Oscilloscope probe ground leads act as FM attennea

[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]

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

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]

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.

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]

Mine looks like this:

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

Peter

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

[ebastler]

I don't think there is cause for concern. Since the noise is low when using the ground spring, the scope itself is obviously fine. Since all probes behave the same, there is no reason to assume that you have a dud there with a bad ground clip connection.

It seems pretty clear to me that you are capturing noise from some source in the environment -- which might include the scope itself or its power supply. A few things to try:

• Use an even longer ground lead as an experiment, to see whether the noise goes up further.
  
• Move the ground lead around, especially making the loop cross section smaller or changing its orientation, to see whether that reduces the noise.
  
• To investigate whether the loop is picking up noise from the scope itself or from its power supply, try moving your probe + ground lead closer or further away from these. (No need to connect to the 1 kHz test signal, just clip the ground clip to the probe tip.)
  
• Zoom in on the noise or do an FFT, to see whether there are pronounced frequencies. Those might give you a hint as to the potential source. Again, that's probably best done without the 1 kHz signal.
  

Edit: I don't think anyone has mentioned it, maybe since most of us have gotten so used to DSOs for a while: Compared to CROs, noise tends to be visually more dominant on DSO displays, even with the "digital phosphor" intensity grading which all modern DSOs provide and which aims to mimic a CRO display. You can play with brightness and persistence settings, but will always see the overlay of many noisy sweeps more clearly than on a CRO. On the bright side, you will also see rare events more clearly! -- Also, when comparing to your 25 MHz CRO, setting the 20 MHz bandwidth limit on the DHO804 makes for a fairer comparison.

[David Hess]

Mine looks like this:

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

A change in sample rate by itself should not affect the noise.  Aliasing will cause the noise to fold over in the frequency domain and the standard deviation of the signal will be unaltered.

[2N3055]

Maybe you have one with the noisy PSU?
There were some reports about that in early specimens.
That is why Rigol changed to better quality PSU.

But mostly that is because new scope has 100MHz + BW and now you can see high frequency old scope didn't show.
Also, CRT scopes show fast signals with less brightness (pretty much invisible) if slow part of the signal is set to normal brightness.
Digital scopes show more detail, because they show faintest parts of the signal still in a way that is visible. You can see difference in brightness, but if there was a signal you will see it.

And then there is probing, once you have the scope that clearly shows more than 100 MHz, you realize that short piece of wire with a clip is antenna and inductive pickup....

[Fungus]

Mine looks like this:

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

Peter

Good point. I just pressed AUTO to make that screenshot, I didn't fiddle with the memory.

Here's mine with ground clip and the same memory settings:

[Fungus]

And then there is probing, once you have the scope that clearly shows more than 100 MHz, you realize that short piece of wire with a clip is antenna and inductive pickup....

Yep. It could easily be something in the room.

[Martin72]

I just tried this out here as well.
At first glance it looks good, but if you dissolve it further, you can see a ripple on the "roof".
If you take advantage of the good properties of the rigol and resolve more vertically, you can clearly see a ripple of about 30khz.
In my case, the ripple remains, regardless of whether groundspring or alligator clip or not connected to ground at all (the probe already has ground).
This is a disturbance that probably comes from the power supply.
But this is really nothing bad...

[ebastler]

If you take advantage of the good properties of the rigol and resolve more vertically, you can clearly see a ripple of about 30khz.
This is a disturbance that probably comes from the power supply.
But this is really nothing bad...

Thanks for giving the "zoom in to look for characteristic frequencies" suggestion a try!

While your cursor positioning for the frequency measurement is debatable (I would estimate ~40 kHz),  the switching supply clearly seems to be the source here.

[dmulligan]

Mine looks like this:

So that was an interesting exercise.  In order to match your conditions I had to upgrade to firmware 00.01.01 as pressing Auto gave me a different memory depth and sample rate.  I also saw a couple more differences in your screenshot pointing to you using new firmware.  Sadly I get no change in behaviour with new firmware. 

Do you have BW Limit enabled for that channel?  Does it change if you set your probe attenuation to 10x since you are running your probe at 10x?

[Martin72]

While your cursor positioning for the frequency measurement is debatable

Argh, new glasses please...

OK, but I don´t repeat this today, also the following not...
Following an idea, I went back to the study and did the following...
Siglent HD and the rigol switched on, first measured the siglent ref. generator with the siglent, then with the rigol.
Then measured the rigol ref. generator with the siglent.
The case is clear, the disturbance comes from the generator, which catches something...
But as I said, this is nothing bad as small as it is, just do not think that would now be a flaw.

[ebastler]

Sadly I get no change in behaviour with new firmware. 

Do you have the switching power supply right next to the scope? If so, does anything change if you move it as far away as the cable allows? (Not too likely because the noise is probably largely wire-bound, not radiated. But worth a try, I think.)

Also, which brand of power supply was shipped with your scope? It seems that Rigol shipped a no-name, fixed 12V supply with early units, but had problems with it. They transistioned to LiteOn or Lenovo supplies -- proper USB-C supplies capable of negotiating the voltage, which must cost them more, and probably are better quality and less noisy too.

[ebastler]

The case is clear, the disturbance comes from the generator, which catches something...

That's even better news! Noise on the 1 kHz reference signal really is irrelevant. As long as the scope input is unaffected and can measure real-world signals with low noise, everything is fine and dandy. 

[dmulligan]

I have a LiteOn PSU.  If the noise is coming from the PSU, wouldn't I get the same noise using the ground clip as with the alligator lead?  The PSU is on the floor and my scope is on my desk.  I will move it to yet another room soon.

I knew to expect extra noise on a DSO especially a higher BW scope, this is more than I expected.  The frequency and amplitude of the noise looks random.

Although the frequency looks to be random, perhaps it isn't.  If I am using FFT correctly, the only peak that sticks out above harmonics of the original signal is 63.0 kHz.

[David Hess]

I knew to expect extra noise on a DSO especially a higher BW scope, this is more than I expected.  The frequency and amplitude of the noise looks random.

It looks pretty normal to me for a 100 MHz bandwidth when using the ground clip and the built in calibration signal.  My 300 MHz DSO would look even noisier.  If I use a probe tip to BNC adapter to connect directly to a BNC, then the calibration signal is clean. (1)

(1) Some old Tektronix oscilloscopes used a BNC for the calibration signal, so this can be done on the oscilloscope itself but a function generator output works just as well for calibration.

[Martin72]

this is more than I expected. 

Once again, and as far as I'm concerned for the last time, you're worrying about this unnecessarily.

[dmulligan]

My #1 concern was that I don't have a dud scope.  I didn't feel any better about that having seen Fungus' and Martin72's relatively clean waveforms didn't make me feel any better. 

I tried moving the scope around to different parts of my house and my 3rd location produced much better results.  So the problem is environmental.  I am not really surprised as I am 100M from a transmission line and there is so much computer and electronics gear around my desk. 

Thank you all for helping alleviate my fears.  Now I need to figure out what I can do to use my ground alligator leads at least sometimes and, more importantly, learn how to use this little thing.

[Fungus]

Mine looks like this:

Do you have BW Limit enabled for that channel?  Does it change if you set your probe attenuation to 10x since you are running your probe at 10x?

Oh, I forgot my probe was in 1x mode. I don't have bandwidth limit enabled though.

In 10x mode I see pretty much the same thing.

[Fungus]

My #1 concern was that I don't have a dud scope.

The only thing that counts is what you see when there's no probe connected to the 'scope.

This is mine with probe connected and the probe's clip connected to the probe tip:


This is mine with no probe connected:

[CosteC]

Do you have grounding lead connected? DHO800/900 series with USB-C PSU introduced new failure mode - lack of grounding and "unnoticed loss of grounding"

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

[ebastler]

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.

[iMo]

..
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..