Checking Signal Generator Accuracy With A Frequency Counter and Scope

[Notimetolooz]

You definitely want to turn the modulation off while you check the RF frequency. Sometimes you will get an unstable or incorrect reading on a frequency counter if the signal level is too low and sometimes if the signal is so big it is causing the counter input circuits to distort. I have a SG-8 and like other generators of that vintage the amplitude varies a lot as you change the frequency. The signal also has lots of distortion on much of the frequency range, the harmonics can be used as a source of higher frequency. BTW, the modulation isn't very linear. I modified the circuit by adding another tube just for modulation. After working in a modern electronics lab I'm surprised that people were able to get radios adjusted right.

[jr_tech]

Originally Posted by Notimetolooz

The signal also has lots of distortion on much of the frequency range, the harmonics can be used as a source of higher frequency. BTW, the modulation isn't very linear.

Mine does not produce a good sine wave even at 455 kHz, and modulation is quite low, but it seems to be an adequate signal source for simple AM radio alignment. Is this typical, or does mine need some attention?
The frequency counter in my 'scope seems to measure it ok, but I suspect that some would have a problem with that waveform.

jr

[Notimetolooz]

Originally Posted by jr_tech

Mine does not produce a good sine wave even at 455 kHz, and modulation is quite low, but it seems to be an adequate signal source for simple AM radio alignment. Is this typical, or does mine need some attention?
The frequency counter in my 'scope seems to measure it ok, but I suspect that some would have a problem with that waveform.

jr

If it works OK for you then you could just leave it.
That IS a pretty bad waveform. It could be that the DC bias on a section of the 12AU7 is off somehow.
One thing I noticed from the schematic is that the output level control pot wiper doesn't have a DC blocking capacitor. That means that the changing that pot changes the DC load on the output section of the 12AU7. I added a 0.05 uF there. My SG-8 wasn't mint, the ON-Off switch was replaced with a toggle switch, so I wasn't concerned about keeping it stock. I did quite a bit of experimenting on it and modified the circuit. I got it to work better but it still is less than ideal. I just use it for AM BC adjustment, 540-1600KHz. I made a separate solid state 455KHz source with modulation.