VFO TRICKS
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- Опубліковано 17 січ 2022
- My Variable Frequency Oscillator is the High Plains Drifter! Are you getting comments like "you are 1 Hz off according to my SDR"? Building an old fashioned analog VFO is an exercise in understanding the secrets of basic oscillator stability.
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This takes me back to 1980 I built a 3.5 to 4.0 MHz VFO. I also opted for JFET in Colpitts. I was lucky to have the best components and a good source of used mil standard parts. There's no substitute for getting your hands dirty and building and testing and seeing what works best
👍Thanks for video. Some excellent advice.
Another all out detailed effort to cover every component in these circuits. I appreciate how everything is explained and makes sense.
Alright Mike, You've hit the jackpot there AGAIN,
It was in March 2001 when I learnt about Ham Radio as a hobby and CW was the bait i followed, Fortunately enough no amateurs where anywhere to find in my area to ask questions or seek guidance. Not long after that I found Homebrew (Beer, Wine or Whiskey??) None of that. It was QRP, and boy i was on my way to a learning Curve.
Crystal Sets to Sideband was THE BOOK that Introduced me to RF design and showed me the way from scratch. I still enjoy the book and refer to it regularly.
TNX FER bringing it up and Beautiful VFO u built there.
73
Wow Khalid!
There is so much to think about here...thanks for the talk through.
Great job, Mike, this looks like a book I need to get! I'm determined to get my old Atlas 210 stabilized (never stops drifting upward) without changing out the vfo (more fun this way😊), but it flunks on many of the important design elements that you mention...wish me luck!
I have an old swan solid state 100MX and it drifts only a bit within the first 15 minutes then is stable.
Your channel is great, lots of cool information. Good luck with your further work!
And thanks for watching Ryan!
Here's a circuit topology which could make a very good VFO circuit should it be applied properly. Of course experimentation will be the key.
Use a cascode amplifier circuit as the basis, bipolar transistors will probably be plenty fine. Use a capacitive divider on the first transistor between the base emitter and ground like in a conventional colpitts (probably 470p top and 220p bottom at 7mhz) and couple a parallel tuned circuit into the base through a capacitor... something like a 220pf NPO at 7mhz.
The arrangement of the cascode amplifier should afford a good stability and well buffed output for the parts count
Thanks for the book recommendation! Snagged a copy!!
My favorite Osc is also the Colpits.
One thing I do differently than most is to take the output via a cascoded BJT buffer connection to the drain of the Osc JFET. This provides very nearly the same zero swing drain voltage as taking it from the source, yet nearly completely isolates the FET from any loading by the buffer. This matters because the junction capacitance of the buffer changes with temperature and bias current level, which typically drifts with temperature.
This also allows the common base RF buffer to serve at DC as an emitter follower to buffer the regulated drain supply. I put a Si diode in series with the zener and JBweld it to the buffer to track the Vbe variation with temperature, providing a low drift voltage at the emitter-drain connection.
Since the cascoded buffer is running CB at RF, it avoids Miller effect, and can have substantial voltage gain and decent bandwidth to boot. If I need/want low Zout, I’ll follow the cascoded pair with an emitter follower.
The cascode pair also lets the buffer piggy back on the FET’s bias current, so you don’t need any biasing resistors beyond the DC degeneration in series with the FET source…just a current limiting resistor for the zener+diode on the base. I find the JFET+BJT cascode to be among the most elegant of circuits, especially when applied to an oscillator.
I first “built” one of these when I transistorized a Johnson Viking VFO for use with my DX-60b. The cascode pair makes a decent approximation of a tetrode if you treat the upper base as the screen grid. (And since there is no secondary emission, you can ignore the suppressor grid of a pentode. I quadrupled the 6.3vac filament supply to get enough Vcc that a 2N3904 gave me enough swing to drive the tubed transmitter. The transistor conversion virtually eliminated warm-up drift, but it still had the same awful yoop as the tube if I tried to key it on and off. I finally left it running and keyed it by diode switching in a few pF to shift it out of the rx pass band.
Love your channel, though I don’t find tubes much fun to homebrew with, but don’t mind trouble shooting commercial tube gear.
The common base amp is a natural for untuned loop antennas, which have a very low impedance. I use a push pull version on one of my loops. Thanks for the nice comments!
Thanks for this presentation. I need several VFOs badly. I need one capable of delivering about 22 volts to the 6GK6 grid oscillator circuit of a CW transmitter I completed 36 years ago, a Johnson Viking Adventurer and a lower voltage vfo for the Tuna Tin II I homebrewed 33 years ago almost completely from Radio Shack parts. Should be fun.
One trick is to use a transmission Line Transformer as a step up device. 50 Ohm side to the VFO and the step up side to the transmitter. This gets you those volts of swing over the whole VFO range.
@@MIKROWAVE1I’ve seen the use of a 2N2219 for a solid state vfo for vacuum tube crystal controlled transmitters in my old 1976 Amateur Radio Handbook but I have seen a site online where the builder uses a transformer to step up the voltage. He basically used a 4 to 1 toroidal transformer. I’ll try that. Thanks!
Большое Вам спасибо за очень интересное видео!
Thanks for another educational video. Funny how varactors showed themselves twice in my last 24 hours. Surprised no mention of using audio taper pot on one to try to straighten it out, but sometimes to curves are worse than one to deal with.
Yeah this is going to take a Part 2.
Thanks for your valuable information
Thanks for watching!
NPzero NP0. Great vid. First time I saw a vid on this. I'll send it to the guys who designed my drifty TS-830S.
Yes NPZero but most excited people like me say it: NP OOOH! OOOH! OOOH! Ha.
If it has one, try pushing the "fix" button again and again. The fix button is for the fixed crystal. They are known to crud up as no one uses fixed crystals and the vfo doesn't fully make contact in the switch and the frequency dances all over the place.
Thanks for video❤❤❤❤❤❤❤❤
Got a couple of his books.great video..
It's amazing what those guys did.
Замечательно. Интересная конструкция.
Надеюсь, вам понравилась вся серия VFO!
I changed the LO caps in my old DX-160 with NP0 type. Cut the drift in half. I can actually listen to SSB with no hands now...
I got a DX-150 all bipolar receiver in high school and it was darned stable. But it had images on the highest band. Still it got me on 15M.
What about using a Tung-Sol 12K5 12V tube that was for use in car radios when the first solid state transistors were not up to standard.
They sure liked those big Germaniums for the power amps though.
Question - are they NPO or NP0 capacitors? Maybe their identified both ways but I always thought the temp caps were NP0 and C0G? Or is it six of one and half a dozen of the other? ;-)
ZERO - Maybe we need a follow up on compensation schemes?
@@MIKROWAVE1 Yes, I think that would be a great topic for you to cover!
Dear Michael: dumb question (maybe not): Why not use a MOSFET rather than JFET for the VFO? Just asking, thanks.
High Frequency Mosfets like the vintage single gate 2N128 make fantastic VFO oscillators. Perhaps even better than JFETS. Any modern Dual Gate RF mosfet lor a vintage part like the the 3N201 will work with both gates tied or with the unused gate biased
@@MIKROWAVE1 I needed that information because I'm about to start building again. I'm not sure which FETs are a good substitute for the MPF102. Thank you.
A single or dual gate Moffet like a 3N128 or 40763 would both work ad good or better than a JFET. But they are rare now in leaded form and are not found in many parts drawers!
@@MIKROWAVE1 Thank you.
Dear Michael, you forgot about the inductance in the source circuits of field-effect transistors. De R0AIW from Russia.
The inductor can be replaced with a resistor usually around 2K - BUT that means that you may have to run higher voltage, so stability suffers. I'm not sure how the resistor effects the oscillator's Q.
@@MIKROWAVE1 , In the original circuit, the coil is connected in series with the resistor, this method just increases the quality factor of the oscillatory circuit and makes it independent of the resistance value of the resistor. In general, with a coil, the quality factor and stability of the oscillatory system will be better.
Yesterday I saw an ad that advertised radio for sale one dollar volume stuck on high. Thought I can't turn that down
Try sending a lower offer.
@@MIKROWAVE1 I thought I can't turn that down (turn down the volume) 😁
@@MIKROWAVE1 I'm sorry I couldn't resist 😁 just wanted to joke around
А о чем там говорится?