HOMODYNE AND SYNCHRODYNE
In that the homodyne is a kind of carrier-reinforcement system, it is important to notice that the advantages of carrier reinforcement had been appreciated rather earlier than the first announcement of the homodyne. In March 1922, Robinson had applied for a patent for a system in which the carrier of the incoming signal is filtered out in a path separate from the main signal path, and amplified in a high-Q regenerative tuned amplifier before being recombined with the original input signal, which has been amplified if necessary by a low-Q tuned amplifier. The combined signal is then detected in the usual way.
Fig. 2 shows the basis of the system. The importance of this patent--and of another by the same inventor, applied for in August 1930--is that it shows an appreciation of how important it is to reinforce the carrier in such a way that highly tuned stages are not used in the main signal path. In this way, the attenuation of the higher modulation frequencies, which was a weakness of the homodyne, can be avoided. The second patent referred to above, while not basically different from the first, is interesting in that in Claim 6 it is stated that the carrier-wave tuned stage may even feebly oscillate, no advantages are claimed for this condition, but it is clearly identical with the improved homo-dyne or synchrodyne to be discussed later. No appreciation of the significance of the synchronization requirement is evident. A much more recent treatment of carrier reinforcement is given by Crosby, in 1945.
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17th September 2001