SigmaDSP crossover bessel filter -3dB point error?

Eric Question

Dec 1, 2009 00:00

Hi,

how can i design the crossover in one channel? that's to say series the HPF&LPF in one channel, in order to realize the bandpass.

i have a try using the 3-way crossover(only using the Mid-way for one channel), but i found that in the Frequency-Reponse curve, there

have an error-----the bessel LPF/HPF crossover point not at -3dB point.

Thanks&Regards,

Eric Wang

Audio Signal Processors

adau1446

etc

butterworth

.

SigmaDSP Audio Processors

crossover

SigmaDSP

type(bessel

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ADIApproved Reply

Dec 1, 2009 00:00

A simple way of making a bandpass filter is to just use the general BP filter as shown here:

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/2761a43eb18d2d77ef3c6e6efbcbcf11.bin

If you want to cascade butterworth/besself HPF and LPF, add a general 2nd order filter to the project.

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/222ac9f2ebdc32b350d893a87a40b53d.bin

Right-click on the cell and select the "Grow Algorithm" option to add a band.

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/cce02dfb0c6f1fb60646acaa9e18eba7.bin

Click the blue filter icon to change the filter's parameters.

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/f72bf1835ce1b73603a7f4385aec8496.bin

Here you can select butterworth or bessel, LP or HP.

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/348e6b5b2ded899e284c0d389aba0d79.bin

Use the simulation stimulus and simulation probe cells to check the frequency response.

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/a63bea9b43f4c6742a08ec6f518717a2.bin

Here's an example of a butterworth HPF at 500 Hz and LPF at 5 kHz.

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/eeccf228c5111b26986c20c8df7ebc2c.bin

Butterworth filters always cross the -3dB point at the cutoff frequency, regardless of the order (image source: National Instruments):

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/23a23d8a0e47dec7e36775a8cbbe23f5.bin

However, with a Bessel filter, this is not always true (image source: National Instruments):

https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/396/d85f861f05d4cebc4e1ce2c3871bea5f.bin

The equations we use to calculate our filters are given in the SigmaStudio help file.

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Eric Reply

Dec 2, 2009 00:00

Hi BrettG,

Thanks for your replay and meticulous guide for the filter design example. there still have some confusion for me:

Yes, the HPF&LPF may cascade, but if we want to realize high order HPF/LPF, cascade those second order filter, how do we set the coeff(adjust related parameters)

For example, one special status, the HPF type is 3rd Bessel filter(Fc=120Hz), but the LPF type is 3rd Butterworth filter(Fc=2kHz).

then those 3cascade 2order HPF Bessel filter Fc=? , 3cascade 2order LPF Butterworht Fc=?

If cascade with the same Fc simply, then the cross point do not on -3dB.

Thanks&regards,

Eric wang

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Eric Reply

Dec 2, 2009 00:00

Hi BrettG,

I have found some related topic about bessel filter, there have two kinds of style.

One is the basic, cross at -6dB point, and another for actual application, cross at -3db, simly mutiply one factor for frequency.

crossover type Low pass high pass

2nd 1.272 0.786

3rd 1.413 0.708

4th 1.533 0.652

For example, for a second-order low-pass and highpass set to 1000 Hz, set the low-pass to 1272 Hz and the high-pass to 786 Hz.

so those bessel filter response is right in SigmaDSP.

regards,

Eric Wang

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EZAdministrator Reply

Apr 29, 2019 00:00

This question has been closed by the EZ team and is assumed answered.

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