(Revised September 1980)
CCIVN 77:103



Worse, PLLs tend to lock on the strongest signal in the bandpass; they are, therefore, sensitive
to QRM. PLL filters have an important place, but not at the bandwidth range discussed here.

     The filtering method described here achieves a bandwidth of only a few Hz, without ring-
ing, and without a tendency to lock on the QRM rather than the desired signal. Such a narrow
bandwidth improves signal-to-noise ratio dramatically. A one-watt signal, copied through a 10-
Hz filter is comparable to a 50-watt signal heard through a 500-Hz filter, or a 230-watt signal,
heard through a 2300-Hz filter.



Nforse cw signals as Digital Signals

     I Morse cw signals may be analyzed into a series of digital units, all of which (at least
approximately) have a unit of time on common. For convenience, I'll call this time unit a
"frame." Each frame contains either a "mark" (key-down) or a "space" (key-up). Figure I illus-
trates this concept.

     The coherent cw signals we are discussing here are like ordinary cw signals, except that
they are sent in regular time-disciplined fashion. Ordinary Morse dits, dahs, and spaces begin
at arbitrary times, depending on when the operator happens to press his key. In coherent cw,
all dits, dahs, and spaces are exact multiples of the basic time unit and occur in predictable
time frames. This includes any pauses in transmission during the QSO. When received by
ordinary Morse cw methods, ccw signals sound just like other cw signals, except that they are
sent very regularly, "with a perfect fist."



     In ordinary Morse. the frame length varies to a considerable degree, so you cannot predict
when each frame starts and ends. The time-disciplined ccw signal, however, has a constant and
accurate frame length; this allows the filter at the receiver to "know" exactly when each frame
begins. The filter uses this capability to achieve a narrow bandwidth which matches that of the
signal without ringing.

     Typically, in Coherent cw the stations agree on a frequency (e.g., 14,049,000 Hz, plus or
minus 2 Hz), and a frame length (usually 0.1 second), and acquire the "framing" - when each
frame starts and ends - as part of the process of tuning in the signal. The receiving filter there-
fore "knows" these three signal parameters (frequency, frame length, frame phase) and uses
that information to help it detect the desired signal and reject undesired signals.



Definition of Coherent cw

     We now see that coherent cw may be defined as the sending and receiving of cw signals
such that the frequency, frame length, and frame phase are all known at the receiver and used
to advantage in the detection process.



The ccw station

     To achieve the necessary accuracy in frequency and frame length, both stations must use
only highly stable oscillators. The oscillator which determines the frame length must also be
highly accurate. The stability and accuracy requirements are within those obtainable by care-
fully build crystal oscillators compared to a reference frequency such as WWV as received.

     To get on frequency within the narrow tolerance of the filter (that is within a few Hz) all
frequency-determining oscillators in both transmitter and receiver must be stable and this has
often achieved by locking them to a reference oscillator.

     To impose the necessary time-discipline on the transmitted signal, a reference oscillator is
counted down to provide a 10-Hz "synchronizing" signal for the transmitter keyer.

     Likewise, the ccw filter at the receiving station uses timing signals derived from that
station's reference oscillator. It is these timing signals which "tell" it when to expect a frame of
the received signal to begin and end.