Subject: Re: simple filters
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Date: Thu, 12 Dec 2002 05:11:16 GMT
NNTP-Posting-Date: Thu, 12 Dec 2002 06:11:16 MET
> I'm designing a simple high pass analogue filter with a 3dB cutoff at
> 30KHz and a 65dB drop by 3KHz. I was just wondering what properties
> will be significant to filter performance? So far I have group phase
> delay, band pass ripple and attentuation curves. Are there any others
> I have missed?
> PS the design is done using Butterworth (4th order), Chebyshev (3rd)
> and Bessel (4th) filters
it all depends on the purpose of this filter.
If you make a passive design and the O/P is loaded with some 50 or 75 ohms
measurement device, you will have to match the output impedance of your
filter to the same value. In this case you will need a series resistor at
the output which will give you some passband attenuation. The R-L-C
combination(s) to gnd will not only equalize the output impedance, but also
alter the transfer function. Without a good optimizer program you will have
a hack of a trouble getting something useful.
Input is not that important, because of the low frequency, here a first
order compensation will do (coil+resistor to gnd).
Very important the pulse response, which can be calculated from the complex
The active approach is also not much easier, here the high frequencies get
f***ed in amplitude and phase. :-((
In any case do not forget to include the driving impedance and output load
impedance in the calculations.
lots of luck
electronic hardware designer