On Fri, 21 Feb 2003, Michael Rigby-Jones wrote:

> > -----Original Message-----
> > From: Scott Dattalo [SMTP:scott@DATTALO.COM]
> > Sent: Friday, February 21, 2003 1:40 PM
> > To:   PICLIST@MITVMA.MIT.EDU
> > Subject:      Re: [EE]:Analog filter design...
> >
> > To achieve 0.01% flatness and 40dB attenuation at 700kHz will require
> > either a Chebychev II or an elliptic filter. The Chebychev type II filters
> > are monotonic in the passband as opposed to Chebychev type I filters
> > (which what most people simply call Chebychev filters) which are monotonic
> > in the stop band.
> >
> I don't know much about the Type 2 Chebyshev, but isn't the phase response
> of an elliptic filter fairly horrible?

Yes. But you have to be more specific. If you designed a 4'th order
elliptic and 4'th order Butterworth filter with the same cutoff frequency
and same stop frequency, then what you'll find is that the elliptic filter
will have a smaller amplitude error (up to the 3dB define cutoff
frequency) than the Butterworth filter. However, the phase error right at
the cutoff freqeuncy will be higher. The phase error "way below" the
cutoff frequency is small.

Now, the original poster asks for 0.01% flatness and 0.01% phase error up
to 70kHz and an attenuation of 40dB at 700kHz. He didn't ask that the
cutoff frequency be 70kHz. So, an elliptic filter with a cutoff frequency
somewhat higher than 70kHz can be designed to have a small amplitude and
small phase error. Determining what is "somewhat higher" is a job for the
software and the original poster.

Scott

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads