COHERENT CW NEWSLETTER (Copyrighted June,1975) CCWN 75:7

FUNDAMENTALS OF COHERENT CW
                                              RAYMOND C. PETIT,
W7GHM
                                              
     Introduction. It has been recognized for a long time that
the bandwidth used for receiving a signal should be no less and
no more than the spectrum width of the signal itself. Wider
receiving bandwidth permits unnecessary noise to reach the output
of the receiver, and narrower bandwidth cuts off part of the
desired signal. In single sideband practice this rule has been
universally accepted. Before sideband came into general use, a
10-kHz segment of a band could accommodate only two a.m. stations
without excessive interference.
Now it is possible to accommodate five stations in the same
segment.

     The 3-db bandwidth of a 12-w.p.m. c.w. signal is only about
9Hz.  At sixty w.p.m. this bandwidth widens to only 45 Hz. Yet
even today no receivers or filters are presently in general use
which have the narrow bandwidth required to optimize the
signal-to-noise ratio of such a signal. Two reasons of rapidly
declining usefulness are usually offered. The first is that
narrow c.w. filters rings excessively, making the c.w. signal
hard to read. The second is that the required frequency stability
is too difficult or expensive to obtain. 

 COHERENT CW is a technique for transmitting and receiving
messages in Morse or other binary on-off code in which time
synchronization and extreme frequency precision are used at both
ends. In use, it typically achieves about a 20-dB improvement in
received S/N, and without ringing. The heart of a CCW system is
the special filter used in the receiver. Understanding its
operation is the key to understanding CCW.

     The Coherent CW filter consists of two input mixers, two
integrate two "sample and hold" circuits, and two balanced
modulators as shown in Figure 1. I will describe the filter by
analyzing each of these sections in turn, from left to right. I
will be making a few approximations in the analysis, and by doing
this, we can avoid complicated math and keep it intuitive. If you
would like to go through the math, you will find that these
approximations will make only a slight difference in the final
outcome.
                         
     We start with the input balanced mixers. Imagine a
transformer with a center-tapped secondary and an s.p.d.t. switch
connected as shown in Figure 2. Suppose we connect a sine-wave
input to the transformer to the input, and we cause the switch to
change position at exactly the same frequency as the input with a
switching signal.  Depending on the phase relationship between
the input and the switching signal, the output will have a d.c.
or average value which can be positive, zero, or negative. Let us
look at the three cases.