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Products => Test Equipment => Topic started by: JPortici on March 07, 2017, 02:26:18 pm

Title: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: JPortici on March 07, 2017, 02:26:18 pm
- Of course one could use a separate programmable wavegen. Let's keep it contained for now shall we?

So, it seems like the bode plot function on the keysight dsox-1000g is (for now) a gimmick because it steps the frequency over a limited number of points. (10 apparently)

Question: If someone had a scope with generator included, this one could make its own (computer) application by sending commands and reading the measurements?

-> Set AWG wave: Sine
-> Set AWG amplitude
-> Set AWG frequency at F start
Repeat n times (for an average)
-> read back measurement: amplitude A
-> read back measurement: amplitude B
-> read back measurement: phase B - phase A
draw point.
Increase AWG frequency by delta (delta being the number of points)
Adjust vertical/orizontal scale.
repeat m times, until F stop is reached.

seems pretty straightforward... not as handy as having it already inside the scope though, but probably more powerful
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: pigrew on March 07, 2017, 02:38:11 pm
- Of course one could use a separate programmable wavegen. Let's keep it contained for now shall we?

So, it seems like the bode plot function on the keysight dsox-1000g is (for now) a gimmick because it steps the frequency over a limited number of points. (10 apparently)

Question: If someone had a scope with generator included, this one could make its own (computer) application by sending commands and reading the measurements?

seems pretty straightforward... not as handy as having it already inside the scope though, but probably more powerful

Yes, very possible. I actually have put together some Matlab code to do this for audio frequencies for the 3000 series of oscilloscopes.

I use the high-res mode of the scope for acquisition (while keeping the sample rate high enough to prevent bandwidth limiting of the signal harmonics). This lets me also make distortion and noise measurements. FFTs are used with the Welch method to calculate the PSD.
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: rstofer on March 07, 2017, 03:07:15 pm
That's pretty much the way we did it in lab class.  Just set a bunch of frequencies and measure the output amplitude and phase.  The data could be stuffed into something like wxMaxima (or Excel, or ...) to get a decent looking Bode Plot.

The quality of the plot will be determined by the number of sample points.  I suppose the data could be fitted to a curve (least squares?) to improve the appearance.
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: voltsandjolts on March 07, 2017, 03:35:07 pm
... a gimmick because it steps the frequency over a limited number of points. (10 apparently)

No, its more like 50 points, enough to be useful I reckon.
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: MrW0lf on March 07, 2017, 04:08:26 pm
https://bitbucket.org/hexamer/fra4picoscope/wiki/Home

Standalone open-source bode plotter, maybe source code can be used as basis for some other scope interface also:
(https://bitbucket.org/repo/jb9bnX/images/2376038125-HPF%20Screenshot%203.png)

I will include it in my Pico 2408B review/test, however my AWG is only 1MHz with standard signal. So this app probably more useful on 5000/6000 models with 20MHz AWG (if works).

On the other hand my relatively expensive handheld Agilent U1733C LCR meter is only up to 100kHz, works in steps and even with these limitations has proved very useful in small electric motor R&D.
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: rstofer on March 07, 2017, 04:17:33 pm
The Analog Discovery will run the Bode Plot out to 10 MHz.
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: djnz on March 07, 2017, 04:39:46 pm
Ever since I read of the Bode plot feature on the new 1000X scopes, this has been on my to-do list. It seems quite doable, and I am pretty sure it could be done a little more intelligently. For example, by seeing the rate of change of the magnitude (derivative / double derivative) and taking data points more densely when that rate is high or changing fast so that you get better resolution around interesting frequencies.
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: JPortici on March 07, 2017, 06:03:51 pm
... a gimmick because it steps the frequency over a limited number of points. (10 apparently)

No, its more like 50 points, enough to be useful I reckon.
the 1000x seems to be doing 10 points at the moment

see: https://www.eevblog.com/forum/testgear/new-keysight-scope-1st-march-2017/msg1153743/#msg1153743 (https://www.eevblog.com/forum/testgear/new-keysight-scope-1st-march-2017/msg1153743/#msg1153743)
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: voltsandjolts on March 07, 2017, 06:21:19 pm
... a gimmick because it steps the frequency over a limited number of points. (10 apparently)

No, its more like 50 points, enough to be useful I reckon.
the 1000x seems to be doing 10 points at the moment

see: https://www.eevblog.com/forum/testgear/new-keysight-scope-1st-march-2017/msg1153743/#msg1153743 (https://www.eevblog.com/forum/testgear/new-keysight-scope-1st-march-2017/msg1153743/#msg1153743)

See for yourself, at 6mins in:
https://www.youtube.com/watch?v=N63dJHbCdbk (https://www.youtube.com/watch?v=N63dJHbCdbk)

Edit: ...and the answer is: it does 10 points per decade

http://literature.cdn.keysight.com/litweb/pdf/5992-1965EN.pdf?id=2832547 (http://literature.cdn.keysight.com/litweb/pdf/5992-1965EN.pdf?id=2832547)

In the 3000X series the points per decade is adjustable, allows at least 30 points per decade.
http://literature.cdn.keysight.com/litweb/pdf/5992-0594EN.pdf (http://literature.cdn.keysight.com/litweb/pdf/5992-0594EN.pdf)
Title: Re: Using a generic programmable scope with wavegen to make automated bode plots?
Post by: JPortici on March 07, 2017, 06:34:48 pm
you're right!
i have a feeling that the delta f in the 1000 is fixed... or reduced according to range. i've seen another video where the guy was plotting an audio filter responce, way less points