[SOLVED] Rf signal with 2 modulations

[jvgt54]

So I have a 144Mhz signal from a transmitter that I am modulating with a 2 Khz signal. But when I look at the signal on my oscilloscope the signal is modulated by 2 signals: my 2kHz signal and an extremely overmodulated signal at about 125 Hz. Has anyone ever seen anything like this before?

Also I am fairly new at rf circuitry and theory so I apologize if my question is mundane or obvious.

Thanks in Advance.

 

[biff44]

probably a poor ground connecdtion somewhere is allowing 2 x 60 hz to modulate something

 

[tony_lth]

maybe power supply bypass capacitor not so good.

 

[Syncopator]

If you switch off the 2kHz modulating frequency, what does the carrier look like then? Is is clean, or does it still have the other modulation?

 

[jvgt54]

If you switch off the 2kHz modulating frequency, what does the carrier look like then? Is is clean, or does it still have the other modulation?

Ya the the approx. 125 Hz modulation is there whether the 2 kHz signal is being sent to the mic or not. As long as the Transmitter is transmitting the long 125Hz modulation is present.

 

[Syncopator]

Ok, that rules out problems with the 2kHz modulation source.

If your mains supply is at 60Hz, are you sure that the spurious modulation is at 125Hz, or could it be 120? If you do have 60Hz mains and the problem is at 120Hz then power supply ripple may be the cause.

Any chance that you could run the transmitter temporarily from a clean d.c. source? If so, do that and see if the nasty modulation disappears.

 

[goal]

Probe your transmitter DC power using an (oscilloscope) probe+DC block to a spectrum analyzer and see if 125Hz appears in spectrum analyzer?
then it may be because of bad AC to DC converter. you have to place a big decap for DC supply in order of 1500uF.
if the above doesn't happen please read about room resonance online

 

[biff44]

"mic". Did you mean microphone?

Jeez, I remember a day when hams would craft their own hardware from bailing wire and spit, and communicate half way around the world. Nowadays, they do not even know what 60 Hz hum noise is.

A brave new world?