CCWN75:74
                                            
PETIT LOGIC SYSTEMS
THEORY OF OPERATION, PCF-1 COHERENT C.W. FILTER, SUMMARY by Ray
Petit

The CCW filter is an extremely narrow-band audio bandpass filter
with
a center frequency of 1 kHz which achieves its selectivity by a
time-
disciplined integrating and sampling process instead of resonant
circuits. IC10, ACll, and IC8 form a product detector which
frequency-
translates the audio input down to the region of "zero beat."
IC12
and IC13 integrate the product detector outputs during precisely
set
time intervals determined by the counting logic, ICl-lC7. At the
end of each interval, the instantaneous values of the integrator
outputs a sampled by IC14c and IC14d. IC15 and 16 are the "hold"

amplifiers which deliver outputs representing the resulting
charge
on the .ol uF input capacitors. The outputs of IC15 and 16, plus
their inverses, generated by IC17, drive the switch array IC18,
the two lOK summing resistors R24 and R25, and an R-C lowpass
filter.
This section is a balanced modulator, operating at audio. If the
outputs of the sample/hold amplifiers are zero, the balanced
modulator
is balanced, and no audio output results. If one or both of the
sampler outputs is non-zero, this offset unbalances the circuit
and
produces an audio output proportional to the offset.

IC1 through IC4 is a frequency divider to provide the synthesized
center-frequency signal for the input product detector, the
output
pitch signal for the balanced modulator, ~e selectable bit rate

signals for S2. IC5,6 and 7 generate the narrow pulses that opens
the sampler gates and then resets the integrators to zero at the
conclusion of each bit interval.

For a detailed description of the operation of this filter, see
Petit,
"Fundamentals of Coherent CW," Coherent CW Newsletter 75:5
(Published by Charles Woodson, 4641 Tolman Hall, Univ. of Calif.,
Berkeley, Ca. 94720.)