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Can anyone help me identify why my data for a bandpass filter does not match the theory curve?

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Lee
Lee on 30 Oct 2016
Answered: cool on 16 Dec 2016
I had my data checked by my professor before we left the lab. It was the expected plot from what he said. It's the phase diagram for my Bandpass filter using 2 capacitors. For reference, this is how it should look. http://www.electronics-tutorials.ws/filter/filter_7.html .
Im not sure why our and/or how our dat is shifted and simple transforms of the data don't seem to work.

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Star Strider
Star Strider on 30 Oct 2016
My only observation is to be certain that your calculated transfer function equation is correct, and to allow for tolerances in your components. Measure their values with an accurate digital multimeter that can also measure capacitance, and use those values rather than the marked values. Unless you’re using precision (1%) components, you can expect some variability.
Lee
Lee on 30 Oct 2016
The transfer function is definitely the expected one. I'm 100% certain of that. We get our data from an ELVIS board using National Instrument's Software and then import the data to Matlab. I think the data is correct. I think that it's the way labview chooses to express the data. If my assumption is correct, there should be something with the way the phase is displayed that I can transform it to fit my data. I think i'm just missing whatever that is.
Star Strider
Star Strider on 31 Oct 2016
Without your code and your data it’s difficult to determine what the problem may be (assuming I could do it then). It looks to me that there’s an unaccounted-for series capacitance that is producing a significant phase lead in your data.
That’s the best I can do.

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Answers (1)

cool
cool on 16 Dec 2016
how did you get the data of different frequencies? by sweep frequency (eg. 1Hz to 100Hz, 10Hz/s variation)? the filter has some delay, if you variation is too fast, maybe the data you get is not correct. you can do some fix frequecy test, sample data when the output is stable. all the components you use may not precise enough.

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