HOMODYNE AND SYNCHRODYNE

7. The Synchrodyne as a Precision Demodulator

As has been previously stated, the author's interest in the synchrodyne was not primarily in its application to radio reception; it was, in fact, more specifically in its application as a very accurate demodulator for highly-selective transmission-measuring equipment. This accuracy arises largely due to the facts that the centre of the frequency band selected is determined almost entirely by the carrier to which the local oscillator synchronises, and that the shape of the band is determined by the low-frequency filter in the output. In ordinary circuits, where the filtration is done at a relatively high i.f., neither the shape nor the centre of the band selected is easily made exactly what is desired. To enhance this inherent accuracy, the author and his colleagues investigated the cause and cure of distortion due to the synchronising process when the natural frequency of the local oscillator and the injected carrier frequency are not exactly equal; this distortion arises as a result of phase-modulation in the oscillator. A reasonably full theoretical study of this effect has been published, and it has been shown that the main result is harmonic distortion on the output of modulation frequency and an error in the d.c. output if this is retained. Since the phase modulation in the oscillator is due to the amplitude modulation of the carrier, there is no such effect if the synchrodyne is used merely to select a tone from a spectrum of unwanted signals, the wanted output being then a d.c. voltage. It was subsequently discovered that this distortion effect had been reported by Curtis, patent specification previously discussed, but he gave no analysis of it. To avoid the distortion discussed above it is evidently necessary to prevent the natural frequency of the oscillator from drifting very far from the frequency to which it is made to synchronise; this can be done by what is really an automatic phase-control arrangement. A reactance valve is used to readjust the natural frequency of the oscillator according to a d.c. voltage obtained by modulating the oscillator output with the injected carrier. The direction of the control is such that the d.c., which is approximately proportional to the phase difference between the oscillator and the carrier, is brought towards zero; and this condition means that the natural frequency of the oscillator is made very nearly equal to the actual frequency at which it is synchronised. This method of control was also described by Curtis in his patent.

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