homebrew single transistor regen (with a stupid resonance question)

Discussion in 'Homebrew and Kit Projects' started by KX6MWS, Sep 12, 2011.

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  1. KX6MWS

    KX6MWS Ham Member QRZ Page

    After looking at several regenerative radio circuits, I decided to build one. The goal is to end up with something that can listen at shortwave and 80/40/20 or maybe even 10 meters. Before venturing into HF, I decided to make an AM broadcast receiver, because its lower frequency and a much stronger signal: it's easier to tell when the thing is working.

    My question revolves around the tank circuit. It's an inductor and capacitor in parallel. Often, I see the output of the tank circuit connects to the base of a transistor, and the base has to be biased. To keep the base bias voltage from shorting through the inductor, a capacitor is placed between the tank and the base. It looks like this:

    (some one else's design) http://cool386.tripod.com/6tr/srrx.html


    I'm made a single transistor version, so I just have the 'core' circuit centered around Q2. L1, C3 and C4 make up the tank circuit, and choosing appropriate values for these for different bands is simple math. In fact, I have it working just fine in the AM broadcast band, just by using a much larger inductor and a larger capacitor. My question, is does the value of C5 contribute to the tank circuit?

    I originally thought not, it's just to keep the bias DC from shorting through the coil. I tried different large values, 1 uF, 10 uF, makes no difference. With values like 1uF, the DC is blocked, but any AC goes through just fine. But when I use smaller values (100 , 50, 20 pf etc) it seems to change the station I'm tuned to by a little bit My question is... why? It's not part of the tank, right? And why does smaller capacitors change it; if this cap is affecting tank resonance, then putting in something huge like 10uF should completely throw me off the band, but it seems the larger this value is, the less it matters. What's going on?

    My other question, is when I push this into oscillation (which is necessary to receive SSB), is the value of C5 going to affect the oscillation frequency? Is there an 'intelligent' way to choose what C5 is?
     
  2. WA6TLP

    WA6TLP Ham Member QRZ Page

    That's a really neat design for a super-regenerative receiver. I would think you would do well to use the values of capacitors called out.

    I am flatly amazed that you used 1 to 10 microfarad capacitors for coupling. I have never heard of anybody using anything that big to couple HF signals and I wouldn't have believed that the circuit would work even at 200 meters.

    OK, here's my take on it. I'm sure there will be others who know a lot more about interelectrode capacitance who will come in and correct me.

    With a coupling capacitor that is way too high, it's like it just isn't there and you are getting the full effect of the interelectrode capacitance of the transistor. With the more reasonable sized coupling capacitors (in the PF range), the overall capacitance of the tank circuit would be much lower and the interelectrode capacitance of the transistor would only insignificantly contribute to the tank circuit since it would be in series with this small value of capacitance. Thus, as you increase the capacitance from 1 microfarad to (say) 1000 MF, yes you will see a change in the resonant frequency as more and more interelectrode capacitance is coupled over, but after you get much past 1K, I very much suspect that you won't see much change because all the interelectrode capacitance is already coupled and very little is lost by being in series with such a huge coupling capacitor.

    Does that make sense to you?
     
  3. WA6TLP

    WA6TLP Ham Member QRZ Page

    Just reading over the article referenced, I'm wondering if a super-regen can be made to demodulate SSB? My whole life, I've never worked with a super-regen, so right now I'm thinking that it can't as it doesn't maintain a continuous oscillation to provide the beat frequency. If this radio you are thinking of building doesn't detect SSB or CW, it might be of only limited use to you as a ham receiver -- no?

    Just for laughs, you might read the saga of how I came to build a little regenerative radio that tunes 80 up to 40 meters. It detects AM, SSB and CW quite well and is cheap and easy to build with nothing critical or specialized in its parts list. The story is on my website at http://www.geojohn.org/Radios/MyRadios/Latest/LatestRadio.html
     
  4. AB9LZ

    AB9LZ Ham Member QRZ Page

    Sure it can, just turn the regen control up until the thing oscillates, it's like having a BFO and detector in one.

    Actually using one to make a contact is another matter ;)

    73 m/4
     
  5. KX6MWS

    KX6MWS Ham Member QRZ Page

    Interesting, I didn't even consider the parasitic capacitance that comes with the transistor. Putting a small capacitor in series with the base would both perform the DC decoupling I want AND help cancel out some of the transistor's parasitic capacitance. Awesome! Thanks!

    SO back to the 'gigantic' coupling capacitor I used: I mostly just plugged in the large capacitor to a) see what happens and b) prove it was there for what I thought it was there for. Why does it work? Yeah at 1 MFD, it's like its not there at all; any AC in the tank circuit goes right into the base of the transistor. I was using the capacitor for only one purpose: To allow me to bias the base of the transistor to the required .6 'turn on' voltage, and to keep that DC bias from shorting through the tank inductor to ground. I'm at work at the moment, but when I get home I'll put up a schematic of what I have so far. Keep in mind I was using this to receive AM band at about 1Mhz, very different from the original circuit at VHF frequencies.

    I noticed that, for this ~1 Mhz receiver, the coupling capacitor from the tank to the base has to be above 50 pf or so. When I made it smaller, the volume dropped off significantly. 200pf worked well. The original circuit runs at 100+ mhz and used a 4.3pf coupling cap; but I don't think 1 Mhz travels through such a tiny capacitor very well. According to http://www.electronics2000.co.uk/calc/reactance-calculator.php

    100 Mhz, 4.3 pf (original in the circuit) : 408 ohms (strong signal gets through to base!)
    1 Mhz, 4.3pf: 40.8 k Ohms (too weak)
    1 Mhz, 200pf: 795 Ohms (decent signal, sound good)

    It appears that as I lower the frequency of the original circuit, I need to lower the value of the coupling capacitor to let in the lower frequency. But.. what happens is the value of the coupling capacitor is actually affecting the station I'm tuned to. If I tune to a station, then replace the 200pf coupling capacitor, with say 100 pf or 150 pf, the station isn't tuned in anymore. I CAN tune it back in by adjusting the tank, but I'm not happy with just covering up the issue. I'm wondering what the math says the resonance should be. I have the formula for LC resonance in parallel... How does it change when you have: L and C1 in a parallel tank, with a C2 in series ?


    I suppose for the ultimate in tweak-ability, I should just make the coupling capacitor a varcap as well, and let the user play with it. But having main tune, coupling 'tune' and regen (plus volume control) seems like a bit many controls for such a single circuit!
     
  6. WA6TLP

    WA6TLP Ham Member QRZ Page

    Using a "quenching" circuit is supposed to prevent the SUPER-regenerative radio from going into oscillation, isn't it? Never experimented with a SUPER-regenerative radio, so I really don't know. Certainly, a straight regenerative radio may be "throttled" such that it easily and stably goes into oscillation and thus provides a "direct conversion" where the beat frequency is at an audio frequency and if it is right where the carrier should be, SSB will be demodulated.
     
  7. WA6TLP

    WA6TLP Ham Member QRZ Page

    If your amplifier is a high impedance device (like an FET) then the impedance of the coupling capacitor may be quite large (small capacitance) and you will still get good coupling. A low impedance amplifier will require more capacitance. This is why I've always liked to work with tubes and FETs. Still, the magnitude of the signal you actually demodulate is not a function of how much signal you couple to the detector, but by how many times the signal is amplified as it is 'regenerated' in the circuit. This is the principle that Armstrong developed in 1914 using early Audion tubes. My suggestion is to pick a capacitor between 50 and 100 PF and go with it. The gain of the radio can then be adjusted with the regeneration control and the audio gain potentiometer. I would think it would be a completely unnecessary complication to try to adapt a variactor diode to this part of the circuit and it very well might make smooth regeneration control difficult or impossible as they would tend to interact.
     
  8. KB7NRN

    KB7NRN Ham Member QRZ Page

    First, to try a take a VHF super regenerative circuit and convert it to a MF regerative circuit is not the way to go about things.
    There are many simple solid state regenerative circuits out there, why you attempted this is beyound me. I mean no offence, it's just not the way to go about it.

    Hmm... where do I start to expalin that what you have created more then likely is not a regen or super regen at all? From what you described you have a variable L/C hartley circuit connected to a transistor, it's no more then a gorified crystal radio. I question if it's really even oscillating.

    If it does oscillate, you have not described how you control the feed back (regeneration). Q6 was what controlled osciilation in the orginal circuit. And yes, super regens oscillate but the oscillation is quenched at sub-audible freqs as John describes in his article.

    "The emitter of the UJT provides an approximate sawtooth waveform, which as it also provides the bias supply for Q2 takes the detector in and out of oscillation at about 50KHz.
    It is necessary to be able to set the optimum quenching voltage and this is done by adjusting Q6's supply by pot VR2. This effectively functions as the regeneration control."

    So without a regeneration control either your circuit is not oscillating or is oscillating out ouf control. I would suspect the former but I could be wrong.


    To learn more about regenerative and super regerative receivers you might want to join the Yahoo regen group
    http://groups.yahoo.com/group/regenrx/

    They have many schematics in the files section and an overflow files section for vacuum tube and solid state regens. You can also discuss regen theory with some of the most knowlegble people on the subject. Charles Kitchin, N1TEV, (Mr. Regen himself), contributes to the group.

    Hope this helps,
    John
    KB7NRN
     
  9. WA6TLP

    WA6TLP Ham Member QRZ Page

    I agree with John (KB7NRN), trying to convert a VHF super regen to use on the HF bands is going about it the hard way and the experiments you have performed to date are highly suspect and (in my mind) of limited worth.

    Before you spend any more time trying to adapt the design you have referenced, you might consider building a radio based on an existing design known to work on HF. Once you have it working, you might try tweaking resistor and capacitor values to try to improve its performance and test your hypothesis' regarding what each component is actually doing in its circuit. I think you will have a lot of fun and will learn a lot doing that. The only thing I would suggest (as I state at length in my article), stay away from Mr. Kitchen's so-called "Scout Radio."

    If you want something simple and cheap to build, uses common, easy to get parts, tunes with smooth regeneration control and that works quite well, I've already linked to my little radio. John's suggestion regarding the chat group is really excellent too if you want to get knowledgeable regarding this subject before actually building something. Of course, I believe that the best way to learn is to just jump in and build something.

    By the way, in my earlier post I said "...as you increase the capacitance from 1 microfarad to (say) 1000 MF..." Boy was that wrong. I meant "...as you increase the capacitance from 1 picofarad to (say) 1000 picofarad..."

    I was wrong when I stated "magnitude of the signal you actually demodulate is not a function of how much signal you couple to the detector." Obviously, the stronger the RF signal, the louder will be the demodulated signal. Coupling can obviously be taken too far and there is an optimum amount. A compromise must be found so that the radio has good 'Q' and the regeneration is smooth and reliable. Most designs I've seen call for something between 5 and 100 PF at HF.

    Anyway, good luck.
     
    Last edited: Sep 14, 2011
  10. KX6MWS

    KX6MWS Ham Member QRZ Page

    Thanks for everyone's comments and advice!

    Whoops, I'm sorry, I forgot to mention I had a tickler coil coming off of the collector. WHen I wrote my initial post I edited it a bit, and I guess I accidentally deleted that part!

    This is a schematic of what I have right now:
    regen-radio.png

    Observations:

    -When L2 is close to L1, this circuit makes a horrible noise. (oscillation)
    -If L2 is turned around the other way the noise stops (oscillation is suppressed)
    -Adjusting R2 can stop/start the oscillations.
    -If tank circuit is tuned to a particular station, right before R2 oscillates, the station is very loud
    -If R2 is adjusted to that the receiver is oscillating, tuning slightly up/down where the station is seems to cause the station to become high or lower pitched.

    I think it's actually regen-ing properly. Using the same tank circuit I built a crystal set, and I don't get near the amount of sensitivity this is getting.


    When I want a good performing radio I will likely end up choosing a design that's known to work and copy it. I tend to learn most things through trial-and-error. (Though there's a big difference between hardware and software: when a computer crashes, you press reset. When a circuit 'crashes', the magic smoke comes out! )
     
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