Civilian Wartime Radio - Further Notes

[pwdrive]

Might be worth checking the AGC swing from weak signal to that very strong local signal, a high impedance (10meg) meter or 'scope switched to DC with x10 probe would be best for that.

Lawrence.

[Sparks]

I connected the oscilloscope to the grid, but there was nothing that I could make any sense of. Either the oscilloscope is not good enough, or, more likely, I am not good enough to figure it out! Whatever radio I have ever built over the past 20 years has had this distortion problem with this particular station, so I can reasonably assume that is the problem.
Bob

[Sparks]

This is the oscilloscope that I used!
Bob
   

[Murphyv310]

Hi Bob.
On this thread you'll see what you may see on the Scope, this is obviously a single tone but on the second on the left picture it shows over modulation, the one on the left is at around 80% modulation, an audio signal is a little harder to see but you'll get lines in the middle if it is over modulating.
The scope you have should be fine for the job.
https://golbornevintageradio.co.uk/forum...p?tid=6069&pid=64442#pid64442

[Sparks]

That is the sort of thing I was looking for, but it didn't bear any resemblance. I will probably try again tomorrow. May put an RF signal from sig gen in the aerial just to find the scope settings.
Bob

[ppppenguin]

It's not the easiest waveform to lock a scope to. When the audio is semi-random, as you would get from a normal radio station, it's almost impossible. If you're using a signal generator it's a lot easier. If the SG has an audio output then you can lock the scope to that. Otherwise many scopes have an HF reject setting for trigger which will pass the audio modulation and reject the RF. Hence it will trigger on the audio.

[Murphyv310]

Fine to use a signal generator but we are looking for a radio station that is overmodulating. I'll post up a YouTube video tomorrow to show how to do it and what to look for with over modulating sources, being fair it is actually quite easy.

[Terry]

... many scopes have an HF reject setting for trigger which will pass the audio modulation and reject the RF. Hence it will trigger on the audio.

Alternatively, connect the Audio from the output stage to the external trigger input

[Murphyv310]

Hi.
As promised a quick video to show what over modulation looks like on a scope.
https://youtu.be/gG_tdvRw3VQ

[ppppenguin]

Good demo, Trevor. While it's not as obvious with off air signal as with tone from a generator overmodulation is still very visible.