(Revised September 1980)
CCWN 77:128



to within I Hz. This receiver is stable to within about 0.1 Hz over a QSO. The IF is similar to
that designed by W7ZOL The performance of this receiver is quite superior to others I have
used for ccw. Best results are obtained when the AGC is controlled by the output from the ccw
filter.



     At JRIZZR a crystal controlled FT-10IBS has been used with a 2980 to 3080 Hz VFO R-
C oscillator built by JAlBLV as the reference for the center frequency of the ccw filter. Oscil-

lators in the FT-101BS were stabilized by temperature compensation and the use of high stabil-
ity crystals.



A Practical Coherent Digital Filter

     Figure 7 shows a practical coherent digital filter. The first CD406OA6 is used as a switch-
ing mixer, the second controls the sample and dump functions. The output from the two chan-
nels may be used to run a digital mixer for generation of audio output. A/D converters can be
used on each channel for computer input and the desired signal amplitude is the product of the
two channels. Figure 9 shows a digital mixer for generating audio output from the ccw filter.
Figure 8 shows a frame reference for the coherent cw filter.



,V ficroprocesser Filter

     Figure 10 shows a logical diagram of the computerized system which has been used at
W6NEY. The switching mixers are essentially the same as those used in the filter described
above. The computer program controls the A/D conversion and dump functions. Computer
control of the mixer has been used but the VFO capability is a convenience during operation.
The internal computer clock generated interrupt is used to define frames. The I MHz internal
clock is stabilized and set to I MHz. Phase is adjusted by the computer program under the con-
trol of a single pole double throw switch which can advance or delay phase. Between control
of the sample and dump functions the computer also converts the received Morse signals to
ASCII code and transfer the ASCII code to a CRT character display terminal or to a paper-copy
terminal.



Practical Results

     When received by an ordinary cw receiver, a ccw signal sounds like an ordinary cw signal
with very regular machine-like keying. If the ccw transmitted is optimized for effective use of
power for ccw, the keying will sound very soft.



     The reception of weak ccw signals is quite different from that of weak ordinary cw signals.
For ordinary cw signals, as the signal gets weaker QRN or QRM remain as "no signal" output
and we eventually end up with noise the bandwidth of the filter. For ccw, noise is a series of
dots in the frames and randomly varying in intensity. The output of the ccw filter is limited to
one frequency by design. The weak signal therefore is characterized by missing and extra dots
randomly interspaced in the desired signal. Noise received by the ccw filter is heard as the
same frequency as the signal but as a random series of marks and spaces of randomly varying
intensity.



     If frame phase is 180 degrees off, frames of the receiver filter overlap frames of the
received signal. The result is that dots are blurred equally into two frames while dashes are
blurred into four frames. The signal is unreadable and might go unnoticed if it is a weak signal.

     As the phase nears correct adjustment, the blurring is eliminated. When tuning a series
of dots, a standard part of a ccw CQ, one can tune framing and frequency for maximum con-
trast between dots and spaces. On a strong signal, even 10% error can be noticed. A slight lead