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SG Frequency Change With Modulation - 462ron - 12-06-2019

Hi all, I have the typical service grade Heathkit signal generator from the 50s, it was my dad's. I have done the typical electronics restoration on it years ago. I usually use AVC voltage as my indicator for peaking things during alignment of a radio. I noticed if I peak things without modulation and then turn on the modulation I need to readjust things just a tad to reach maximum response again. It's as if the signal generator's RF frequency changes some with modulation. I have a frequency counter that I use but when I turn on the modulation the counter goes crazy and I can't read the true frequency. Anyone ever notice this happening? I guess one of these new digital generators might be better but they look to be beyond this old brain's comprehension level to understand. I'm still stuck in an analog world!

Ron

RE: SG Frequency Change With Modulation - John Bartley - 12-06-2019

I have had a couple of Heath SG series signal generators come thru' my shop over the years. I completely redid one (just for the heck of it) in an attempt to make it usable, a model SG8, and it had to have been the absolute worst generator I have ever seen as far as lack of stability. I don't recall any issue with the frequency changing due to the addition or removal of modulation, but then I couldn't keep it on a frequency long enough to do an alignment anyway. I went back to my General bench oscillator ....

EDIT : for the last decade or so I have been using a BK4040A. .... love it !!

RE: SG Frequency Change With Modulation - rfeenstra - 12-06-2019

Perhaps the counter is confused by the side bands??

RE: SG Frequency Change With Modulation - W8ZV - 12-06-2019

Hi All!!

The counter isn't confused.  It's trying to read the RF AND AF signals together.  You can't set a signal generator with a freq counter when the generator is running a modulated signal, AM or FM.  IF you have expensive equipment, i.e. HP or TEK, THEY will give you accurate freq measurements with AM modulation.  That's because the Counter, built into the generator, is looking at the oscillator output, before it's AM modulated.  That said, if your power supply is being loaded more when the modulator in the generator is running, then I'd expect the oscillator to change frequency a tiny bit.  Try looking at the power supply line in the generator and see if it's stable between loads.  IF it's not, then you've found your culprit.  I suspect that's what's going on here.  The frequency is changing due to power supply fluctuation.

You can try pulling signal from the oscillator tube via a cathode tap.  You might have to amplify the signal to get your counter to read, but THAT will give you a freq reading that should be accurate.  That said, I haven't looked at the SG-8 circuit to see where the modulator is at, in relation to the generator output.  The freq read scheme I just described was used by HP in their 606B signal generator.  You could hang a freq counter on that port and read freq while running the modulator.  That generator was using an RF amp for output with a modulator and mixer stage between the oscillator and the RF amp tubes

RE: SG Frequency Change With Modulation - 462ron - 12-06-2019

Thanks all, this is a cheap basic generator from back in the day and isn't in anyway stable. That's one of the reasons I bought a counter. I figured the counter was getting confused with the modulation and side bands and all. So I have no real way of checking if the RF osc. is changing when adding modulation but I suspect it is by the fact that I need to slightly retune the radio when adding modulation. It's been awhile since I had it apart but if memory serves, it has 2 tubes. One is the RF osc and the other the audio osc with a selenium rectifier! 

Ron

RE: SG Frequency Change With Modulation - TV MAN - 12-07-2019

You have to have a strong enough signal to the counter to be stable. If it is too weak, it jumps around. I use an Eico and it is very stable. I think it was one of the better units back in the day. The radio repair books will tell you to have the gen on for about 30 minutes before alignment. Now, in the days before those digital freq counters, all you had to go by was a known good radio and the pointer on your dial, and seems like they did pretty good. 

I note that on my AM radio transmitter that the freq jumps around some when I hit it with modulation. Steady when things are quiet.