Author Topic: Setting a scope scale  (Read 1157 times)

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Offline bitman

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Setting a scope scale
« on: August 08, 2018, 03:58:52 am »
Thanks to Eevblog I've gotten a lot of valuable information from using my basic scope. I keep finding new features, but there's one feature/issue I cannot seem to resolve.

Let's say I have a measuring point where I want to see changes in the millivolts in the 24v range. Meaning, the signal averages 24v but there's fluctuations and I want to "zoom in" on those.  I can set the Y scale to let's say 5v but that doesn't really allow me the granularity and I'll see mainly a straight line.  If I set the Y axis to .5V - the line goes WAY off and finding it may or may not be possible with the "adjust zero" option on the Y axis.  How do you get the granularity of changes if it's not close to the 0v line? Is there a trick here, or do you just turn the know 100 times until you get there?

If it's relevant, my scope is a Rigol DS1054Z.
 

Offline nugglix

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Re: Setting a scope scale
« Reply #1 on: August 08, 2018, 04:02:18 am »
See one of Alans great videos:


Hope that helps!
 
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Offline jcw0752

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Re: Setting a scope scale
« Reply #2 on: August 08, 2018, 05:21:14 am »
Set your input to AC and turn the Vertical Scale down to the level you want to see. The trace will center and only display signal fluctuations from the average DC voltage.

John
 

Offline bitman

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Re: Setting a scope scale
« Reply #3 on: August 08, 2018, 01:28:17 pm »
Absolutely! Learned something new - offsets :D
 

Offline drussell

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Re: Setting a scope scale
« Reply #4 on: August 08, 2018, 01:32:20 pm »
Set your input to AC and turn the Vertical Scale down to the level you want to see. The trace will center and only display signal fluctuations from the average DC voltage.

That only works if you don't care about the DC level itself.

The advantages and disadvantages are explained well in the video linked above.
 

Offline bitman

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Re: Setting a scope scale
« Reply #5 on: August 08, 2018, 02:31:14 pm »
See one of Alans great videos:


Hope that helps!

So it looks like my scope doesn't have an offset on the Y axis?  Let me rephrase: I cannot find it on the front panel. It's not listed on the options when I  select a channel.

Strangely, https://media.readthedocs.org/pdf/ds1054z/stable/ds1054z.pdf indicates there's an API call to set the channel offset and there are even long threads here on eevblog about the offset being limited (documented in the linked docs too), but I cannot find it on the front panel/menu? I can change AC/DC coupling, the resistance in the probe etc. but not an offset.. The only place I found offset was under references (cool feature btw - I definitely need to use that more). 

EDIT: Let me rephrase again. I can adjust the POS - ie where the "zero" point shows up, and if I keep turning for 30 seconds I can get it to the limits mentioned in the guide. From the video POS != Offset - but it looks like on the Rigol it's one and the same thing?
« Last Edit: August 08, 2018, 02:47:20 pm by bitman »
 

Online rstofer

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Re: Setting a scope scale
« Reply #6 on: August 08, 2018, 07:02:35 pm »
I guess it shouldn't be a real surprise that a $17,400 scope has a few features that a $350 scope doesn't.

If you have a clean voltage of appropriate level, you can always do the Math A-B operation.  You don't necessarily have to have the 'same' voltage because you can mess around with probe x1 x10 or whatever.  Use a 1.5V battery with an x1 probe and tell the scope it is an x10 probe.  Now you have a clean 15V to subtract.  Not perfect of course.


 

Offline JS

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Re: Setting a scope scale
« Reply #7 on: August 09, 2018, 01:44:57 am »
I guess it shouldn't be a real surprise that a $17,400 scope has a few features that a $350 scope doesn't.

If you have a clean voltage of appropriate level, you can always do the Math A-B operation.  You don't necessarily have to have the 'same' voltage because you can mess around with probe x1 x10 or whatever.  Use a 1.5V battery with an x1 probe and tell the scope it is an x10 probe.  Now you have a clean 15V to subtract.  Not perfect of course.

Even if you do math it does it after conversion, so the clipped signal will remain clipped and the "granularity" or bit depth of the signal won't change. If you apply an offset prior to the conversion you can amplify more the AC before conversion and get much greater detail of the signal. Now, if you are interested in high detail of the AC mounted on a DC you can use two channels, one DC coupled with low gain and other with AC coupled and higher gain and get the info from two different places. If you add both rescaled then it doesn't make much sense as the DC coupled will have too much noise for the other to mean anyting on top of it.

JS
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Offline David Hess

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Re: Setting a scope scale
« Reply #8 on: August 09, 2018, 02:00:59 am »
Most oscilloscopes which lack a separate offset function use the position control to do the same thing over a more limited range.  If the range of the position control is not great enough, then you will have to find another method.  For oscilloscopes which have both functions, the difference is that the position control signal is added late in the vertical amplifier chain and the offset control signal is added early.  This does not include DSOs which move the trace in software; they are toys.

Some old oscilloscopes had a massive offset range for applications which could take advantage of it.  For instance the Tektronix 7A13 has a 1mV/div sensitivity over a calibrated +/-10 volt offset range which is like having a position range of 20,000 divisions.  These were called differential comparators.

The same thing can be done with an external differential probe and variable reference.
 

Offline JS

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Re: Setting a scope scale
« Reply #9 on: August 09, 2018, 02:15:40 am »
Most oscilloscopes which lack a separate offset function use the position control to do the same thing over a more limited range.  If the range of the position control is not great enough, then you will have to find another method.  For oscilloscopes which have both functions, the difference is that the position control signal is added late in the vertical amplifier chain and the offset control signal is added early.  This does not include DSOs which move the trace in software; they are toys.

Some old oscilloscopes had a massive offset range for applications which could take advantage of it.  For instance the Tektronix 7A13 has a 1mV/div sensitivity over a calibrated +/-10 volt offset range which is like having a position range of 20,000 divisions.  These were called differential comparators.

The same thing can be done with an external differential probe and variable reference.

The nice part of the external variable reference and a differential probe is that, in some special cases, you could use some AC for your reference, in sync with the signal composed of a similar AC and some smaller amplitude signal on top. I wander how useful it might be but I guess some cases would appreciate this, like measuring distortion in audio signals, where you generate the original signal and then some other stuff appears on top of it.

JS
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Offline David Hess

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Re: Setting a scope scale
« Reply #10 on: August 09, 2018, 03:28:28 am »
The nice part of the external variable reference and a differential probe is that, in some special cases, you could use some AC for your reference, in sync with the signal composed of a similar AC and some smaller amplitude signal on top. I wander how useful it might be but I guess some cases would appreciate this, like measuring distortion in audio signals, where you generate the original signal and then some other stuff appears on top of it.

AC bridge measurement is a good example of what you are suggesting.

I doubt it would be useful for audio distortion measurement because limited common mode rejection limits precision.
 

Offline JS

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Re: Setting a scope scale
« Reply #11 on: August 09, 2018, 04:01:01 am »


AC bridge measurement is a good example of what you are suggesting.

I doubt it would be useful for audio distortion measurement because limited common mode rejection limits precision.
I don't think THD measurement would work, but characterization of the distortion, like clipping recovery time or windup effcts, or more extreme distortion situations. 0.001% is out of the reach of an oscilloscope for sure, but the shape of a 2%THD might if you can get rid of the fundamental.


JS

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Offline David Hess

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Re: Setting a scope scale
« Reply #12 on: August 09, 2018, 07:11:40 pm »
I don't think THD measurement would work, but characterization of the distortion, like clipping recovery time or windup effcts, or more extreme distortion situations. 0.001% is out of the reach of an oscilloscope for sure, but the shape of a 2%THD might if you can get rid of the fundamental.

Assuming a perfect 8 bit DSO, distortion measurements down to 0.3% could be possible.  In practice, at least 1% should be feasible with the FFT function on a real instrument.

 

Offline JS

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Re: Setting a scope scale
« Reply #13 on: August 09, 2018, 10:58:49 pm »
I don't think THD measurement would work, but characterization of the distortion, like clipping recovery time or windup effcts, or more extreme distortion situations. 0.001% is out of the reach of an oscilloscope for sure, but the shape of a 2%THD might if you can get rid of the fundamental.

Assuming a perfect 8 bit DSO, distortion measurements down to 0.3% could be possible.  In practice, at least 1% should be feasible with the FFT function on a real instrument.

You could be able to say it has about 1%THD but you are a long way to say something useful about that distortion, like what to do in order to minimize it, or make it less audible, like the recovery time or windup I said.

JS
If I don't know how it works, I prefer not to turn it on.
 


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