Products > Test Equipment
Oscilloscope probe ground leads act as FM attennea
dmulligan:
--- Quote from: ebastler on October 29, 2023, 10:09:12 pm ---As Martin said, the frequency range almost exactly matches the FM band (at least as defined in this part of the world, 87.5 to 108 MHz). And in your later post the signal does look modulated, although it looks more like AM modulation -- is that being used in this frequency range in Canada?
If you can visit a friend out of town, further away from the transmitters, it's something I would try to get a better understanding and confirm the radio interference theory.
Also -- does the noise appear specifically when you measure the calibration signal? What happens when you just ground the probe tip (clipping the probe's little ground lead to it)? What happens if you do the same, but grounding via e.g. a 100 kOhm resistor? If it should just be the 1 kHz reference output that catches the radio interference, that would be odd but would not affect your use of the scope.
--- End quote ---
Yes, I said that it looked like FM band interference in the first message I posted an FFT capture. No we don't have AM at 87.5 to 108 MHz in Canada. We have the same FM band as the US.
With the probe tip grounded using the aligator clip and lead, I do indeed have the same noise. It changes drastically as I move the probe around. I even found a place on my desk, with the cable looped a bit with only 45mV of noise. The noise is about 75-125 mV with a 100k resistor depending on where I wave it around. It's mostly 75-80 unless I let go of it.
If I use the ground spring to the calibration out and ground the signal is nice and clean.
Fungus:
I tried using the worst cable I could find and it was all good.
If I touch the clip with my finger I get ripple on the signal.
tautech:
--- Quote from: dmulligan on October 29, 2023, 10:36:20 pm ---If I use the ground spring to the calibration out and ground the signal is nice and clean.
--- End quote ---
This ^ :clap:
Now you know how to work around the probe reference (gnd) lead making a nice RF loop, you have all you need to go forward .....but don't forget about that nice little RF loop as non-contact frequency measurement can at times be useful.
ebastler:
--- Quote from: Fungus on October 29, 2023, 10:41:51 pm ---I tried using the worst cable I could find and it was all good.
--- End quote ---
Any change if you also connect the black GND wire to the scope's GND clip (next to the reference output)? We want a ground loop here!
Edit: Well, normally we don't want it. But for testing...
Fungus:
--- Quote from: ebastler on October 29, 2023, 10:50:53 pm ---
--- Quote from: Fungus on October 29, 2023, 10:41:51 pm ---I tried using the worst cable I could find and it was all good.
--- End quote ---
Any change if you also connect the black GND wire to the scope's GND clip (next to the reference output)? We want a ground loop here!
--- End quote ---
Nope.
The GND is connected the BNCs internally so no chance of a loop. ;)
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