CCWN 75:58
                                                               
If the Morse signal is transmitted as coherent pulses, a digital
filter may be designed to match the characteristics of the signal
which we are seeking to receive. Figure 1 shows a simple
idealized matched digital filter which may be used to illustrate
the capture of digital filters.            The input to the
filter is several hundred Hz wide with the desired signal exactly
at 1000 Hz. Switch 1 is an electronic switch which switches back
and forth at exactly 2000 Hz and exactly in phase with the
incoming signal. (In practice, this could be done by tuning the
phase of the switch for maximum reception. However, in the most
widely used CCW filter, the Petit filter, this necessity is
overcome by having two switches working 90~ out of phase and
adding their outputs. This gives the desired result no matter
what the phase relationship between the input signal and the
switches.)           Switch 2 is a switch which is operated in
phase (coherent) with the pulse period of the modulation
(keying). If we are using a pulse length of .1 second, then at
the end of each .1 second time period, the switch is pushed to
the sample position for a fraction of a second. The sampler takes
an instantaneous voltage reading and holds it until the next
reading is taken. In the diagram I have indicated a high
resistance voltmeter which can be used for long pulse periods. In
the Petit filter, this voltage is used to control the intensity
of a tone for the next pulse period, thereby converting the
signal to an audio tone convenient for humans.           A
fraction of a second later, switch S2B is closed and discharges
the charge of C1 to ground. This is called a dump. Then the next
pulse period begins.           The voltage on C2 is a function of
the total (or average) power being received on the desired
frequency during the last .1 second period. This may be written
down as a number, and then a series of these interpreted. Or,
this voltage may be used (as it is in the Petit filter) to
control the intensity of the output fo a desired frequency for
the next .1 second period.           To illustrate the bandwidth
of this digital filter, consider the following examples: