[W8ZV]

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