Passive Fifth Order Bessel Filter Circuit Realization

Farah 18 years ago

Hi All,

I am trying to design a fifth order Bessel Low Pass Filter with a cut- off at 48.8e6 Hz.

I calculated the transfer function and checked it in Matlab to make sure it met my specs. I got: Transfer function:

945

--------------------------------------------------------------------

3.05e-041 s^5 + 5.802e-032 s^4 + 5.15e-023 s^3 + 2.612e-014 s^2

7.452e-006 s + 945

How do I realize a low pass filter circuit from the above transfer function? I know how to do it for a second order filter cos the standard form of the transfer function would be: (1/LC) / (s^2 + s (1/CR) + (1/LC)) which could easily be designed for a R and L value after selecting a C value.

But, how to do it for a fifth order transfer function. Is there any such equation for a low pass filter of Nth order from which I could find the values of the L,R,C components?

Thanks, Farah.

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The Phantom 18 years ago

Consult a good book on network synthesis.

It would be easier to get a trial version of a filter design program (such as

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and just design a filter to your specs. The program will give you the realization.

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miso 18 years ago

In the dark ages, you would use a filter table. The table has a diagram with the topology. The columns are L and C. Actually L1, C2, L3 or C1, L2, C3 etc.[For a lowpass, you could start the filter with a series L or a parallel C, hence there are two solutions.

All the filters are designed for 1rad/sec. You scale the frequency and impedance as needed. Sometimes the Bessel is scaled to 1 second rather than a 1 rad/sec corner frequency.

I did a bit of googling and this topic is too boring for the net. However, any good engineering library will have a book on ladder filter design.

I will say anytime I tried to build a passive filter higher than a few Mhz, I didn't get the expected response. The inductors are very non- ideal. I suspect people that build these high frequency ladder filters have a great deal of practical experience in choosing components.

BTW, you are going to need a LCR meter to measure the inductors you wind.

This is the best book on the subject:

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