Oscilloscope constant time base with varying amplitude?

[W4PJB]

Is there a way to set an oscilloscope to maintain the amplitude on the zero axis while measuring in the time base? Case in point; measuring noise on a DC motor over varying voltage / amperage. I’d like to be able to increase the voltage while monitoring the noise feedback, which is the critical measurement.

Scope is an Aligent 54621A, if that matters. But this should really apply to all scopes, really.

[dmulligan]

Change the input from DC to AC.

[2N3055]

Only scope I know that has built in autoranging (like a multimeter) is Picoscope.
I also know that Siglent touch scopes that have DVM option, you can enable DVM on a channel and enable autoranging in DVM and it will, as a side effect, auto-range scope display for that channel.

Your scope cannot automatically auto-range..

[csuhi17]

There is a description of the channel coupling in the PDF below.
You need the AC coupling to cut off the DC component and only see the AC, i.e. the noise.
Did you think of this?



My mistake, I only wanted to upload pages 122-123

Maybe Dave's video can help you.
It's true that the video is about testing the noise of the DC power supply, but I think it can help you too.

[pdenisowski]

Is there a way to set an oscilloscope to maintain the amplitude on the zero axis while measuring in the time base? Case in point; measuring noise on a DC motor over varying voltage / amperage. I’d like to be able to increase the voltage while monitoring the noise feedback, which is the critical measurement.

I'm not sure I completely understand the measurement you'd like to make, but I'm wondering if XY mode might be useful here

[bson]

You can also measure the standard deviation.  This will be the RMS value of the excursion around the mean.

[David Hess]

That sounds like XY mode to me.  Apply the noise signal to the vertical amplifier and apply the voltage or current to the horizontal amplifier.  Now sweep the voltage or current to get a display of noise versus voltage or current.

[ebastler]

So far, we have three very different interpretations of the OP's question. ("You want AC coupling", "You want auto-ranging, but that is not commonly available", "You want XY mode".) Maybe it's time for the OP to clarify?

[Fungus]

Only scope I know that has built in autoranging (like a multimeter) is Picoscope.

Micsigs can do it, in horizontal and vertical (axes are selectable).

So can my $30 Zeewei.

[csuhi17]

Based on the picture that the OP uploaded, I came to the conclusion that he needs the "AC coupling" on CH1.
I think he wants to look at the noise of the DC motor, fed with different voltages (at speed?).
Amperage? load the DC motor?
But really, it would be nice if the OP would clear it up.

I can clearly see that CH1 is set to 20mV/Div and the trigger to 3.09V? It is possible?

[W4PJB]

Thanks for all of your replies! Sorry I'm so late getting back to this; I thought I had set e-mail notifications but I don't recall ever seeing one.

I'm traveling so it will be some time before I can get back to the bench, but this is some great brain-food in the meantime!

[W4PJB]

So far, we have three very different interpretations of the OP's question. ("You want AC coupling", "You want auto-ranging, but that is not commonly available", "You want XY mode".) Maybe it's time for the OP to clarify?

I'd like to be able to raise the voltage on my adjustable power supply while keeping the signal centered vertically on the screen, so I can watch the noise signal change over various RPM. At low enough voltage division to see the noise signal, a small rise in voltage causes the trace to fly off the screen and I have to bring it back down using the vertical alignment.

I'd like the trace to stay on the 0 line of the Y axis regardless of amplitude. Thanks, I hope this makes more sense!

[ebastler]

Thanks for the clarification. Setting the scope's input to "AC coupling" should do what you need then. It will remove the DC component and only pass the AC part, i.e. the noise.

[W4PJB]

Thanks for the clarification. Setting the scope's input to "AC coupling" should do what you need then. It will remove the DC component and only pass the AC part, i.e. the noise.

Thanks! Still very much learning this!