Author Topic: Channel cross-talk issue on Rigol 1054Z ?  (Read 3799 times)

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Offline mrnuke

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Channel cross-talk issue on Rigol 1054Z ?
« on: June 05, 2016, 04:03:15 AM »
I've been playing around with my Rigol 1054Z, and I noticed that the waveform on Ch1 changes depending on whether Ch2 is enabled or not. This doesn't seem right.

Is this a normal phenomenon, or do I have a faulty unit?

 

Online Fungus

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #1 on: June 05, 2016, 04:39:22 AM »
Probably a PEBKAC. If you look at the top of the screen you'll see the number for sample rate has changed. That might be enough to do it (ie. you've got half the sample rate you had before).



« Last Edit: June 05, 2016, 04:45:07 AM by Fungus »
 

Offline mrnuke

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #2 on: June 05, 2016, 04:53:06 AM »
That might explain the waveform changing when Ch2 is enabled, but it still doesn't explain why the waveform on Ch1 changes when Ch2 is physically connected and disconnected from the scope.
 

Online Fungus

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #3 on: June 05, 2016, 05:04:56 AM »
Sorry, I didn't watch the video before.

But: PEBKAC. You're triggering on channel 2.   :)

 

Offline mrnuke

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #4 on: June 05, 2016, 05:07:27 AM »
Trigger is set to Ch1, 0V, falling edge.
 

Offline saturation

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #5 on: June 05, 2016, 05:20:21 AM »
There should be ~ no cross talk between channels, but you should test for it explicitly under controlled conditions to be certain your DUT isn't causing it for some unclear reason.  Rigol claims at >= 40dB channel isolation.  The best way is two separate signal sources one per channel, not say from one source with a dual output unless you are sure the dual-output source is problem free.
« Last Edit: June 05, 2016, 05:22:26 AM by saturation »
Best Wishes,

 Saturation
 

Online Fungus

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #6 on: June 05, 2016, 05:21:56 AM »
Trigger is set to Ch1, 0V, falling edge.

Screenshots and video say different. A cyan trigger line appears on the screen when you enable channel 2 in the video (and there's no trigger line visible when only channel 1 is enabled)



If you're triggering on channel 2 then it's not surprising the waveform changes when you connect/disconnect a probe on that channel.

« Last Edit: June 05, 2016, 05:27:06 AM by Fungus »
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #7 on: June 05, 2016, 05:22:56 AM »
By using a long wire to probe the signal you add inductance. And when you enable channel 2 you add extra capacitance with the ground of the probe. Probably these two resonate and that's why you see the signal change. I'm pretty sure this is normal. You shouldn't probe the signal the way you probe it. The scope probe is a transmission line and it properly cancels inductance/capacitance normally. By using a long wire it's no longer cancelled.
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #8 on: June 05, 2016, 05:25:41 AM »
Certainly you wouldn't see this effect if channel isolation had been infinitely large. But it's not.
« Last Edit: June 05, 2016, 07:55:15 AM by Earendil »
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #9 on: June 05, 2016, 05:34:04 AM »
Hmm. You're probably right. I still wouldn't probe any signal the way he does and expect to get proper results.

Trigger is set to Ch1, 0V, falling edge.

Screenshots and video say different. A cyan trigger line appears on the screen when you enable channel 2 in the video (and there's no trigger line visible when only channel 1 is enabled)



If you're triggering on channel 2 then it's not surprising the waveform changes when you connect/disconnect a probe on that channel.
« Last Edit: June 05, 2016, 08:02:20 AM by Earendil »
 

Offline mrnuke

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #10 on: June 05, 2016, 05:35:44 AM »

Screenshots and video say different. A cyan trigger line appears on the screen when you enable channel 2 in the video (and there's no trigger line visible when only channel 1 is enabled)



The trigger indicator on the left side of the screen. The cyan line you refer to is the Ch2 signal, which is, 0V + noise, since there's nothing hooked up to Ch2. If we were truly triggering off channel 2, then the ch1 waveform would be a line of yellow.
 

Offline mrnuke

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #11 on: June 05, 2016, 05:44:11 AM »
Case in point.

Also, on the original captures, trigger settings are displayed in upper-right corner of screen.
 

Online Fungus

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #12 on: June 05, 2016, 05:47:44 AM »
The trigger indicator on the left side of the screen. The cyan line you refer to is the Ch2 signal, which is, 0V + noise, since there's nothing hooked up to Ch2. If we were truly triggering off channel 2, then the ch1 waveform would be a line of yellow.

Oh, you're right. PEBCAK at this end.  :palm:

Still, it looks like a triggering problem.

Try moving the trigger level up/down a bit. If it goes away then 'crosstalk' isn't the problem, it's the probing setup (as others have said).
 
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Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #13 on: June 05, 2016, 05:50:35 AM »
Could you probe the signal without the long wire and see if anything changes?
« Last Edit: June 05, 2016, 05:53:56 AM by Earendil »
 

Offline rf-loop

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #14 on: June 05, 2016, 05:58:49 AM »
This is normal, if we look CH1 alone signal is "clear" thin trace and after CH2 is there CH1 signal come "fat".
Ch1 alone samplerate is 1GSa/s and CH1+2 samplerate is 500MSa/s

This is one form of aliasing. Its name is "corner wobbling"

But there in image can see also other effect (so careful to inspect so that do not mix different things)  what come from trace fine positioning and then also some false triggers due to signal shape and not optimal trigger setting. But it is other case.

Whole talk about "cross talk" in this case is wrong. In your image CH2 do not show anything, least with my poor eyes. My eyes "best before" day have gone long time ago)

Cross talk mean that other channel signal is leaking to other channel. If you set high level fast signal to CH1 and then not connect anything to CH2 you can see CH1 signal also in CH2 when you select enough sensitive setting. Try 5Vpp 100MHz  to Ch1 (1V/div) and nothing to CH2. Then select 10mV/div for CH2. Now you may see signal also there and this is cross talk. Typically attenuation is less with higher frequencies.  40dB cross talk attenuation (voltage) means 1:100.

About different kind of displayed aliasing...

Principle is also explained here (I can not find now this much clear and better example)
http://cp.literature.agilent.com/litweb/pdf/5989-5732EN.pdf

In this paper look Figure 4. (note that display mode there is dots)

Quote
Figure 4 shows an example of a
500-MHz bandwidth scope sampling
at just 1 GSa/s while operating
in a three- or four-channel mode.
Although the fundamental frequency
(clock rate) of the input signal is well
within Nyquist’s criteria, the signal’s
edges contain significant frequency
components well beyond the Nyquist
frequency (fN). When you view them
repetitively, the edges of this signal
appear to “wobble” with varying
degrees of pre-shoot, over-shoot, and
various edge speeds. This is evidence
of aliasing, and it clearly demonstrates
that a sample rate-to-bandwidth ratio
of just 2:1 is insufficient for reliable
digital signal measurements.
« Last Edit: June 05, 2016, 06:05:47 AM by rf-loop »
If practice and theory is not equal it tells that used application of theory  is wrong or the theory itself is wrong.
It is much easier to think an apple fall to the ground than to think that the earth and the apple will begin to move toward each other and collide.
 
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Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #15 on: June 05, 2016, 06:24:39 AM »
Well, I reproduced the same setup with a 25Mhz square wave. I can see the waveform shape change on channel 1 by connecting/disconnecting the probe on channel 2. What is somewhat surprising is that I can't see any difference just by turning channel 2 on/off (with the probe connected).
« Last Edit: June 05, 2016, 06:26:48 AM by Earendil »
 

Offline mrnuke

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #16 on: June 05, 2016, 06:32:33 AM »
Well, I probed the thing the hard way. I can still see the Ch1 waveform change as I enable or disable Ch2, but I can't see any change when I connect or disconnect Ch2. As rf-loop noted, this is most likely due to aliasing. There "appears" to be some 200MHz ringing there, and we're only probing it at 500MS/s.

I'm still not seeing how the previous probing method could affect Ch1 when Ch2 is connected. Could that be due to the return current traveling down the ground of the second probe and reflecting odd the end?
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #17 on: June 05, 2016, 06:41:35 AM »
Also I couldn't see the effect in DC coupling mode. So the AC coupling capacitor might something to do with it.
« Last Edit: June 05, 2016, 07:38:15 AM by Earendil »
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #18 on: June 05, 2016, 06:53:05 AM »
Anyway, I think this would be an excellent topic for a future video for Dave.
I believe you can find this effect on other scopes too, not just on the Rigol 1054Z.
 

Offline MarkL

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #19 on: June 05, 2016, 06:58:58 AM »
Look at the number of sample points.  Using Ch1 it's 120pts.  Using Ch1 and Ch2 it's 60pts.  That's very few points spread across the entire width of the screen.

The sinx/x interpolation is having a difficult time with so few points.  You could try turning it off, or going to dots mode.

Because the interpolation is off, and (I think) this scope has a digital trigger, it would be throwing the trigger off too, hence the horizontal smudging around Ch1 trigger point.
 

Offline mrnuke

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #20 on: June 05, 2016, 06:59:49 AM »
Anyway, I think this would be an excellent topic for a future video for Dave.

I couldn't agree more!!!
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #21 on: June 05, 2016, 07:10:27 AM »
You add a parallel capacitance to the ground by connecting the probe on channel 2. I think this in itself can explain the effect.

I'm still not seeing how the previous probing method could affect Ch1 when Ch2 is connected. Could that be due to the return current traveling down the ground of the second probe and reflecting odd the end?
 

Online Fungus

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #22 on: June 05, 2016, 07:37:58 AM »
I'm still not seeing how the previous probing method could affect Ch1 when Ch2 is connected. Could that be due to the return current traveling down the ground of the second probe and reflecting odd the end?
You add a parallel capacitance to the ground by connecting the probe on channel 2. I think this in itself can explain the effect.

That would (maybe) explain why connecting/disconnecting the probe makes a difference but it wouldn't explain why simply turning channel 2 on/off makes such a big difference.

 

Offline alsetalokin4017

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #23 on: June 05, 2016, 07:41:33 AM »

Screenshots and video say different. A cyan trigger line appears on the screen when you enable channel 2 in the video (and there's no trigger line visible when only channel 1 is enabled)



The trigger indicator on the left side of the screen. The cyan line you refer to is the Ch2 signal, which is, 0V + noise, since there's nothing hooked up to Ch2. If we were truly triggering off channel 2, then the ch1 waveform would be a line of yellow.

The trigger level indicator is on the _right_ side of the waveform display area, but the actual trigger channel, type, and voltage level are always indicated at the very top right of the screen, and even though you can't always see the level in the video, you can at least always tell that this indicator remains yellow, thus indicating that CH1 is the trigger channel used in the video. The indicator at top right does not show the _trigger_ coupling setting, and there is no trigger level indicator or trigger level line displayed if the trigger coupling is set to AC or Low-Frequency Reject, but the trigger level is still displayed in the trigger indicator box at the very top right of the screen.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline Earendil

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Re: Channel cross-talk issue on Rigol 1054Z ?
« Reply #24 on: June 05, 2016, 07:52:46 AM »
I'm still not seeing how the previous probing method could affect Ch1 when Ch2 is connected. Could that be due to the return current traveling down the ground of the second probe and reflecting odd the end?
You add a parallel capacitance to the ground by connecting the probe on channel 2. I think this in itself can explain the effect.

That would (maybe) explain why connecting/disconnecting the probe makes a difference but it wouldn't explain why simply turning channel 2 on/off makes such a big difference.

Yes. I was referring to connecting/disconnecting the probe.

I can't reproduce the effect when he turns ch2 on/off. I'm not sure what's behind that.
 


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