Wednesday, September 26, 2018

Spice sim of an amp and detector

The circuit was designed for 300mv pk-pk input. Here is the result.
I divided the circuit into modulator, Amp, and detector. In this shot I have the modulator set to 300 mv pk-pk and am looking at the RF amp output. To see what's happening I will narrow the time line view.
The amp is doing to good a job and is clipping the signal. So I'll reduce the input.
I cut the signal to 30 mv pk-pk and now have better demodulated signal. The RF amp output is starting to clip but is still producing a decent output. What is the cause of the clipping?
One last shot with 30mv pk-pk showing the RF amp output and the audio output.
Looks like an envelope detector output should. This is with 10% of the designed input and I changed the AF filter resistor from 640 to 10k. I'm using a "high" percent modulation for the sim. What would happen if the modulation was only 1%? So the question is when does the signal get strong enough to over drive the amp and how to prevent it?

I can across this last night and wanted to store it before I lost it. I will have to look up the patent for the rest of the story.
Note the RF decoupling in the collector circuit. Keeping RF out of the power supply can be very important. I must do a new sim with this new found info!

300mv pk-pk input-Clean RF out-Good detected signal with low modulation.
I added a 200 Ohm resistor and 100n cap to decouple the power supply and the signal cleaned up nicely with the 10% modulation. It will probably go crazy at higher modulation levels. An RF choke would be better but that will be a later post.

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