Chapter
23 Contents
Chapter 25
As discussed in Chapter 23, keying speed is usually expressed in bauds rather than in Hertz, or cycles per second. One Baud is one keying element per second, so one square wave keying cycle per second equals two Bauds. Using the standard word as 50 units, then (wpm) / 1.2 = Bauds. (Since 60 seconds divided by 50 units = 1.2)
Harmonic analysis
of the on-off keying wave shows that strong odd-numbered harmonics and
weak even-numbered harmonics are present. It has been found that
under good conditions, adequate readability results when the 3rd harmonic
is present , but under poor conditions we need up through the 5th harmonic.
(Really good quality, however, will include up through the 7th
harmonic.)
International regulations have specified accordingly that minimum acceptable
bandwidths should be at least three
times the
keying speed in bauds for good conditions and five times for poor conditions.
Thus, working from standard wpm, convert to Bauds by dividing by 1.2, then multiply by the highest harmonic (3, 5, or 7) desired. (Since this modulates the carrier frequency, the transmitted bandwidth will be twice this value because of sum and difference frequencies.) Accordingly, e.g., for 20 wpm, covering the 3rd harmonic requires a 50 Hz. bandwidth filter; for the 5th harmonic coverage a 83.3 Hz. bandwidth filter.
A perfect square
wave will generate strong transient overtravel, both initially and
at the end of each pulse. These spikes are
especially
objectionable, as they generate a host of harmonics which will interfere
with other transmissions. For the receiving operator they produce
an unpleasantly harsh quality. Shaping to round off these sharp corners
of the wave by making a 5 - 7 millisecond delay gives satisfactory reception,
but if it is lengthened too much it tends to blur the signals and make
them hard to read. This situation can be taken care of only at the
transmitter, of course. It can be seen that there is a delicate balance
between "good quality" and troublesome harmonics. Refer to the handbooks
for corrective measures.