Oscilloscope probe ground leads act as FM attennea

[CosteC]

..
What is possible far easier in DHO800/900 is to add ferrite core on PE/grounding lead which may help in some cases.

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..

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...
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?

[Martin72]

This is a disturbance that probably comes from the power supply.

No:

https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5139402/#msg5139402

Must come from the "inside"...

[ebastler]

Do you live near a local FM radio station?

[...] 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.

[dmulligan]

Hm, look like the full FM band 87.5Mhz-108Mhz...
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

Thank you for making me look back at the way you zoomed in.  My noise is much higher frequency than yours.  The first three captures are as similar to yours as I could make them.  I didn't set the vertical cursors until the last two captures.

[Martin72]

Modulated signal....Your scope is receiving...

[dmulligan]

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

The signal in question is the 3v calibration signal where I am measuring 300-500mV of noise on it, depending on conditions I have yet to narrow down.  Not quite true, 2 meters to my left and the scope turned to a different orientation I was seeing only 150mV of similar looking noise.  I am trying to determine if the noise is something I can avoid or if I live in a noisy area and all signals require careful probing technique.

[tautech]

I can get 100's on mV doing the same under LED lighting....this is only the additional sensitivity a DSO can provide.
Trick is to know what it is and where from so to not focus on it and only on the part of the signal that matters.
20 MHz BW limit is very useful at times.

[ebastler]

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.

[dmulligan]

I can get 100's on mV doing the same under LED lighting....this is only the additional sensitivity a DSO can provide.
Trick is to know what it is and where from so to not focus on it and only on the part of the signal that matters.
20 MHz BW limit is very useful at times.

Agreed.  I am trying to determine the sources of the noise I am seeing in my work area.  I was amused at and wanted to share the large commercial FM broadcast interference I am seeing, that or asking for advice about how I am measuring incorrectly.  Sadly I am having trouble narrowing down just what it is if it isn't the FM broadcasts.  It's not in this room and I've been waiting for a time that I can shut down the entire house and bring it back up one breaker at a time.

[Fungus]

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.

Get a battery and go for a drive...

[tautech]

I can get 100's on mV doing the same under LED lighting....this is only the additional sensitivity a DSO can provide.
Trick is to know what it is and where from so to not focus on it and only on the part of the signal that matters.
20 MHz BW limit is very useful at times.

Agreed.  I am trying to determine the sources of the noise I am seeing in my work area.  I was amused at and wanted to share the large commercial FM broadcast interference I am seeing, that or asking for advice about how I am measuring incorrectly.  Sadly I am having trouble narrowing down just what it is if it isn't the FM broadcasts.  It's not in this room and I've been waiting for a time that I can shut down the entire house and bring it back up one breaker at a time.

Your FFT clearly shows peaks at each FM station.
Engage Peak markers and compare results to published local FM frequencies.

We had a customer that was convinced his new SDS1104X-E was broken but after assurance it wasn't he eventually found a unknown wallwart behind a curtain across the room. Some of these things just spew EMI.

[dmulligan]

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.

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]

If I use the ground spring to the calibration out and ground the signal is nice and clean.

This ^ 

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]

I tried using the worst cable I could find and it was all good.

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]

I tried using the worst cable I could find and it was all good.

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!

Nope.

The GND is connected the BNCs internally so no chance of a loop. 

[ebastler]

Nope.
The GND is connected the BNCs internally so no chance of a loop. 

Well, there is a GND connection between BNC and Ref GND clip inside the scope, and one outside the scope (if you make the cable connection). That's a GND loop, right?

But when you say "Nope", you mean you tried and it changes nothing? Then that loop is not where the scope picks up noise -- or you live in an electrically quiet area.

[Fungus]

But when you say "Nope", you mean you tried and it changes nothing?

Yep. It all looks great either way.

or you live in an electrically quiet area.

Looks that way.

[dmulligan]

..
What is possible far easier in DHO800/900 is to add ferrite core on PE/grounding lead which may help in some cases.

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..

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.

I found a snap on ferrite bead on an unused cable.  27mm long and 9mm ID (has a plastic cable holder).  I tried looping the ground cable and the USB-C cable or straight through with the AC cable with no noticeable difference.  I even tried the probe ground lead for fun which made a significant difference for the worse.

Oh well. I know how to work around the noise by using the ground spring which is good enough for now.  I still hope to find out how to avoid it all together.  Maybe I'll have to move in with Fungus since he lives in a exceptionally EMI quiet area.

[CosteC]

Ferrite on ground lead - as expected
Any luck with turning OFF some stuff in room/home? LED lighting can be very noisy and LED strips are naturally long, good antennas.
Another classical issue source are variable frequency drives. Now in washing machines, fridges, air conditioners and other stuff with electric motors.
EV cars are also horrible. I would be surprised if EV chargers would be different. Many kW SMPS always emits.

[dmulligan]

Ferrite on ground lead - as expected
Any luck with turning OFF some stuff in room/home? LED lighting can be very noisy and LED strips are naturally long, good antennas.
Another classical issue source are variable frequency drives. Now in washing machines, fridges, air conditioners and other stuff with electric motors.
EV cars are also horrible. I would be surprised if EV chargers would be different. Many kW SMPS always emits.

A few days ago I reported that I turned everything off and unplugged all the things in this room with no significant change.  I am waiting for an opportunity to shut off all breakers in the house and turn them back on one at a time.  There are so many devices in my home, it could be coming from anywhere.  Is this why people wear tinfoil hats?