HOMODYNE AND SYNCHRODYNE

2. The Original Homodyne

The circuit arrangement of the original homodyne, as published by Colebraok in 1924, is shown in Fig. I. This is nearly as old as the superheterodyne, which appears to date from 1918. It is an oscillating detector, the anode and grid coils being lightly coupled so that the valve circuit oscillates gently in the absence of input signal. When the input is applied, the tuning is adjusted until the harsh bent-note disappears. This is the condition of synchronisation, and the modulation signal is heard clearly. Colebrook points out the benefit which is obtained from the fact that this system gives effectively linear (i.e., distortionless) rectification. However, it is clear that the frequency response of the highly regenerative tuned circuit (so highly regenerative that it oscillates) is imposed on the received r.f. signal, and it is inevitable, therefore, that there will be frequency-response distortion of the audio output, since most of the higher frequency sidebands will have been attenuated. Colebrook states that "the quality of the reception was very full and rich in tone", but it is likely that this was due to the relatively enhanced bass response, which may have been advantageous with the headphones and loudspeakers used in those early days. He points out also that the amount of input signal relative to the local oscillation amplitude is critical, and that it may be necessary to reduce the latter, or to provide a r.f. amplifying stage to increase the former.

Following this article, a letter was published in the same journal under the initials F.G.G.D., and this referred to Appleton's earlier paper on the synchronisation of oscillators and to the fact that this method of reception had been in use for some time at Cambridge. This letter also pointed out that the demodulation was not equally effective over the whole frequency band of the synchronized condition, but that the output was zero at the edges and a maximum in the middle of this band. Colebrook had rather inferred that the output was constant over the band, but this may not have been intentional.

The homodyne in this form had the same circuit arrangement as the "autodyne", which was an "automatic heterodyne" receiver of c.w. signals, having its oscillating detector tuned to a frequency different from the incoming carrier by an amount sufficient to give a good heterodyne note. It is clear that the relation between the homodyne and the autodyne did not extend beyond their circuit diagrams. One cannot now avoid the suspicion that in those days of reaction coils and tuning whistles, homodyne reception was almost certainly used far more often by accident than by design!

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