Highly stable frequency-shift-keying exciter

The provision of a frequency-shift-keying system to meet present-day requirements presents the designer with three main difficulties; (a) the attainment of sufficient stability of the centre frequency, (b) the provision of a system suitable for keying at high digit rates, and (c) the prevention of out-of-band radiation. The paper indicates some problems inherent in systems commonly used in which the solution is often a compromise. A system is described in which a crystal-controlled master oscillator is used, and half the required shift frequency is added to, or subtracted from, the oscillator frequency by means of a pick-up coil turning in a rotating r.f. field. The method has the advantage that the stability of the frequencies of transmission is not appreciably degraded as in the case of frequency-pulled oscillators. Also, because the mark and space frequencies do not exist together, the problem of suppressing the alternative frequency does not arise, as in systems in which both frequencies are generated separately. After the f.s.k. signal has been generated at a convenient frequency, a second crystal-controlled oscillator converts it to the particular carrier frequency required. Out-of-band radiation is reduced by synchronizing the shift frequency with the basic square wave of the telegraph digits and correctly phasing it: this can easily be achieved with automatic telegraphy. In conclusion, a practical system including some design features is given.