Hi there,
This is a kind of follow-up to my previous thread in relation to switching between Rx and Tx (see
https://www.eham.net/community/smf/index.php/topic,135367.msg1252049.html#msg1252049). Back then, I noticed that my transceiver build was oscillating on transmit and I put that down to shabby wiring and layout.
However, since then I've cleaned up my act somewhat and the problem persists (although I have a solution - read on!). I decided to take it to you guys for consideration and advice. The question here is: why is this happening and why does my solution actually work. I'll also gratefully accept other mitigation techniques, since my solution does impact the output power somewhat.
Anyway, lets get into it. Here are some images to start with:
The transmitter chain block-->
https://imgur.com/mBprgHsBirdseye of the overall layout (complete with mitigation hacks for the eagle eyed)-->
https://imgur.com/i1MuzqiThe offending oscillation; no audio input; 106Vpp-->
https://imgur.com/KqeJ6eLOk, so taking a breath, you can see the issue. No audio and the transmitter takes off on its own. When it does this, there is some power supply ripple and this despite all the mitigation techniques I know of: Q killing resistors, chokes, capacitors, etc. I will say that out-of-the-box, the W6JL would seem like an angry oscillator waiting to happen, but the problem doesn't go away when I choked the DC feed to the output transformer and added additional components to stabilize the bias circuitry.
Supply ripple-->
https://imgur.com/xURKpoahttps://imgur.com/eDkL3CwAnyway, here is what actually worked for me-->
https://imgur.com/1aL2KwpA -3dB pad at the input to the pre-amp, so that's in between the 40dB pre-pre amp and the pre-amp (see block diagram point A). Note, the attenuation is critical, it most be *at least* -3dB of attenuation.
Also interesting is that if I attenuate the signal earlier in the chain, it isn't as effective (it still oscillates, but there's an improvement).
So, here are my questions: why did this work and are there any other solutions I might try based on what you see in the images?
Additional images-->
The final amplifier:
https://imgur.com/HKdmOZsPre-amp circuit:
https://imgur.com/3LfgAbAPre-pre amp circuit:
https://imgur.com/rJ2KqQ4SSB audio transmission, no oscillation:
https://imgur.com/Phr1KqDRegards.