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Crossover weird gain variations

Question asked by Nando.One on Nov 24, 2012
Latest reply on Nov 27, 2012 by Nando.One

Hi There! How are you?

 

I'm developing a project with ADAU1701 and I'm using BrettG's formulas to calculate the cross-overs. They're described here:

http://ez.analog.com/servlet/JiveServlet/download/1819-2-6143/FilterMathCalculations.pdf

 

Well, everything is working fine, except that I get a strange gain variation as the Xover frequency moves up and down (but the Xover is centered correctly).

 

Here's the test I made: Sampling at 48Khz, I injected the DSP with a 50Hz signal to test Low-Pass Xovers. In bypass, it outputs 100mV rms. Using a Xover, it changes the gain as the frequency is altered. In my test I stated that all frequencies are boosted or attenuated from the Xover point and bellow, so for example, if my Low-Pass Xover is set at 12Khz, doesn't matter if I'm injecting 50Hz, 200Hz, 4Khz or some other frequency lower than 12Khz. They're all boosted or attenuated the same.

 

The gain alterations seem to be pretty linear. For Xovers that use 2nd order biquads, its very linear and has the maximum boost at 1/2 Nyquist frequency, which is 12Khz. Xovers that use 1st or "higher order" biquads behave differently. It is all nicely ploted and measured on the attached PDF.

 

Any help is greatly appreciated!

 

Best wishes,

 

Fernando

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