Filter
Bill Perry
I am looking to design a filter that is bandpass in nature. Fo should be
7Hz, with 6.9Hz and 7.1Hz being rejected. I don't know if analog filters
will do the job. I am sure DSP will, but know nothing about it, except a
basic knowledge of how the lowpass algorithm works. How does Highpass
algorithm in DSP work?
Thank you.
Bill
John Popelish
You need to nail down the specifications a bit more. If you are
looking for a filter that passes all frequencies between 6.9 and 7.1
Hz with no attenuation, and blocks all frequencies lower than 6.9 Hz
and above 7.1 Hz with infinite attenuation, you are in for
disappointment. If you mean to say that the attenuation is 3 db at
6.9 and 7.1 Hz and is bigger for frequencies further from 7 Hz, then
you need to think about the transition band between your 3 db limits
and some large deviation, and how fast you need the signal to fall as
you traverse these bands. you also need to specify how much
attenuation (minimum) is required for all frequencies outside of those
transition bands.
Once these specifications are in hand, you might get some useable
advice on the feasibility of analog and digital filter mechanisms.
A single opamp can provide a .2 Hz 3 db band and 12 db roll off per
octave on each side. Two or three opamps could do quite a bit better,
but you have to start to worry about stability of capacitors.
Switched capacitor filters that depend only on the accuracy of a clock
have poorer noise and precision, but stay on frequency better.
--
John Popelish