(Revised September 1980)

                                                  CCWN 77:104
The ccw filter



     The coherent integrating filter makes possible the efficient reception of a ccw signal. Fig-
ure 2 shows a block diagram of a ccw filter. The major blocks of the filter are: Input Mixers,
Integrators, Sample-And-Hold Circuits, Output Mixers, and the Timing and Control Circuitry.

     There are two filter chains consisting of an Input Mixer, Integrator, Sample-And-Hold,
Output Mixers. The reason for the two chains will be examined later; for now, we will follow
the signal through one chain.
     Input iVfixer. The first part of each filter chain is a switching mixer where the desired sig-
nal (along with adjacent QRN and QRM) is mixed in a digital mixer with a reference signal of
the same frequency as the desired signal. The reference signal is obtained from a stable source
such as the timing and control circuitry. This reference determines the center point of the ccw
filter. A signal at the desired frequency comes out of the mixer as a d-c voltage; the stronger
the signal, the larger the voltage. A signal off-frequency, however, comes out of the mixer as a
low-frequency ac voltage. Thus, we are mixing the desired signal right down to zero-beat;
undesired signals will be distinguished from the desired signal by the fact that they are not
exactly zero beat.



     Integrators The second part of each filter chain is an op amp integrator circuit. We use the
integrator to distinguish the desired signal (the zero-beat d-c voltage) from undesired signals

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(low-frequency a-c) coming from the mixer.

     An easy way to think of the integrator is as a moderately large capacitor (e.g., 1 uF). A
synchronizing "dump" signal from the Timing and Control circuitry shorts out this capacitor at
the start of each time frame. Any desired signal (d-c voltage) during the time frame causes the
capacitor to charge. The resulting voltage at the end of the time frame is a function of the
strength of the desired signal received during that frame.
     QRM and QRN, being off frequency, appear as ac signals to the integrator capacitor.
These charge the capacitor for part of the time frame, but discharge it for other parts of the
time frame. Consequently, signals off frequency do not have as great an effect on the integra-
tor output as do signals exactly at the desired frequency. That is how the ccw Filter achieves its
selectivity.
     For instance, consider an interfering carrier which is 10 Hz above or below the desired
signal. Following.the switching mixer, this QRM appears as a 10-Hz ac voltage. If the filter is
set to the ccw standard frame length of 0.1 second, then the 10-Hz interfering signal goes
through one complete cycle during the 0.1 second integrating period. (Half a cycle of positive
voltage, half a cycle of negative voltage.) Thus, at the end of the time frame, the QRM pro-
duced voltage at the integrator output is zero. Thus the ccw filter has a null just 10 Hz above
and 10 Hz below its center frequency. There are also similar nulls at other multiples of 10 Hz.

     Sample-And-Holds, and Resetting the Integrator. At the end of each 0.1 second time frame,
a "sample" signal from the Timing and Control circuit transfers the voltage at the integrator out-
put to the Sample-and-Hold circuit. That circuit "remembers" that voltage for the following 0.1
second time interval.
     Once the Sample-And-Hold has acquired the integrator output voltage, a "dump" signal
from the Timing and Control circuitry shorts out the integrator capacitor. It does this by a
CMOS analog switch connected across the capacitor. This allows the integrator to start over
a.-lain with zero voltage at the start of the next time frame.
     Resetting the integrator at the end of each time frame lets the ccw filter avoid the "ring-
ing" (or "intersymbol interference") common in other narrow filters. Note that this is possible
only because the ccw filter "knows" when each time frame begins and ends. It is here that the
time discipline of the transmitted signal is used to advantage in the detection process.

     Output mivers. The last block in each filter chain is the Output Mixer. This is another
switching mixer, much like the input mixer; it functions as an amplitude modulator, using the
Sample-And-Hold output voltage to control the amplitude of a sidetone. The purpose of this
mixer is to construct a side-tone for the human operator to hear.