(Revised S(@PL(2rnb(_@t, 1980)
CCWN 77:102



Coherent cw: Conceptual Basis (Part one of a
              two-part series).



           Charles Woodson, W6NEY
                2301 Oak St.
             Berkeley, CA 94708



 Regular keying, accurate frequency control,
 and an integrating, matched digital filter
 can be used to give over 20 dB improvement
       in communication effectiveness
          over ordinary cw methods.



     The more we know about something we seek, the easier it is to find. This is true not only
in general, but also when we seek to detect weak radio signals among noise and unwanted sig-
nals. This article describes an application of this principle to Morse cw communication. to
Morse cw signals. The technique is called coherent cw or "ccw," but can be used with other cw
signals, such as RTTY and ASCII, and with other keying methods, such as fsk. This discus-
sion, however, will focus on on-off keying.

     As with all good things, there is a price. Here there are three aspects of the "cost": we
must set frequency with greater accuracy and stability than has previously been practiced, we
need co-operation in the form of time-synchronized keying from the transmitting station for the
receiver filter to work, and tuning the signal is more complicated and difficult.

     However, the benefits are worth the price: on-the-air trials have shown more than 20 dB
improvement in communication effectiveness over ordinary Morse cw methods. As one of my
DX operating friends put it, "If you will give me another 20 dB, I'll grow another hand." Well,
here is another 20 dB for cw communication.



Filters for cry

     For ssb it has long been established practice to match the bandwidth of the receiver filter
to the bandwidth of the desired signal. In general, receiver filters much wider than the
bandwidth of the desired signal are less effective than filters of bandwidth near the bandwidth
of the desired signal because they allow reception of additional noise and undesired signals. On
the other hand, a clean 12 wpm Morse cw signal occupies only few Hz of the spectrum; gen-
erally about 10 Hz. Yet the custom is to use 500 or 2300 Hz wide filters for cw reception.
Listening through a 500 Hz filter, one hears the 10 Hz-wide desired signal, and 490 Hz of noise
and QRM. By analogy, a ssb operator with a similar approach would listen to some 100 kHz of
the band at one time.

     Analog cw filters are not very useful at bandwidths approaching the bandwidth of a 12
wpm Morse signal. Typically, a high-Q circuit is used to provide a bandwidth of a few hundred
Hz. There are two reasons why analog filters narrower than 500 Hz are not widely used. First,
they tend to ring. High-Q filters continue to give output on the filter center frequency after the
signal ceases, and the human ear is confused by such ringing. Second, the stability and reseta-
bility required, on the order of a few Hz, is considerably beyond the hundred Hz or so common
in commercially-build receivers and transmitters.

     One might use phase-locked-loop filters. Unfortunately, PLLs with time constants long
enough to give bandwidths of only a few Hz would take tens of seconds to achieve lock.