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.
The Art &Skill of Radio-Telegraphy
©William G. Pierpont N0HFF
This page last updated August 02, 1998
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