CCWN 75:57
INTRODUCTION TO COHERENT CW BY CHAS. WOODSON

          Morse code CW signals have generally been handled as
analog signals and decoding has generally been done by skilled
human operators who distinguish between the signals at the
desired frequency and the accompanying interfering signals and
nolse. Filtering has generally been with filters having a
bandwidth of around 2100 Hz or 500 Hz centered near the desired
frequency. The short term stability and resetability of the
transmitter and receiver have generally been such that variations
of 100 Hz are common durinq communication.            Morse code
sent is a very regular digital fashion can be analyzed into a
binary (on-off) pulse code with a pulse length equal to the
length of a dot. The length of the key units of the Morse code,
dots, dashes, one-unit spaces, three-unit spaces, and longer
word-spaces, all have in common the unit of time equal to the dot
length. Each Morse code message therefore converts into a series
of signals of the pulse unit length. This allows the use of
digital methods of processing Morse signals.            If Morse
code is transmitted in a regular way, the resulting coherent CW
(CCW) may be detected by a coherent digital matched filter. This
technique is considerably more effective than the usual analog
methods. Coherent digital methods require that the pulse periods
be equal and begin at sufficiently regular times that this timing
may be obtained by the receiver (pulse phase acquisition) or is
known so the receiver filter can be set to be in shase with these
pulse periods.           As transmitted the CW signal can be
conceived of as a binarx,~ coded signal, that is a series of
on/off pulses. If the pulse l~ngth of .1 second is selected, the
operating speed is about 1- words per minute. Note that the
signal is not of zero b?ndwidth. The modulation produces
sidebands of energy, though they are much narrower than those
produced by voice modulation. For example, if a series of dots is
being sent, the sidebands are at 5 Hz and harmonics of 5 Hz as
the signal is being modulated by a 5 Hz off and on signal. If we
are sending a series o' t's as individual words, modulation would
be a 1 & 4/6 Hz. Clearly if we could design a filter which was
only a few cycles wide and did not ring, a great deal of
improvement could be made in CW reception.           The above
paragraph described the CW signal as transmitted. As received, it
is changed considerably. Obviously it is much weaker. Secondly,
other signals, QB1 and noise have been added. Frequently the QRM
and noise are stronger than the desired signal. Thirdly, less
well known, the signal is subject to changes produced by the
propagation path such as phase modulation. This modulation which
may be conceived of as FM modulation I have found to often be
about .5 Hz per skip by HF signals using the F layer, although it
can be much less or more. I conclude that a CW signal may be
considered to have a bandwidth of something like 7 or 8 cycles.