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Using (and living with) Siglent SDS1000X-E Series Oscilloscopes
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StillTrying:
Are you saying that the scope's CH 1+2 are not in phase with CH 3+4. :-//

If that have a look at SKEW while you're there. :)
bdunham7:
Keep it simple and use two probes, one for CH1 and one for CH3 and display only CH1 and CH3.  Offset them a bit if it helps.  Connect both probes to the same channel of your SDG, eliminating all doubt as to phase.  Pick one to trigger from and then observe which one leads.  Leave the probes connected at the SDG and swap them at the scope.  If the same channel still leads, your scopes two acquisition units are not aligned, which is appears to be the issue.  If the channels change relative positions, the issue is in the delay characteristics of your probes. 

Now press CH1 and then Next Page.  The second button should read DESKEW and a time indication.  If this reads 0 and the channels are that far off, IMO your scope is defective, unless there is some way to calibrate the base deskew number.  You can press the DESKEW button and turn the small universal knob to adjust the position of the traces, but you should not have to do that with identical probes, certainly not for a correction of several nanoseconds.  Do check the DESKEW setting on all 4 channels to make sure someone hasn't set them this way.

FWIW, a quick check on my SDG2042X-E and SDS1104X-E show that the scope has a 1-3 skew of about 90ps and the sig gen about 270ps when the channels are copied but not coupled.

EDIT:  After a 30-minute warmup and a self-cal, my scope's 1-3 skew seems to have gone away altogether, best match is at zero now.  The sig-gen still is 270ps, which is about 12 degrees phase difference at 100MHz.  I'm not sure why that would be.

SimonM:

--- Quote from: tautech on June 29, 2021, 10:31:47 pm ---
--- Quote from: SimonM on June 29, 2021, 10:14:40 pm ---Would I be wrong to think that the scope needs some extra adjustment to bring everything back "in phase", or am I expecting too much?
--- End quote ---
Yes
Channel Phase locking is done in the SDG.....investigate the settings.  ;)

Another tip if/when you need to trigger on a channel but don't need to see that waveform is to hide that channel and that functionality is in every channel's menu on P2.

--- End quote ---
Thanks. I can confirm:

I have channel 2 locked to channel 1. So I cannot change channel 2.

If I probe to channel 1 and channel 2 of SDG using scope channel 1 and channel 2, then they are both "in phase".

I can, of course, turn channels on and off, but I left them all on to show that the settings are the same and only switching inputs, to show a phase change.

I now have channel 2 of SDG off and both probes 1 & 3 on SDG channel 1 after a 50-ohm terminator. I get a phase shift.

So with two probes on the same source, I wasn't expecting any phase shift.

I can also replace probes 1 & 3 with probes 2 & 4 (all set to 10x) to show the same phase shift. Or swapping 2 & 4 to get the same phase shift.

So, I'm sure it's not the probes and there is only one channel of the SDG being used.

FWIW, SDG with one or two channels works very well - that's why I bought it!

Simon
SimonM:

--- Quote from: StillTrying on June 29, 2021, 10:51:14 pm ---Are you saying that the scope's CH 1+2 are not in phase with CH 3+4. :-//

If that have a look at SKEW while you're there. :)

--- End quote ---
Thanks.

Yes, I'm saying 1 & 2 of scope are in phase and 1 & 3 or 1 & 4 are not in phase.

I didn't know about the skew... until now.

I can get 1 & 3 to be in phase by adjusting the skew of channel 3.

The two traces appear to be completely aligned when it is set to -29.35ns.

I can also measure the skew difference, which averages about 400ps.

The skew can be set to a number by depressing the adjust button. There is also a default, or clear, that puts the skewback to 0.

On the function generator, I can increase the frequency e.g. from 10MHz, by 1MHz increments, to 15MHz.

When I change the frequency, the scope trigger still functions and the sine waves get closer together, as you would expect.

They also stay in phase (with skew set to -29.35ns) across different frequencies (10MHz, or lower and up to 15MHz).

If I switch to using channels 1 & 4, I can set the skew of 4 to stay in phase and it's also -29.35ns to get them in phase.

Simon
SimonM:

--- Quote from: bdunham7 on June 29, 2021, 11:15:03 pm ---Keep it simple and use two probes, one for CH1 and one for CH3 and display only CH1 and CH3.  Offset them a bit if it helps.  Connect both probes to the same channel of your SDG, eliminating all doubt as to phase.  Pick one to trigger from and then observe which one leads.  Leave the probes connected at the SDG and swap them at the scope.  If the same channel still leads, your scopes two acquisition units are not aligned, which is appears to be the issue.  If the channels change relative positions, the issue is in the delay characteristics of your probes. 

Now press CH1 and then Next Page.  The second button should read DESKEW and a time indication.  If this reads 0 and the channels are that far off, IMO your scope is defective, unless there is some way to calibrate the base deskew number.  You can press the DESKEW button and turn the small universal knob to adjust the position of the traces, but you should not have to do that with identical probes, certainly not for a correction of several nanoseconds.  Do check the DESKEW setting on all 4 channels to make sure someone hasn't set them this way.

FWIW, a quick check on my SDG2042X-E and SDS1104X-E show that the scope has a 1-3 skew of about 90ps and the sig gen about 270ps when the channels are copied but not coupled.

EDIT:  After a 30-minute warmup and a self-cal, my scope's 1-3 skew seems to have gone away altogether, the best match is at zero now.  The sig-gen still is 270ps, which is about 12 degrees phase difference at 100MHz.  I'm not sure why that would be.

--- End quote ---

Thanks. I do have it all simplified down to only using one source which is SDG channel 1. channel 2 is off.

I was using both channels of the SDG to avoid having to put both probes into one BNC and hold them there.

With only one SDG output enabled, I can manually de-skew (is that a verb?) to align them.

I can swap the probes to show that it is independent of a phase shift in the probes.

I can switch off unused channels.

-29.35ns seems a lot to adjust something. A few 10's ps would be acceptable.

So, is there a way to "calibrate" the skew, so that the channels are in phase with a skew on all channels set to 0?

That's also how I would expect it to come from the factory.

There is also a "Quick Cal" in Utility, that is set to "On". Setting it to "Off" doesn't seem to make a difference.

Simon
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