HOMODYNE AND SYNCHRODYNE
A system was patented by a Frenchman named de Bellescize, with a convention date of November 1930 (U.K. application date November 1931), and also published in a very full and thorough paper in 1932, in which the advantages of carrier reinforcement combined with the homodyne scheme of obtaining the enhanced carrier from a synchronized local oscillator seem to be fully recognized. The synchronization in this system is, however, not obtained by the injection of the input signal into the oscillator as described by Appleton and used by Colebrook, but by a more complicated method using a separate valve as a control impedance.
The local oscillator, as shown in Fig. 3, is applied with a large oscillation amplitude to one grid of a two-grid valve, to the other grid of which the input signal is applied. The difference-frequency between the local and carrier frequencies (which is ideally zero, i.e., d.c.) is applied to the grid of the control valve in such a way that it varies the impedance produced by the control valve across the tuned circuit of the oscillator in a manner which leads to the attainment and maintenance of exact synchronization. This method is nowadays quite well known. A second patent application" in June 1932, by the same inventor, merely clarifies some of the points of the first, and was not accepted as a complete specification by the British Patent Office. Another system which seems to be a sort of homodyne is described by Walton in a patent applied for in December 1930. This patent specification is a rather difficult one, the method of operation is not made very clear. However, the system uses a local oscillator synchronised to the incoming carrier to produce an oscillation which fully loads a balanced valve circuit the balance is upset by the application of the in- coming signal, which must be of opposite phase to the local oscillation. This seems very much like the use of a balanced modulator in place of the usual "detector" valve in the previous homodynes. The r.f. circuits have very flat tuning. The advantages of this system are not fully set out, and the inventor does not appear to be aware of the homodyne as previously published, or alternatively considers his scheme to be quite different. Another form of homodyne receiver was described by Reimann in 1932, but the author has been unable to obtain details of this. A system which was in all essential respects identical to the "Synchrodyne" described by the present author in 1947 was patented by a German named Urte in German Patent No. 670,585, applied for in December 1932. A British application for a patent based on the same specification was made in February 1934 but this did not mention the inventor's name. A switched linear modulator and synchronised local oscillator are specified, although the method of synchronisation is not absolutely clear--the local oscillator is "triggered" by the carrier wave. However, this can hardly mean anything other than synchronisation by direct injection, on Appleton's basis. Some further analysis of the signal discrimination in a homodyne system was published by Groszkowski in 1933. A very detailed homodyne design is described by Jarvis in U.S. Patent No. 2,166,298, applied for in November 1933. Synchronisation is by plain injection, and the local oscillation is added to the incoming signal at a very high level. The homodyne is used as an i.f. stage in this design, and this appears to be the first record of its use in this way. Automatic volume control is provided in the usual way--i.e., the output of the i.f. amplifier is rectified to provide the control bias. For the first time, too, an effort is made to remove the objectionable tuning whistle which occurs when the local oscillator is not properly synchronised. However, this is accomplished at the expense of using a highly tuned stage preset at the wanted carrier, the rectified output of this being used as a muting bias. This arrangement, if it is really supposed to work this way, seems hardly an advance, since if this stage can be so accurately preset, why not the whole receiver?
Fig. 4 shows one of Jarvis's circuit arrangements. An interesting version of the Homodyne is a single-valve circuit described by Starnecki in Polish Patent No. 28655, applied for in August 1934. The one valve serves as local oscillator, r.f. amplifier, and modulator. The circuit arrangement is shown in Fig. 5.
It is doubtful if this circuit gives enough control of factors such as synchronisation to be satisfactory over any range of conditions. Although normally an effort is made to obtain linear action in the receiving circuit of a homodyne, Oltze, in a German patent of 1938, claims additional selectivity by the use of suitably non-linear demodulation. This has not been investigated further. One further homodyne scheme was patented by Curtis, an American, with a convention date (U.S.A.) of March 1939 and a British application date of February 1940. This arrangement has not only the desirable features of a separate path for the synchronised oscillator and a balanced modulator for the signal demodulation, but also the features, later described for the synchrodyne by Carlick of using both injection and a reactance valve method for synchronisation-i.e., using both Appleton's and de Bellescize's principles. Although the advantages of this combination are not described in the patent, it is, in fact, a very effective combination, as shown by Garlick.
Fig. 6 shows the circuit arrangement, and it will be seen that a.v.c. is provided by using the rectified input signal--not a very suitable method if strong unwanted signals are present. An important feature of the patent is that it reports the production of non-linear distortion due to the phase-modulation of the synchronised oscillation by sidebands of the input signal; this effect was snalysed by the present author and a colleague in 1949-50. Finally there was the important, but anonymous, article in 1942 that gave a very lucid and well-reasoned discussion of the position and prospects of the homodyne, although it gave no references to previous work.
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17th September 2001