Oscilloscope probe ground leads act as FM attennea

[Martin72]

Is this why people wear tinfoil hats?

 

Do you have a metal box ?
You could put the scope in there and then see if the irradiations become less.
But I am afraid that the probe will still be the "antenna".

[dmulligan]

Is this why people wear tinfoil hats?

 

Do you have a metal box ?
You could put the scope in there and then see if the irradiations become less.
But I am afraid that the probe will still be the "antenna".

Tinfoil hats and metal boxes are just two different styles of faraday cages.  I am thinking about wrapping a cardboard box in foil, grounding the foil then using the web interface to look at the results.

[ledtester]

[quote author=dmulligan link=topic=398355.msg5143530#msg5143530
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?
[/quote]

Just don't go too crazy trying to find it...

Coppola - The Conversation - ending
https://youtu.be/SEo7FGkmRx0

[dmulligan]

I finally found the opportunity to shut down the power to the rest of the house.  I didn't set the scope up in the same location due to travel time to my normal work area.  I regret not doing that as the last step.  I was only seeing 35mV of noise with my scope plugged in beside the breaker which wasn't good for testing for lower noise.  I tried a few more outlets around the basement with similar luck.  Eventually I decided to use an extension from the outlet where I saw the lowest noise from and located my the DHO804 at the top of the stairs.  I was only seeing 80-100mV of noise but at least the FFT showed the same peaks across the FM band.  Shutting down all other circuits yielded no change.

Learnings:
1) Noise source doesn't seem to be local.
2) Noise improves the lower down I get.  The other way to think of that is the noise improves the further I get the scope away from line of sight to the towers.
3) (Separate from this test) Twisting the ground leads around the probe lowers the noise, often significantly.

I would really like the convenience of both working upstairs and using ground leads at least sometimes.  I wonder what other mitigation methods I can use.  I have some more split ferrite beads of unknown value coming from Amazon today that I can try on my bench power supply cable, ground, scope AC and scope DC cables.  I've read that a grounded antistatic mat has helped with some types of probe noise, I don't know if that could help mine.  I suppose that I could simulate that with some foil or a cookie sheet and a ground wire.

Are shorter or lower inductance ground leads available?  LeCroy makes a "HF Compensated Ground Lead", I wonder if I can find them for Rigol probes or if I can find affordable probes with some.  I wonder what sort of sorcery makes them special and how effective they are.  From the probe datasheet "A passive network in the lead itself reduces the loading effect of any resonance."  At 35USD I really hope they are special and at that price I don't think I'll be buying one to test if it even fits my probes let alone helps with my problem.

[ebastler]

Are shorter or lower inductance ground leads available? 

Do the Rigol probes come with ground springs which you attach right to the front of the probe (after pulling off the hook attachment)? If so, have you tried what difference these make? They are not applicable in every situation, since a ground connection needs to be available very close to the measurement point -- but should work on the 1 kHz reference output, and will give you another data point.

EDIT: Ah, sorry -- just saw your older post a page up where you already mention that you successfully tried these.

[dmulligan]

Are shorter or lower inductance ground leads available? 

Do the Rigol probes come with ground springs which you attach right to the front of the probe (after pulling off the hook attachment)? If so, have you tried what difference these make? They are not applicable in every situation, since a ground connection needs to be available very close to the measurement point -- but should work on the 1 kHz reference output, and will give you another data point.

Yes, the problem goes away using the ground springs.  I mentioned that a few pages back.  It's less convenient but it works.  The inconvenience gets worse the more channels I need to use, especially as I go over 2.  I'm going to 3d print some probe holders but as I am clumsy I suspect they won't help as much as probe clips would.

I don't think I've mentioned before that I see the same noise on any circuit I probe as long as I am using ground leads.  Sadly my probe noise makes I2C decoding fail to work most of the time.

My options are now 1 or more of the following:
1) Use ground springs
2) 20MHz BW filter
3) Use probe at 1x

[ebastler]

I don't recall all the steps you have already taken on this journey. Did you try some (partial) shielding already? Maybe try a grounded metal plate oriented vertically, relatively close to your device under test, in the direction of the incoming radio waves? Or a grounded plate (with an insulation layer on top) as your workbench surface?

[macboy]

Based on a quick eyeball of the frequency peaks and the description of the transmitters on a hill across the river, I'd guess you live in the NCR? I do as well, but much further south and so further away from most of the transmitters. My lab is also in the basement, which helps. I have never experienced an issue with noise as bad as yours.

The FM peaks are certainly a smoking gun. There is no question why you pick them up; any conductor will pick up RF radiation to some extent. The spring is very short compared to the ground lead, so it picks up less. Being so close to multi-kW transmitters is certainly a mixed blessing; great for picking up clear radio or TV stations, but bad otherwise. Try holding the ground lead parallel to the probe rather than letting it form a big loop. You could try adding a ferrite on the scope probe lead very near the probe (not on the ground lead!!, but on the coax from probe to scope). As before, this may help or hinder, but is easy to try.

Reducing the sampling speed does not eliminate high frequency noise, as you have found. The scope does not apply a low-pass (anti-aliasing) filter to the signal, and the sample-and-hold prior to the ADC is very fast. It will capture the instantaneous voltage of the waveform at the sample time regardless of the sampling period. This means all HF noise is still there, just aliased to a different frequency.

As for other mitigations, you could try the scope's built in filters: low pass or band stop. In either case, the sampling frequency will need to be high enough that the noise at 87 - 107 MHz appears at that frequency and is not aliased to a lower frequency. So sample rate will generally need to be very high. Filters are effective with single-shot or repetitive captures, as long as you have lots of samples to spare (high sampling rate). Obviously, it will reduce HF content of the signal in addition to noise.

Since the FM radio noise is not coherent with the input signal, using averaging can help dramatically, even with a small number of samples like 4 or 8. This requires a repetitive signal of course, but when that is what you are observing, I recommend averaging at a low rate (4/8) in general. Averaging helps to enhance resolution and clean up noise so you can see the real shape of the signal under the unavoidable noise, and it does not reduce HF signal content like a filter (or eres mode) does.

Finally, if you have a quality USB-C PD charger (like from a laptop or chromebook), try powering the scope from it, to see if some of the noise is actually conducted from the power supply.