Viewing Audio Hum

[Mike/W8BAC]

I saw a question in another thread that interested me as well. Since it went unanswered I thought I would post in a new subject.

I use a Flex SDR 1000 and power SDR software. Viewing the Pan adapter when listening to an AM signal with hum I can zoom in and see clearly the steady hump that represents the hum. I assume it would be possible to tell if the hum is 60 Hz or 120 Hz hum by adding or subtracting from the center (carrier) frequency. If I'm viewing a carrier at exactly 3880000 Hz and viewing 120 Hz hum I should see a hum bump at 3879880 and 3880120 Hz. Is this correct? If so 60  Hz should be viewed the same way and a combination of 60 Hz and 120 Hz hum should give a total of 4 bumps, two above and two below the carrier.

Viewing my carrier, and understanding what I see on an oscilloscope isn't clear. If I can view a clean carrier and adjust time/division and volts/division and bracket the clean DC carrier between vertical lines and add the hum, is it possible to simply count the AC cycles on the display and multiply by time/division?  Thanks!

Mike

[KF1Z]

Yes, you can!

Right on both counts.

[N2DTS]

Yes, if things are calibrated.
On the flex, if you move the mouse pointer over the bump, it should read out the EXACT frequency its at on the screen below the spectrum display.
The right side shows the tuned freq I think, the display on the left shows the mouse pointer, and does freq and amplitude.
Its like a piece of test equipment...

Brett

[Steve - WB3HUZ]

Most S/As made in the last 30 years will allow you to display the difference between to markers. You shouldn't even need to do any math. Put one marker on the carrier and the other on your modulation spectral line. The difference is shown (60 Hz or whatever).


If your scope has a line sync selection in the trigger section, you can use this to quickly see if there are any AC power related componenent in your modulation waveform. Once again, no math or calibration needed.

[Mike/W8BAC]

Thanks all for the help. I wasn't clear, especially, on how to measure AC riding on a DC trace. Again I hope Clark finds this useful.

Steve/HUZ wrote

If your scope has a line sync selection in the trigger section, you can use this to quickly see if there are any AC power related component in your modulation waveform. Once again, no math or calibration needed.

Steve, dose this mean regardless of DC dead carrier being present you will only see the AC component? I'd like to learn more about this one Thanks

Mike

[Steve - WB3HUZ]

You will still see the entire waveform on the scope. But anthing related to 60 Hz will be synchonized with the sweep. In other words, the 60 Hz components will be in the exact same place on the horizontal axis every sweep - no movement or drifting across the screen.

[Mike/W8BAC]

In other words, the 60 Hz components will be in the exact same place on the horizontal axis every sweep - no movement or drifting across the screen.

NOW, THAT'S HELPFUL! I'll have to check to see if I have line sync on my scope. You hit this one out of the park Steve. The AC (without line sync) is on the move and very hard to sync for counting. Thanks

Mike

[WA3VJB]

Most S/As made in the last 30 years will allow you to display the difference between to markers.

Are there "from" markers too ?

[Steve - WB3HUZ]

No problem Mike. Good luck tracking down the hum.

In other words, the 60 Hz components will be in the exact same place on the horizontal axis every sweep - no movement or drifting across the screen.

NOW, THAT'S HELPFUL! I'll have to check to see if I have line sync on my scope. You hit this one out of the park Steve. The AC (without line sync) is on the move and very hard to sync for counting. Thanks

Mike