured across 50 Ohms on the secondary side I am seeing 3.2 Volts Peak to Peak. If you perform the rigorous calculation that transforms into 25.6 Milli-watts or 14 dBm. Now we are cooking! That is better than the rig on Bill's blog.

See Photo addition at the end --just to posit where 60+ years have taken us --and I have personally seen the amazing change to our tools, techniques and toys!

Recently my friend Bill, N2CQR posted data on his blog ~ soldersmoke.blogspot.com about a vintage late 1950's early 1960's 10 milliwatt 10 Meter transmitter. That was quite a feat!

But given my Italian heritage I could not let that pass without building my own solid state transmitter using a transistor from 1955. My rig operates on 14.060 and produces 0.4 milliwatts with a 3 volt collector supply using a Germanium transistor from Philco. The SB-100 was one of the first RF transistors that could work all the way past the 10 Meter band. The max Pout was 10 milliwatts --so mine is just loafing along.

 

 

 

 

 

 

 

 

This was fun.

 

73's

Pete N6QW

 

It must be easy to misunderstand when someone says wire wrapped it is decoded as wire tapped. Hmm wonder who recently did that?

New Rigs ?????

Buying a New Car and that New car smell!

 

Almost makes you forget that you just paid $55,000 for a 9 passenger Toyota SUV. It looks cool but scary to park. The ride is no better than a bucking bronco! I am a small framed guy and my visibility is like looking at a picture framed between the steering wheel and the top of the dashboard. No I didn't buy one --just looked at one while my Avalon was being serviced. But the new smell is much like the buzz of building a new transceiver!

 

Pete, N6QW

 





Mystery Transceiver

Or is it?

 

3/5/2017

 

A special shout out to active duty  US Navy Seabees and former Seabees world wide. Today is the 75th Birthday of the founding of the Seabees. Happy Birthday fellow Seabees! Our motto "Can Do" resonates even today! The first Seabees were civilian construction workers who were formally organized as Naval construction forces shortly after the start of World War II. Seabee's were instrumental in building the advanced bases that made the Pacific island hopping strategy a reality. Today that legend lives on in places like Iraq and Afghanistan. We Build, We Fight is the Mission Statement! 

 

If you followed the prior post on Transceiver Architecture look at item #3. The circuit below IS the hardware described in number 3.