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Oscilloscope probe ground leads act as FM attennea

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CosteC:

--- Quote from: iMo on October 27, 2023, 12:21:47 pm ---
--- Quote from: CosteC on October 27, 2023, 11:43:42 am ---..
What is possible far easier in DHO800/900 is to add ferrite core on PE/grounding lead which may help in some cases.

--- End quote ---

That will not help you in any cases, imho. The ferrite bead put on a grounding wire will introduce serial impedance, like 10-100 ohm based on frequency.. You want the opposite - to have as small as possible impedance towards the ground..

--- End quote ---
In general you are right, yet in this case environment seems very noisy, maybe PE is noisy too. Putting ferrite on cable is easy experiment. Probably it shall be on emitting device, yet it may be celling light, so somewhat difficult.

dmulligan:
I understand now that there is nothing wrong with my DHO804, now I am trying to locate the source of the noise.  It feels inconsistent.  I tried shutting everything off in this room, which meant unplugging everything except my scope.  No significant difference on the original nor a nearby wall outlet.  An outlet in the next room seems better but an extension cord back to the original location seemed maybe a little better but hard to tell.  I clearly need to find a way to measure this better so that I can more immediately see if it's improving.  It doesn't help that I am still learning how to use this scope and haven't had much time to spend on it.

I tried FFT again, and this time I found something different.  I set the trigger point to just a little left of the screen and changed the horizontal scale so that I was just looking at the top part of the pulses.  Note: I definitely don't know how to use FFT properly, however this time I found a collection of peaks which, unless I did something very wrong, seems to be the sources of my noise, at least today.  Also I've moved the scope about 2 meters from where I had it on my desk due to something else I was working on and in this orientation/position the noise is less than half of what it was before, only ~150 mV.  Here were the results.









If I haven't made any glaring errors, my problem seems to be coming from a few locations with line of site through my window from 5 to 8 KM away.  All (most) of the FM and TV transmitters in my city are arranged along a long hill across the river valley from me, all visible from my window.  Could this really be the source of my noise or am I have I really messed up how I am using FFT?

Martin72:
Hm, look like the full FM band 87.5Mhz-108Mhz... ;D
Won´t believe that this is the cause, but you could try it like I did:

https://www.eevblog.com/forum/testgear/new-dho804-does-this-compensation-signal-look-noisy/msg5134284/#msg5134284

tautech:
~90-100 MHz is FM broadcast.
Careful probing technique is required with low level signals.

Fungus:
Do you live near a local FM radio station?

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