Hi,

I'm trying to build a programmable bandpass filter whose center
frequency is controlled by a PIC16C84. I need to filter audio signals
between about 200Hz up to 1500Hz under software control. I was hoping
to be able to use a switched capacitor filter, e.g. National
Semiconductor MF-8 or the equivalent Linear Tech Part. For these
devices you control the center frequency via an external clock signal
running at either 50x or 100x of the desired center frequency (which I
had hoped the PIC could provide). The PROBLEM is that 1500 Hz * 100 =
150000 which is too fast for a 4 or 10 or 20 MHz clocked PIC to do, I
think. I've tried various types of codes from the EASY PIC'N book and
from frequency generation ideas using phase accumulators by Scott
Dattalo within the MPSIM simulator and I can only get a clock to go at
about 22 kHz using any of these methods -- not nearly fast enough.
The phase accumulator method is preferred, since the application is
musical note identification and the frequencies are oddball multiples
of the PIC clock frequency.

THEREFORE -- If I'm going to continue on this tack, I need a clock
multiplier to hang on the output of the PIC to boost up the frequency
a bunch (several clock doublers ??). Any good ideas here?

By the way I did run across a programmable sine wave generator chip
good from DC to 500 kHz from MicroLinear -- I suppose this could be
squared up and used as a clock. the problem is that I can't seem to
find small quantities for sale, they want to sell tubes of 25 for
$10.00 each.

Any help or suggestions are greatly appreciated.

Steve

--
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Stephen Brown, Ph.D.
Applied Research Associates
RR 1 Box 120A, Waterman Road
South Royalton, Vermont 05068

(802) 763-8348 voice
(802) 763-8283 fax

sbrown@ned.ara.com
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++