Applying regeneration in the audio stage of the gnat40

[neazoi]

Hello,
This little transceiver www.home.earthlink.net/~christrask/Gnat 40.pdf uses a reflex topology for the receiving section, with applied crystal based regeneration,
Since it is used only for CW, I was thinking that audio regeneration could be applied to it, leading in a dramatic increasing of the sensitivity and audio selectivity.

Audio regeneration means that the audio bandwidth will decrease dramatically, but because this receiver is only used for CW (single audio tone) this is not a problem.
Audio regeneration also means that there must be a means of controlling the positive audio feedback to prevent the audio amplifier to oscillate.

The pdf file, describes the different sections in detail.
I would like to give me general advices of where to start, if I want to apply audio positive feedback to the transistor?

 

[betwixt]

You can't easily apply audio feedback in that circuit but if you want to try, take the signal from the top of T3 secondary and pass it through a variable resistor (~500K) then a capacitor (~10uF) then an RF choke to the base of the transistor.

It would be difficult to adjust because the blocking action as it reached oscillation threshold would block the RF regeneration. Selecting the audio frequency would also be a problem, bear in mind the transceiver isn't tunable so the tone would be dependent on the transmitting station frequency. A better solution would be to use a twin-T filter and small amplifier after the existing audio output.

Brian.

 

[neazoi]

It would be difficult to adjust because the blocking action as it reached oscillation threshold would block the RF regeneration.
Brian.

Thanks for the useful starting points. So positive feeback is taken out of the top part of the secondary of the audio transformer, nice.
I do not really understand why this audio feedback could block the RF regeneration. I assume that the audio feedback is set to a point before audio oscillation (maybe long before).

 

[betwixt]

The feedback problem is because in order to work at both frequencies the stage has to be linear. The RF signal would have to sit on top of the lower AF signal as they pass together through the transistor. When oscillation occurs, the transistor will be driven in and out of conduction so the other signal will become chopped up.

If you try it, bear in mind that the phase of the audio transformer has to be correct to give positive feedback, it may be necessary to reverse one of the windings to make it work.

My point about not being tunable is that the receive frequency is fixed by the crystal so CW signals nearby to it will have different audio pitch depending on their exact relationship to the crystal frequency. If you narrow the audio passband you will start to lose signals away from the audio peaking frequency so you will only be able to pick up transmissions in a very narrow range of frequencies. For example, if the crystal was (example) 10MHz, the audio peaked at 1KHz you would hear a CW transmission at 9.999KHz and 10.001KHz only, any other frequencies would not be heard. Holding a two way QSO would be difficult as each station would have to transmit exactly 1KHz apart.

Brian.

 

[neazoi]

The feedback problem is because in order to work at both frequencies the stage has to be linear. The RF signal would have to sit on top of the lower AF signal as they pass together through the transistor. When oscillation occurs, the transistor will be driven in and out of conduction so the other signal will become chopped up.

If you try it, bear in mind that the phase of the audio transformer has to be correct to give positive feedback, it may be necessary to reverse one of the windings to make it work.

My point about not being tunable is that the receive frequency is fixed by the crystal so CW signals nearby to it will have different audio pitch depending on their exact relationship to the crystal frequency. If you narrow the audio passband you will start to lose signals away from the audio peaking frequency so you will only be able to pick up transmissions in a very narrow range of frequencies. For example, if the crystal was (example) 10MHz, the audio peaked at 1KHz you would hear a CW transmission at 9.999KHz and 10.001KHz only, any other frequencies would not be heard. Holding a two way QSO would be difficult as each station would have to transmit exactly 1KHz apart.

Brian.

Thank you, very useful points!
Do you think that the linearity problem could be improved by the use of a more poerful transistor, so that it's saturation point is higher? I have seen norton noiseless feedback RF preamplifiers that use power transistors and I think this has to do woth improving their linearity as well.

 

[betwixt]

A power transistor would be able to handle a larger signal but in the case of an oscillator, the normal mode of operation would be for the feedback to almost or completely cut the bias current off over part of the cycle so the amplifiction of the other frequency would be affected as well. This is why the more usual approach is to use an oscillator stage and mixer stage as two different circuit blocks.

Brian.