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 Calculating frequencies for Spectrum Analyzer

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jmc2385 Posted - May 09 2014 : 09:14:16 AM
Hey Aaron,

I came across your spectrum analyzer (http://www.aaroncake.net/circuits/3chspec.asp) via this site (http://makezine.com/2010/10/18/circuit-skills-led-color-organ-spon/), and I'm currently working on making my own version of it.

Could I ask you if you have a formula for calculating the frequency and bandwidth of each of the bandpass filters you made? I wanted to add more bands and adjust the frequencies and bandwidth of each band, but I need a way to determine which parts I should use to get the results I want.

I found a calculator here http://www.muzique.com/lab/gyrator.htm , but it is a different circuit design so I can't use it to calculate the frequency or bandwidth (Q) with your circuit.

Any help is much appreciated.

Thanks!

- Joe
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audioguru Posted - May 12 2014 : 11:05:56 AM
The project uses an odd LM3900 quad amplifier IC and a Japanese LED driver that is obsolete.

The filters are not gyrator but are called Multiple Feedback Bandpass Filter and the calculation of parts and frequency are shown in Google.

The bandpass is wrong for a spectrum analyser since the peak is narrow-band and the simple skirts are gradual and wide-band.
A spectrum analyser needs filters with a response that has a flat amplitude for its bandpass then sharp skirts.

Here is a simulation of the response of a Multiple Feedback Bandpass Filter:

Download Attachment: multiple feedback bandpass filter response.PNG
26.44 KB

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