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Oscilloscope probe ground leads act as FM attennea

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dmulligan:
This is my first DSO so I am not sure if my expectations are at fault or if it's my new oscilloscope.  I really hope it's something I am doing wrong.  I hooked each of my probes using the alligator ground clip to the compensation signal output terminals, clicked auto and I see a lot more noise on the signal than I expect.


However I get a very smooth signal when I use the ground spring instead.


I have tried all 4 channels with the same results.  The calibration signal from my DHO804 looks smooth when viewing it from my 25MHz CRO using the alligator ground clip.  However the calibration signal from my CRO looks even worse when viewed on my DHO804 with the alligator clip but clean using the ground spring.

I also tried taking the DHO804 to another room and it was no better viewing it's own calibration/compensation signal.  I am nearly certain that AC circuit would be from the opposite phase as the one at my desk, certainly a different circuit down the the breaker.

I am using the ground lead provided by Rigol through my benchtop power supply.  In the other room I loosely put it into the ground plug.  I have also tested with the scope floating also with no difference.

What might I be doing wrong?

Martin72:

--- Quote ---However I get a very smooth signal when I use the ground spring instead.
--- End quote ---

Complete normal behaviour, no need to worry.
The lead with the alligator clip is acting like a "antenna" receiving high frequent signals.
You will see a difference between bandwithlimit on/off.

dmulligan:

--- Quote from: Martin72 on October 24, 2023, 11:24:58 pm ---Complete normal behaviour, no need to worry.
The lead with the alligator clip is acting like a "antenna" receiving high frequent signals.
You will see a difference between bandwithlimit on/off.

--- End quote ---

I forgot to mention that I saw a lot less when setting the bandwidth limit to 20MHz.

This is expected behaviour even with about 250mV p-p noise riding on both the high and low parts?

Fungus:
Mine looks like this:


It could be your environment. Try it somewhere else ... eg. at a friend's house.

Also try unplugging all the little wall warts that you have near the 'scope (ie. The Phone chargers, etc.) and turn off the lights.

dmulligan:

--- Quote from: PeDre on October 25, 2023, 04:37:39 am ---
--- Quote from: Fungus on October 25, 2023, 01:44:30 am ---Mine looks like this:

--- End quote ---
Here the difference is more because of the sample rate, 312 MSa/s vs. 2.5 MSa/s.

Peter

--- End quote ---

Excellent observation.  Sadly reducing the sample rate to 2.5MSa/s made no difference what so ever.  So far only the BW limit brings down the noise when using the ground lead.  Which makes it odd that reducing the sample rate even to 500K Sa/s didn't help.  I am going to upgrade to the latest firmware tomorrow to see if that will help.

David

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