Accurate, variable, reference frequencies can be derived from accurate, fixed frequency
oscillators by several means. Direct frequency multiplication uses the generation of
harmonics of the reference frequency waveform. The reference waveform is passed
through an amplitude comparator to generate a TTL or ECL logic waveform of the same
frequency. This logic waveform is then passed through a one-shot multivibrator to create
a train of narrow pulses having a Fourier spectrum containing many harmonics.
The pulse train is then made the input to a high Q tuned circuit which has its center
frequency equal to that of the desired harmonic. It is practical to recover harmonics up to
order 9 with this technique. One can also use synchronous binary logic ‘rate multipliers’,
such as the 7497, to reduce the oscillation frequency to fR, given by the relation,