What is a VFO (variable frequency oscillator)?

The Variable Frequency Oscillator ~ aka "Grief in a Box".

Whether you have an appliance radio (store bought) or one that you made with your own hands somewhere buried in the innards is a variable frequency oscillator.  However if you are running a single channel fixed frequency device such as a crystal controlled transmitter or receiver then maybe that is not the case. The acid test is if you move a knob or mouse pointer and you are able to change the frequency, then you have a VFO. A special case may be the VXO which is a variable crystal oscillator which uses the properties of a quartz crystal to shift its frequency of oscillation over a small range. But the VXO usually has a knob adjusting a variable capacitor or a pot controlling a voltage variable capacitor.

 

But I really want to focus on the knob turning, mouse pointing variety of variable frequency oscillator. There are various ways to generate a variable frequency starting with the very early approach involving analog circuits where an inductor and capacitor formed the very heart of the VFO. When an inductor and capacitor are linked up, either in series with each other or in parallel, a resonant circuit is formed. If this combination is placed in an amplifier circuit and certain conditions are met with respect to internally generated feedback the circuit will oscillate at a frequency determined mostly by the  inductor and capacitor values. Other factors that impact the actual frequency of oscillation include stray capacitance and/or inductance as a result of component lead lengths. For these reasons most VFO construction tutorials stress short direct connections and isolation from other circuit elements.

There are many types of analog VFO circuits with names like Hartley, Colpitts, Clapp, Vacker, Seiler, Franklin, ECO. The difference among these types basically is how the L and C are combined (series or parallel), the method of achieving feedback, where the load is placed and finally the coupling to the load. Some of these circuits are very load sensitive and you will frequently see some sort of follow on buffer amplifier to isolate the actual load (such as a mixer stage or convertor stage) from the oscillator itself. Below are two examples of oscillators with the first being a Hartley and the second called a Vackar. [The naming typically follows the person who invented the circuit.]

The Basic Hartley circuit (parallel L and C)

Vackar Circuit with L and Co in Series

 

 

For those who have an insatiable desire to know where every nut and bolt is hiding there are many factors concealed in the bushes as to why the circuit oscillates. Some factors have to do with the physics of inductors and their response to  being charged and collapsing electric fields while others have to do with capacitance discharge rates and flywheel effects. I do not plan to cover these other than to recognize that certain conditions have to be met for an analog circuit to oscillate.

 

Here is part of why I used the term "grief in the box "-- getting a circuit to oscillate is but a slice of the pie as then the builder is confronted with keeping the circuit in the oscillating state over the desired range. When you twiddle the knob or move the mouse pointer the objective is to keep it oscillating over the entire tuning range. That does not always happen.

 

Another pie slice is oscillator stability which has several subsets. There is the "drift factor' manifest as either "initial turn on drift" where as the circuit elements heat up from a cold start the frequency will drift. This can be a very large amount in the range of several hundred hertz to kilohertz. Capacitors especially are subject to temperature drift and the inductor as well will respond to temperature changes.

 

Another drift factor is "long term drift" where after the initial "turn on drift", over time the frequency will shift albeit usually a smaller amount. Turn on drift as stated may be in order of magnitude of kilohertz whereas the long term may be 100 Hertz. Fifty years ago a few kilohertz drift was common. Today the Flex Radio Operators on 40 Meters will scream at you if your rig moves 10 Hertz -- so with progress there are some penalties.

Yet another form of drift is the result of poor mechanical construction of the VFO. If your inductor flops around --the VFO will change frequency. If you move your hand near the inductor it will change frequency and even blowing cold or hot air on the inductor will change the VFO frequency. You get the drift (pun guys).

 

Other issues include "FMming" a term often applied to certain older boat anchor inexpensive commercial appliance radios that lacked good voltage regulation. Typically these vintage radios featured 500 watt sweep tubes and with a marginal power supply on voice peaks the VFO regulation suffered. The result is the SSB signal frequency that varied with voice peaks --thus frequency modulation. Before we leave VFO voltage regulation that is an issue in itself as the lack of adequate voltage regulation results in frequency drift.

 

A similar problem was when the VFO was built  "al fresco" with no shielding such that the output RF signal was being picked up in the VFO circuitry and this results in distortion. 

 

Hopefully now you understand why I call it "Grief in a Box"! But in examining all of these maladies there is a general approach that cures many of these problems and most of the cures are physical in nature. So for those contemplating building an analog VFO behind the schematic is a host of factors that if properly addressed  will assure a success. Lets us examine some of the physical factors.

 

1. The components themselves are one factor. Use NPO temperature coefficient capacitors and use multiple caps in parallel. Capacitors have AC current passing through them which cause a heating of the cap. If you have say a 100 PF cap in parallel with a 50 PF variable cap to set the band range then use ten 10 PF caps in parallel as then each caps is drawing 1/10 the current and the heating of each individual cap is dramatically reduced. While you are at it the 50 PF variable must be a double bearing type (supported at both ends). This keeps the cap linear and not subject to vibration. Old style brass capacitors work the best but may not be readily available.
2. Inductors are very subject to temperature and mechanical impacts. Air wound inductors seems to be the best BUT how you mount them may negate their otherwise excellent properties. Frequently the inductors are wound on a grooved ceramic core or on a cardboard form that has been varnished. Keeping an inductor away from the chassis or walls of an enclosure is an art. Ceramic pillars are often used for this chore. One other approach is to super glue the air wound inductor to a 1/4 inch thick piece of plexiglass which is then mounted on the pillars.
3. Physical isolation is another factor. High quality VFO's are built in shielded enclosures and power is fed to the VFO via "feedthrough" capacitors. The signal output is best done utilizing SMA connectors. Real die hard VFO builders will put the Inductor and Capacitor in one box and the electronics in another box where the two boxes are interconnected with a short length of RG174/U 52 Ohm coax. This approach isolates any heat from the frequency determining elements.
4. The build itself should utilize short direct connections over a common ground plane. This is where a Manhattan style build could result in an extremely "solid" VFO. For best results do not utilize Zener regulators but instead a three terminal type such as the 78L08. That is another point use the  minimum amount of voltage to sustain reliable operation. This reduces  circuit heating thereby impacting the drift in the least amount possible. Further amplification of the VFO signal can be done externally.
5. Mechanical factors must be addressed wherein any vibrations or mechanical movement will cause movement of  components in the VFO itself. One the earliest frequency agile ham transmitters was the self-excited Hartley oscillator which frequently used a type 45 tube. Essentially this jewel was a keyed VFO built on a wooden breadboard. This was a pure example of "al fresco construction" . Literature of the time cautioned that the operator must place the transmitter on a shelf above the operating table as the mere act of striking the Morse key would transmit vibrations to the transmitter and the signal would vary in frequency based purely on the movement of the key. The physical isolation solved that problem. The second caution was to insure the antenna was taut --- if the antenna moved with a breeze it would present a variable load to the oscillator and the frequency would shift. A hefty mechanically sound enclosure serves two purposes: one is to shield the VFO and secondly to reduce the impact of mechanical vibrations.
6. Voltage regulation is key as mentioned earlier. Resist using Zener diodes and resort to stout three terminal regulators. You ask why? -- Most of the three terminal regulators feature internal temperature compensation to keep the output constant --Zener diodes do not. Very sophisticated regulation circuits will keep the output constant even when subjected to wild swings in the input side. Lack of voltage regulation and heating of the circuits (especially with tube type vfo's) frequently resulted in a chirpy signal especially if one were keying a VFO for CW operation. The message here is to use the lowest voltage practical to sustain oscillation as this facilitates maintaining the applied voltage and reduces the device dissipation which in turn generates less heat.
7. Once you have the VFO built typically some sort of gear reduction drive mechanism coupled with a mechanical readout like a circular dial is fixed to the capacitor shaft. Careful alignment is required so that no stresses are introduced into the capacitor shaft (ie binding) such that the assembly lacks smooth tuning. This takes time to get it right!
8. Finally --don't be greedy! Select a reasonable tuning range like 500 kHz for your VFO as this satisfies several factors one of which is stability and another is linearity. The ultimate goal is to have the same degree of movement at one end of the VFO range produce the exact same increment of change at the other end. Did I mention that a smaller range facilitates the use of reduction drives to give you "fine tuning". 

The above cover much of the well known cautions and tribal knowledge in analog VFO construction and if you are looking for a simplification in building an stable analog VFO --there is none! There is much effort involved if you want to have a stable VFO. Translate this to hours and days of work versus a hour with some wire wrap tools and 30 Minutes writing some code for an Arduino. Unless time is well spent on analog VFO construction you will have grief in a box!

Above all feel free to add your additional comments on how to avoid "Grief in a Box".


73's
Pete N6QW

Taking a Break!

Are you tired of seeing me post about my rigs?

9/28/2016 ~ Some recent updates

I have often said that most of my rigs are experimenter's platforms and the Junk Box Rig stands as an example of that approach. Everyday I look at possibilities for improvement --some changes are in the negative column but many are in the positive direction. Usually I employ the concept of TFMS when "peaking and tweaking" a rig. Oh almost forgot TFMS is an old timer term for Tune For Maximum Smoke. Well in this case when I had the rig outfitted to run 700 Watts out (Yes, Virginia, that is not QRP) I found that I got a report of splatter. One of those Flex Guys saw me pop up on his 60 inch color display and he reported I was very broad and splattering over onto other frequencies.

 

He was absolutely right! If you are running QRP at one watt --there may be splatter that is hardly noticed but magnify that 700 times and you will notice that your signal is broad and spilling over the normal bandwidth. A simple adjustment of the microphone gain can cure that problem. So out comes the scope and I was indeed flat topping and lowering the gain didn't actually impact the PEP output but did immeasurably improve the signal quality and cleaned up the spectral purity. For reference purpose a 1 watt signal is 30 dBm and a 700 watt signal is  58.5 dBm. So you will hear that very large elephant in the room!

Once I reduced the microphone gain, the Flex guy was  happy and so now I was happy too! WE have a responsibility as hams to put out the cleanest signal possible! I am now ever vigilant to not let that happen again. The positive side was that my PNP audio amp was no wimp and I had a gentle reminder about flat topping and splatter.

 

Rummaging through the junk box I found a 2SC2166 on a board and swapped out the 2SC2075.  Now 5 watts out is an easy goal. I also made a change on the driver board where normally the EMRFD circuit has a 2N3904 driving a 2N3866 but in my circuit I have a 2N2222 driving a BD139. In the emitter lead of the 2N3904 (my 2N2222) there is a 22 Ohm resistor to ground. I replaced that resistor with a 200 Ohm pot. The center wiper is connected to ground and the top lead is connected to the 2N2222. This enables me to adjust the stage gain. When I drive the 100 watt intermediate amp ahead of the SB200, I have it set so the Junk Box Rig outputs 1 watt as that is about the max level input to the intermediate amp. In the TFMS QRP mode (without any amplifiers) I can adjust that pot for 5 watts output from the 2SC2166.

 

On 9/27/2016 I made a 400 mile hop to San Francisco from my QTH near Los Angeles running 1 watt out on 40 Meters. Not a lot of signal strength but readable. But I must confess that 40M QSO's to the mid-west are much better running 700 watts!

 

Recap: Beefier RF Transistor, Watch for Flat Topping, Variable RF Output, QRP QSO's.

 

73's

Pete N6QW

9/21/2016 Revised Microphone Amplifier and plot. (See Comments)

Revised Schematic to limit the frequency response.

Output Plot of the revised shematic

Here is a chance to tell me what is on your bench and if you have attempted to build any of my projects.

73's
Pete N6QW

40M Junk Box SSB Xcvr in a Blue Case

The Blue Junk Box 40M SSB Is Alive!

9/15/2016 ~ Continued -- Listen to the Junk Box Radio...

9/15/2016 ~ Junk Box Experimenter's Platform

Ever wary of unwanted frequency mixing schemes and picking an IF too close (harmonically) to another band, I decided to change out the Yaesu 3.180 MHz Crystal Filter. The second harmonic of the BFO is at 6.4 MHz which is in my opinion too close to the 40M band with just the simple Band Pass Filter I am using. Adding a better BPF (more poles) or changing the IF would of course provide better solutions to my concerns. Still another solution would be to use dual conversion approach to mitigate the problem.

I opted to change the IF frequency as I had a spare 9.0 MHz GQRP Club SSB Filter. The change from 3.180 involved unsoldering two wires and the mechanical removal of the filter from the board. The Z in/out of either filter is 500 Ohms, thus requiring no change in the matching transformers. Insofar as Arduino code changes, about a total of eight entries and that was it. Since the LO is above the incoming there is a sideband inversion so the lower frequency BFO frequency is used to receive LSB (8.998500 MHz) and USB uses the upper 9.001500 MHz Carrier Frequency. The changes went FB and we now have an IF of 9.0 MHz. I now would feel more comfortable in moving the rig to 20 Meters. BTW total elapsed time was about 30 Minutes for the hardware change out and the software modifications / loading on to the Arduino Pro-Mini.

This is the beauty of my approach to building a rig. Changes both in hardware and software can be easily made without a complete disassembly of the radio. It is truly an experimenter's platform.

73's
Pete N6QW

The Junk Box SSB Transceiver is now fitted with a 9.0 MHz Crystal Filter from the GQRP Club



A Close Up of the Junk Box SSB Rig Front Panel -- You gotta love that blue!

9/12/2016 ~ Making Contacts

The one nice facet of our beloved hobby is to make contacts and hopefully by what I have done with the last several transceiver projects is to inspire others to "take up the iron". Perhaps one of the most powerful ways that can be done is by using the rigs on the air. I have actually spent more time on the air in the last several weeks than I have all year long. True much of my operation has been in short bursts like 20 minutes here and there -- but the results have been many fold. Not only have I met some really great people; but the interest level has ramped up with many inquiries about the rigs I have built. Perhaps one or two will be inspired to build their very own rigs.

 

73's

Pete N6QW

 

 

 

9/8/2016 ~ Ramping up the Power!

Yesterday I added some switching circuitry so I could simultaneously key up the Atlas 210X "brick" and my SB200 Linear Amp. I am happy to report I can now put out about 600 Watts into my droopy dipole. This now should give me a shot at working coast to coast with the droopy dipole.

 

Another critical part of the tests were to make sure I wasn't tripping (or ripping up)  the security alarm, telephone system, sprinkler system or the cable TV. All is clean and so the grounding, shielding and good wiring practices have paid off.  

 

I am very pleased at the performance of this radio and now with winter coming on want to do some serious 40M operation. I note that in the early morning hours I am routinely hearing stations in Japan and Indonesia on 40M SSB. So that is another goal to make contact with these station and I feel possible with this rig and the amps. I hardly ever run it at 3 watts so I guess my QRP award is in jeopardy of being revoked.

 

73's

Pete N6QW

9/7/2016 ~ Some observations and updated information

I continue to be amazed at the performance of this "Junk Box Rig". Signal reports affirm its good quality signal and nice sounding audio which does seem to have some added "sparkle" to the sound. I attest the audio reports to spending some time in improving the frequency response of the PNP microphone amplifier. A bit of time with LT Spice pays off in big dividends.

 

You can't imagine my surprise several days ago where about six stations in a row contacted me for details about my rig and how it was built. There is no finer compliment than to hear your station being called by others. With a bit of tuning and some improvements I now get about 115 watts out using the linear amplifier "brick" liberated from an Atlas 210X SSB transceiver. This power level now enables easy QSO's into the mid-west and western Canada with my "droopy dipole" installed on my postage stamp sized Southern California lot. A better antenna would definitely extend the range into the east coast on a regular basis. So the antenna does count. In fact typically the second question after the usual first question about the rig is what kind of antenna are you using?

 

Extensive use of a radio will often identify "issues" and so it was with the Junk Box Rig. I found that after an hour or so of operation the rig would not return to receive after a lengthy transmission and at times even though the rig would go into transmit -- there was no output signal. The problem was traced to the solid state switch. I found that when this occurred the SN7400 IC which was wired as an inverter was "hot"  to the touch. Not just warm; but burn your finger hot. So my fix was to install a small 12VDC SPDT relay in lieu of the switching board. Problem fixed and a simpler solution. The radio no longer suffers from that hang up. But it does beg the question as to why? I will do some further bench evaluation --but in keeping with the theme of this rig --the relay is what I found in the junk box -- and even though the relay is small its contacts are rated for 3 Amps DC at 32 Volts --so plenty of head room.

 

To seem repetitious often when we challenge ourselves to find solutions by using what we have an end result which is often better than simply flipping out the plastic and buying new. I have quite a few boat anchors and aside from the KWM-2, this rig hears better than most of those radios of old. With a bit of scrounging and some horse trading this rig can be built for less than $150. The frequency generation part, BFO, encoder and the LCD can be bought for about $20. The SBL-1 Double Balanced Mixers can be purchased from RF Parts for about $16 and the Iron Powder and Ferrite cores add about $10. The real bang for the buck is the pleasure of saying my rig is homebrew built by me!

 

So what is on your bench???????

 

73's

Pete N6QW

It has been one week since we made the first contact with the 40M Junk Box SSB Xcvr and I am happy to report that the hunk of junk has now been given a proper and fitting home.

This is how it all started!

Using my Harbor Freight metal break I made some brackets to provide support to the front panel

The rig base is a piece of copper clad board 10.5 inches long by 8 inches wide. Here we see the front panel/support brackets installed on the base plate assembly. I used my manual mill to make the front panel.

This was an exercise to determine how all of the boards would fit in the case

The boards are all mechanically installed and the wiring is everywhere.

Here the wiring is about 85% done

Oblique view showing the various boards.

The Junk Box Rig sitting next to its older brother the Nu-Rig. Love that Blue!

The Junk Box Rig is now fully functional.

Front Panel close up!

Innards: Driver Board, Final and Solid State switch

Band Pass Filter, Bi-Directional Amp, Main Board, Si5351 and Microphone Amp.

Left Oblique View.

Right Oblique View: Audio Amp, TR Relay,Linear Amp Switch and Low Pass Filter

Power Amplifier. The device is mounted directly to the bottom copper plate (insulator used). We have 84 square inches of copper heat sink.

 

I now have connected this rig to the "brick" liberated from an Atlas 210X. With 3 watts of drive I get over 100 watts out. Since finishing this rig today I have made 5 contacts running the amp --- all excellent reports. Three of those contacts were into the mid-west.

 

What are you waiting for --get cracking on your junk box rig! Soon I hope to make a video so you can listen to this wonderful radio. This also has been a great experience for me in using what you have -- my 1st thoughts were to buy a chassis --ouch at $30 I figured there had to be another way and there was.

 

73's

Pete N6QW

40M Junk Box SSB Transceiver Microphone Amplifier

Exploring PNP Transistor Microphone Amps

This is a first for me in terms of using PNP transistors for microphone amps. In fact more recently it was a first for me to use a single NPN transistor as a microphone amp. For the longest time my standard building block was the NE5534 for microphone amps.

 

On the air signal reports for the Junk Box rig were very good with the added comment that there seemed to a favoring of the highs with very little in the way of low frequencies. Well Duh, a couple of minutes with an LT Spice Simulation confirmed the why this was so. This post will explore the why.

 

[I should note that I have a bag of vintage PNP audio type transistors that I have often wondered what can be done with these treasures of old? Now I know!]

 

I ginned up the circuit I was initially using and it worked and just moved on but given the reports of the lack of lows I actually simulated the original circuit and found that there was a substantial lack of gain (like 10 to 15 dB) for frequencies below 1 KHz. Above that frequency things were much better.

 

Since there were no LT Spice Library entries for the 2N996 I picked the 2N4403 and then verified the same results with a 2N3906. Two capacitors play a key role in boosting the low end response and those are C1 and C4. Initially I had C1 at 100NF and C4 was 10 ufd and of course we could see that lack of the low end response. With a bit of cut and try these final values of 10 and 100 ufd, really boosts the low end. Now there is less than about a 3 dB change from 300 to 3000 hertz. I used a generator with a 100 millivolt output and swung the frequency from 10 hertz to 5000 hertz.

 

For those picknitters in the readership the + side of the 10 ufd is toward the signal input and the minus toward the base. On the 100 ufd, the + side is connected to the emitter of the device. I installed a socket on this board so I can actually turn this into a PNP transistor checker. For those who are still uncertain about electrolytic cap polarity-- the answer is to purchase and install non-polarized electrolytic capacitors.

 

A little more than the 15 minutes I spent with LT Spice could even further improve the low end so that the circuit is flat to 5 kHz. At this point I say Basta and just move on.

Schematic of the PNP Microphone Amplifier

Finally for the doubting Thomas crowd here is the expected performance of the circuit. I should also caution that R4 is really a 10K Trim Pot and the 220 NF (0.22 ufd) cap is connected to the center wiper. The output end of the 220 NF is paralleled with the audio amp input where both are connected to pins 3& 4 of the SBL-1.

Projected Output from the PNP Microphone Amplifier Stage.

 

The bottom line is that the PNP Microphone amplifier stage is good for about 20 dB of gain over the audio range and confirms the suitability of the device in homebrew SSB transceiver projects. Somehow many of us sort of gravitate to NPN devices --but good circuits can be built using PNP devices.

 

73's

Pete N6QW

40M Junk Box SSB Transceiver

You Too Can Build A Junk Box SSB XCVR!

8/31/2016 Results Update
8/31/2016 -- Photos of the Boards Added

This is one great radio and following my success at 500 Milli-watts I added the output stage using a 2SC2075 where I am getting about 3 Watts PEP with the device  biased for 100 Ma. This in turn can drive my SB-200 to about 45 watts output. With my backyard droopy dipole on 8/30 I worked stations in Arkansas and Texas running 45 watts. So I am pleased.

 

At this point I am torn between putting it in a box or just leaving it "junk style" on the bench. I fear taking it apart may ruin the magic of what has been done so far. Of course if it goes into a box it will get the Juliano Blue treatment.

 

I am really impressed at how well the receiver is working and perhaps some of the additional refinements have helped. The transmitted signal has received excellent reports. In the bi-directional amp stage that started with 2N3904's and then went to a 2N1711 and 2N3868 has now moved on to a 2N2102 in each leg. This seems to be a very good final choice. What I like most is that in  some of the circuit blocks I have included sockets where I can test various solid state devices -- the radio after all is an experimenter's platform.

 

For a bit of a trip if you can find a 1966 publication entitled "The Transistor Radio Handbook" authored by Les Earnshaw ZL1AAX and Don Stoner W6TNS, there is featured a complete 40M SSB transceiver designed and built by Jo Emmett Jennings, W6EI. [Jennings invented the Jennings Vacuum Variable Capacitors].

 

This rig used circuit boards made by International Crystal Manufacturing that were modified by W6EI for this application. It puts out about 1/2 watt, similar to the first debut with my radio, and in the mid 1960's used a lot more parts. This book can still be found on Amazon for $12USD. [For those who are scratching their heads that is $12 dollars in US dollars. I was recently admonished that my portrayal using $12USD was incorrect as being redundant! Twelve bucks guys is what it is.]

 

[Follow on note: Earnshaw started several electronics companies here in the USA including Kachina which was one of the 1st computer controlled ham transceivers. Stoner teamed with Pierre Goral to found SGC which produced a line of ham equipment and of course linear amps as well as auto tuners.]

 

To date I have had about 20 QSO's with this radio and states worked include; California, Arizona, Nevada, Arkansas and Texas. The frequency stability using the Si5351 adds immeasurably to making contacts and getting good reports. However an Analog type VFO and separate BFO could produce good results and that is left to the builder as an option.

Other filter options include the readily available Heathkit filters at 3.395 MHz (Zin/out = 2000 Ohms) or the GQRP Club 9.0 MHz Filters (Zin/out = 500 Ohms) or Inrad sell a 4 pole 9.0 MHz Experimenter's filter (Zin/out = 200 Ohms). In fact the photo above the masthead featuring the LBS II, that radio has the 4 pole Inrad filter.

This is the Main Board with the MCL Double Balanced Mixers

 This Board is the Solid State Switch designed by me.

This Board is the Audio Amplifier Board



This is the 2N2222 & 2N3866 Driver Board

This is the 2N996 PNP Transistor Microphone Amplifier Board

This is the 2N2102 Bi-Directional Amplifier Board

This is the 40M Band Pass Filter Designed by N6QW

This board is the 2SC2075 Final Amplifier

This board is the Arduino Pro-Mini with the Si5351 PLL Clock Oscillator

This board is the TR Switch, Linear Amp Switch and the Low Pass Filter from W3NQN

 

The fact that there are individual photos of the boards (as above) should make it obvious the rig is headed for a Juliano Blue case.

A side note here, initially these photos had no captions wherein I was asked to include captions of just exactly what were the boards. In the spirit of cooperation I have done so. But if you could not determine what were the boards based on the schematics presented in this thread then I caution you "do not build this radio"!

 

73's

Pete N6QW

 

This current detour in the road was the result of awaiting parts for the LDMOS Amp which are now here and the second piece was I smoked something on the Germanium PNP Transceiver. So back to the junk box to find more boards and parts to work on whilst I wait.

 

Having lots of old boards lying around makes it a pretty easy task of simply soldering up a bunch of wires and you are there --well almost!

 

First let me say that on Saturday August 27, 2016 running 500 milli-watts (1/2 watt) I made the first contact with the rig below by having a QSO with Jim W1WY, located in San Diego about 173 miles from my QTH in Newbury Park. So once again QRP does work.  But Hey I just wanted to see if it could work --more than once! You can clearly see  this is just a bunch of boards hay-wired and screwed down to the work bench. This is not how to build radios!

 

 

But lets start in a more organized fashion and we do this with a block diagram and some description of the circuits.

 

The architecture of this rig is not unlike many of the radios I have built in the past--it works so why not use it! Of note is that most of the boards have been recycled. The Solid State switching board was one I built for the 30M CW transceiver and the driver stage board was actually the prototype board for the BFO/CIO used in the 2012 KWM-4 build. I had made an error on the layout and just used the available islands for this build. The Bi-Lateral amp stage was another prototype build from about three years ago. The LPF board was in the junk box. The Band Pass Filter board was built on a board that originally housed some external low pass filters used with the softrock SDR transceiver.

 

Some notes the bi-lateral stage mainboard as I mentioned was built several years ago and while it worked seemed a bit deaf. Now I know why -- I had failed to install two 100NF caps on 2N3904 Emitters. Boom fixed that and the receiver is really hot!!! Normally the supply voltage is 6 VDC -- I upped it to 8 Volts by replacing the on board 6 VDC regulator with an 8 VDC regulator. Big improvement. I heartily recommend that change. The bilateral stage was developed by Plessey and is a direct lift from EMRFD -- finally found something useful in that publication.

 

A special comment about the audio amplifier. When I built it and connected earphones to the output at first I thought it was not working as there seemed to be no sound. I used the Juliano special audio test which consists of placing your finger on the input and BOOM -- A Loud Sound! This is a great amplifier circuit as it has a lot of gain with very little noise. FORGET building those discrete part amplifiers --you are wasting your time and using a lot more parts!

 

 In the bidirectional stage which is something I developed I wanted to try some transistors other than the 2N3904's as they get a bit hot. On the receive leg I used a 2N1711 and on the transmit leg a 2N3868. These are TO-39 devices and a bit more amenable to a heat sink versus the plastic 2N3904.

The driver stage is another useful lift from EMRFD --that makes two now. However I substituted a TO-18 style 2N2222 for the 2N3904 as that will work with a heat sink.

For the Band Pass Filter I used what I designed with the help of LT Spice for the Simpleceiver project. It worked there so why not here?

C1,C2 in this schematic is comprised of a 150PF NPO in parallel with a 15-60 PF Trimmer Cap. C3,C5 are fixed 47PF NPO caps and C4  is a 5 to 15 PF Johnson Air Trimmer Cap.

Cautionary note here many of the schematics were used in other projects and the basic circuitry is solid --some of the references to in/out connections may not apply.

The IF amplifier is similar to that shown below. Changes include using the Yaesu Filter at 3.180 MHz and the matching transformers are 6 turns #26 on the primary (50 Ohm side) and the secondary is 19 Turns of #26 and connects to the filter (500 Ohms) [ 6^2 = 36 and 19^2 =361 -- 361/36 = 10 and thus a 10:1 transformation that step up 50 Ohms to 500 Ohms.] So just replace the home brew crystal filter with the commercial unit and rewind the transformer for 10:1. Finally Change the source voltage to 8 VDC and don't forget the 100 NF caps on the 2N3904 emitters.

Solid State Switching Circuit is shown below. Ignore the circuitry below the 4N35 as that was used in the KWM-4 for CW operation. That part is however useful if you would like to use this circuit for CW break-in operation.

I am pondering building an IRF510 final RF stage or using a real RF amplifier transistor. The Si5351 uses basic code from AD7C which was modified by me for this application. Send me an email to n6qwham@gmail.com if you would like the sketch. I am almost tempted to switch over to a color display -- but I had the LCD in the junk box.

The front end use the TUF-1 and the back end an SBL-1-just what I had. The Low Pass Filter is a direct lift from W3NQN.

Finally the Microphone amplifier --yes Virginia there is a Santa Claus and you can build audio amplifiers with PNP transistors.

I decided to build a real RF amp stage found a 2SC2075 lying in the junk box and that is what I used as the circuit for the final amp is the same one I used in the JABOM built about 5 years ago and is reproduced below. The 2SC2075 has the same pin out as the 2SC2166.

 

 

 

Thee rig now puts out about 3 watts and tickling the SB-200 is good for about 40 Watts. Plenty good.

 

 

That is it -- have fun building this 40M SSB rig.

 

 

73's

Pete N6QW



 

Homebrew Germanium PNP Transistor 20M SSB Transceiver

UHF Germanium PNP Transistor SSB Transceiver

Whilst I await some parts for the LDMOS amplifier and Arduino Control system, I began to search my large box of projects that "sorta worked" , worked only once or never worked looking for something I could fix in short spurts of time. Somehow those who possess the "knack" for not having a project work or work properly are always drawn back to the bench in hope of finding the magic pill to bring the "pile of parts" back to life.

 

Mention of Germanium PNP  transistors must seem odd today. For some it must seem like a cruel April Fools joke suggested by Bill, N2CQR. Yet for other old timers who really do know about such devices invariably must think N6QW has passed into senility.  Yes there really are (or were) Germanium transistors (the CK722 was one of the first) and most early transistors were of the PNP variety. One of the early RF grade Germanium Transistors was the Philco SBT-100.

 

Interestingly enough today audiophile homebrewers and DIY musicians love germanium transistors as applied to "fuzz boxes".

 

Today by and large most transistors are silicon and NPN. Some 60 years ago most transistors were limited in power and frequency range; but today cheap UHF silicon NPN  transistors can be had for pennies.

 

In about 2011 I had a grand idea to use Germanium PNP  transistors in a SSB transceiver. I was not the first, as Ben Vester, W3TLN in a 1963 QST article did just that, and it was a small radio, roughly 5X7X2 inches. His rig was on 20M and ran about 1/4 watt. W3TLN's article is what prompted me to build the now "Juliano Blue" shirt pocket SSB transceiver. My project was 2X2X4 inches. In preparation for that project which ultimately used no Germanium PNP  transistors, I did purchase a 100 piece box of Russian Germanium PNP UHF transistors. At the time I paid about 10 cents a piece for them. These transistors were weird looking and the in line pinouts were BCE.

 

In 2013 after moving to Southern California, I decided to attempt to build the Germanium PNP UHF transistor SSB transceiver using the Sideband Engineers Model SBE-33 as a template. You might be surprised to know this radio vintage 1963 used bilateral circuitry. I did build the radio and the results were less than spectacular --so into the large box it went. Fast forward to today and I found that radio and cannibalized the parts. Using Vester's approach I built a new Germanium PNP UHF Transistor SSB transceiver. There are still lots of bugs but I am on the path to success. If you look closely in the video you will see some of the strange Russian transistors.

 

 

 

Some changes I made to Vester's deign include the following:

 

1. Vester used a tuned Germanium transistor RF Amp Stage that fed a Germanium Transistor Mixer Stage for the front end. Using a 4 Pole Slide switch whereby using one set of the poles, he switched that front end output to the Crystal Filter on Receive and on Transmit fed the Balanced Modulator output  into the filter. In my scheme on receive I have a broadband Germanium transistor RF amplifier feeding into an SBL-1 Mixer stage. Utilizing a pair of ultra miniature relays ahead and following the SBL-1 on transmit the Microphone amplifier (a Germanium PNP audio transistor 2N996) is switched in place of the RF amp. On receive the LO (an AD9850) is fed into Pin 8 of the SBL-1; but on transmit the BFO is now switched into Pin 8. Cheating a bit? I don't think so --just using what I have.
2. Following the SBL-1 is a post mixer amplifier (Germanium UHF PNP transistor) and that feeds the 9.0 MHz commercial crystal filter. Both the broad band amp and the post mixer amp are simply my standard 2N3904 general purpose amp stage that has been modified for the Germanium PNP transistors. The output is adjustable so we don't overdrive the follow on stages. Works perfectly!
3. In W3TLN's design he has a two stage IF amp chain following the filter and that feeds a diode ring. On receive it is used as the product detector and as the transmit mixer on transmit. Another small relay will switch the BFO/LO signals into this diode ring.
4. I currently have some dreaded feedback/oscillation issues in the IF amp stage which I hope to cure. Shielding and grounding are key no matter what kind of transistors are used.
5. The video was made in the early evening and 20M signals were not too strong. The radio does have AGC (one of the few NPN devices --and not Germanium) so it does have some frills.

Stay tuned.

 

73's

Pete N6QW

LDMOS Amplifier ~ Next Steps

More Revelations about the LDMOS Control System.

Rev 8/19 Added a second video of the three light sequence.

My efforts over the past week or so were to further refine and explore the control system functionality. It is has truly been an exercise in "non-linear" thinking which by my definition is not a simple straight line from step A to B to C and so on. Instead it is the need to see that it is A to B & C followed by D and then perhaps and E & F & G. What may seem straight forward and direct often is not that simple based on the complexity of what you want to happen.

 

Above all, the complexity of the control systems lies in the three basic functions of the Arduino control system those being: 1) Pure Control, 2) Supervisory Oversight and 3) Hardware Protection. The Supervisory and Protection aspects frequently drive and override the Control functionality. Overlaying this is the speed at which events must occur to failsafe the very expensive RF device.

 

I cannot overstress the value of having built a breadboard of the Arduino Control system as this has been an invaluable tool. From this effort I uncovered that the 48 VDC power supply had to be essentially shut off immediately through the use of a FET power switch.

 

Using the on board LED indicators helped in "physically seeing" sequenced actions. To this end there is a sequence where the output of the Low Pass Filter banks are connected to the antenna (step one), a bias is applied to the LDMOS amp (step 2) and finally the transceiver output is connected to the LDMOS amplifier input (step 3). These steps are reversed when going back from transmit to receive. The Arduino Code has the timing (time delays) and forward/reverse sequencing built into sketch. However the time delays were more or less randomly picked based on a notional gut feel. When the hardware was built and the LED's sequenced I could see one of the delays was too short and the other too long.

 

Exercising the keypad buttons has shown where conditions could exist to essentially cause an unexpected event to occur. This is valuable in that such events are discovered and corrected before any expensive hardware is actually connected.

 

To date we have the following in operation:

 

1. Basic Start and Stopping of the 48 VDC Power Supply
2. Low Pass Filter Selection
3. TR Circuit Tied to LPF Selection ( No Filter selected, no TR)
4. Sequencing of the TR Relays and Bias Circuit.
5. The FET Power Switch is built (awaiting the heat sink)

 

 

 

See the latest progress as of 8/18/2016

 

73's

Pete N6QW