17-04-2012, 09:57 PM
Having done this, it's really quite simple...
Picking up the signal post-detector seems to be pretty standard practice in these sorts of sets, and works well. I've got a diode probe that does the same job, though I can't remember the last time I used it...
I mentioned earlier that I don't use a wobulator; instead I use two oscillators. One is a Marconi TF2008, which generates the 470kHz. This is swept by a Wavetek 162, which is a lovely old function generator. This also drives the X input of the 'scope, of course.
The Wavetek can be stopped at any point in the cycle, and that's really convenient. It means that I can set the output to 0V, then centre the spot on the centre of the screen using the X-shift, then adjust the Marconi to give exactly 470kHz. Then, I can move the Wavetek output to the peak of its output, then adjust the amplitude so that the Marconi is giving a sensible frequency - say 460kHz. Then I adjust the X-gain on the 'scope, so that the spot has moved 4 divisions left. That means 1 horizontal division equals 2.5kHz. Once all that is done, the Wavetek is set to oscillate...
Estimating the -3dB point is quite easy, as you can see the peak at the centre, and the baseline. If the difference between the peak and the baseline is 8 divisions, then the -3dB is 5.6 divisions above the baseline.
It's harder to explain than to actually do
I've never played with a wobulator, but do they offer the option to stop the sweep at any frequency so you can check calibration?
Picking up the signal post-detector seems to be pretty standard practice in these sorts of sets, and works well. I've got a diode probe that does the same job, though I can't remember the last time I used it...
I mentioned earlier that I don't use a wobulator; instead I use two oscillators. One is a Marconi TF2008, which generates the 470kHz. This is swept by a Wavetek 162, which is a lovely old function generator. This also drives the X input of the 'scope, of course.
The Wavetek can be stopped at any point in the cycle, and that's really convenient. It means that I can set the output to 0V, then centre the spot on the centre of the screen using the X-shift, then adjust the Marconi to give exactly 470kHz. Then, I can move the Wavetek output to the peak of its output, then adjust the amplitude so that the Marconi is giving a sensible frequency - say 460kHz. Then I adjust the X-gain on the 'scope, so that the spot has moved 4 divisions left. That means 1 horizontal division equals 2.5kHz. Once all that is done, the Wavetek is set to oscillate...
Estimating the -3dB point is quite easy, as you can see the peak at the centre, and the baseline. If the difference between the peak and the baseline is 8 divisions, then the -3dB is 5.6 divisions above the baseline.
It's harder to explain than to actually do
I've never played with a wobulator, but do they offer the option to stop the sweep at any frequency so you can check calibration?
