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Bode Plotter

From the instructions it sounds like the "Bode plotter" is acting like a dynamic systems analyzer or control systems analyzer -- it appears to be injecting a reference signal into the system, then measuring the amplitude and phase of one or two points within the system to determine the transfer function between the two points.

This is basically what I describe here:

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except that in the paper I'm describing how you embed this functionality into a microcontroller that is also implementing a control system, where you want this in a stand-alone instrument.

I don't know how far you're going to be able to get away from the math

-- you have to get the signal processing right or the information you want will be buried in noise, and getting the signal processing right demands doing the math.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim Wescott
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Hi,

again thank you for the previous help. I will start building the simple PSU next week.

This question is little on the edge whether it belongs here. But Scope are used in electronics, so there I go.

Now I am back to my main project, which is to make Linux-software for the Velleman PCSGU250 USB-Scope. This has been uphill battle because I not that fluent with C++, the user interface lib Qt (Nokia.com) is new to me and I don't know the scopes very well. But as this is totally my hobby there is no time limit, so I learn as I go. Now I have done the channels and trigger and part of the Function generator.

The part of the scope I don't have the faintest idea yet, is the Bode Plotter. I would really need some hand holding with it. Maybe there is some free code somewhere to do it, which I can borrow. I tried to google but it is too mathematical to my taste (I am middle of re-learning math). The bode plotter seems quite important part of scope so I would like to implement it.

Any help, comments and web-links appreciated. If I have to make it all by myself it is going to take 2-3 months to learn, I think....

Btw. the software will be free with either GPL license. But I might consider paying something for help....if that's the only way.

Best Regards Kari

Below what the Windows-version gives as instructions.

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BODE BLOTTER This text is lifted from the Windows-version - it is not implemented yet

To get the amplitude plot

- Connect the output of the Function Generator to the input of the device under test.

- Connect the output of the device under test to CH1 of the Oscilloscope.

To get the phase plot:

- Connect the output of the Function Generator also to CH2 of the oscilloscope.

- From the View menu select Phase Plot.

To use the option "Normalize Output by input".

- Connect the output of the Function Generator also to CH2 of the oscilloscope.

- From the Options Menu select Normalize Output by input.

- See Help for more information.

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--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com   (I am just a satisfied customer)
Reply to
Kari Laine

Simply output your data as a CSV and use the spreadsheet of your choice to do your plotting.

Reply to
krw

Kari:

You need to measure frequency response for a Bode plot. While you could do this via a slow sweep of the signal generator while you watch the magnitude of the response, it's much faster (and usually more accurate) to get the spectrum with an FFT.

I have some Help topics at that discuss various strategies for frequency response, including the slow sweep. The other strategies mentioned are stepped frequency sweep (that hits only the spectral lines of the FFT), impulse response, step response (with correction back to impulse response), and white, pink, or arbitrary noise spectra.

In all of these you usually need to average multiple responses together. (In principle, you only need a single impulse or step, but averaging improves the accuracy in the presence of noise.)

But the first step is to get a working FFT. I don't have any handy reference code to point you to, but I'm sure there are plenty of sources out there. FFT source code is typically far from optimized, but the first step is to get the system working, then go back and fine-tune.

With FFTs, ideally you will want a routine that accepts real input... but almost all of the sample code you will find is for real+imaginary input. That will work (you just put your input samples into the real slots and leave the imaginary slots empty), but it's wasteful. Again, something you can deal with after you have it working.

A Bode plot is always shown with log-log axes. The Y axis is no problem: Just take the log of the magnitude value you get from the FFT. The X axis is more of a problem because the FFT returns data only at linearly spaced frequencies. (The spacing is equal to the sample rate divided by the number of raw data points used in the FFT.) So you will need a routine to plot those linear frequencies logarithmically, giving unevenly spaced display points (stretched apart at the low end and squished together at the high end).

Best regards,

Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!

Reply to
Bob Masta

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