TWO POWERFUL CIRCUITS

Evening Post, Volume CVIII, Issue 83, 4 October 1924, Page 22

TWO POWERFUL CIRCUITS

THE "SUPER-HET" AND ITS OFFSPRING. " "Curious" has asked for some information regarding the (recently-developed) "second harmonic" receiver of Armstrong's invention. Not much information about this receiver has been made public, one reason being, no doubt, lhat it is still in the experimental stage, though it seems to-have been placed on the market; and another that it is hardly worth while for the average amateur to attempt to build it, owing to the difficulties of ensuring its proper action. It is a development of the now well-known super-heterodyne, from which it diffors in two important respects; it causes less interference by radiation (a !sin of which the 'super-net.'-' is often guilty), and it is more economical of valves. To begin with, it is necessary to know how the simpler form of super heterodyne

Wlnlo it is not difficult to obtain efficient radio-frequency amplification of signals at or even below the broadcast wave-lengths, using one stage only, the operation of an ordinary .r.f. amplifier becomes enormously more difficult" as the number of stages is increased. The inherent difficulty of short-wave-amplifica-tion is magnified, and each stage requires, a tho set is to be an efficient one, a separate control. These drawbacks were brilliantly overcome in the super-hetero-dyne system, for'•the development of which Armstrong has been chiefly responsible. By using one valve'as" "a" plain detector, and another as a separate oscillator, with the circuits coupled, there results the well-known phenomenon of beat reception," exactly as in the case ( .ot a single valve in an oscillating state. •itl is well-known that if a valve is oscillating and is detuned from the sig°f *i. beat note rises in Pitch > tut at the same time, as the circuit is no longer m tune with the signal, the reception is weakened. With a separate heterodyne valve,the'receiver can be kept strictly tuned, and-the note varied by altering the' tuning of the extra valve circuit. Thus the beat note, whatever its pitch, retains maximum strength. By a proper tuning of the heterodyne circuity tho beat becomes inaudibly rapid, and its frequency soon becomes,so nigh that it is really a radio-frequency corresponding, to a long wave-length. Now long waves are quite easily amplified; and it will' be evident that, no natter what, the .'wave-length of the received signal, it is possible to secure a beat frequency of, a fixed value. The superheterodyne thus works with only two tuning controls—one for the first detector, and one for the heterodyne circuit. No matter how many stages of radiofrequency amplification follow, they are permanently, tuned to a fixed wavelength. The operation of a multi-stage radio-frequency amplifier, otherwise a very difficult operation, is thus reduced to extreme simplicity, and the apparatus is a most powerful instrument for long distance reception—probably, taken all round, tho most efficient known to the general user of wireless.

„ xiw prime' distinction between the second harmonic" receiver and the super-heterodyne is that the oscillator valve is not arranged to oscillate anywhere near the frequencyr of the received wavoj but at approximately half the frequency. 'JJhere is then no tendency for ono frequency to "pull the other into step, and the risk of radiation likely to interfere with other receivers, due to tho action of the oscillator, i s alsq abolished. Tho heterodyne frequency is then provided by the presence of tho so-called second harmonic" (some call it the first) in "the oscillator's wave. It is very difficult to abolish harmonica completely from an oscillating circuit, and cjuite easy to make them strong. Working on these linos, Armstrong developed a very interesting variation. Ho found that it was quite possiblo to make one valvo rocoive a signal at one frequoncy, on ono circuit, and keep a second circuit in oscillation, at a different frequency, at tho samo time. Thus tho separato oscillator valve could bo abolished and the boat frequency intended for amplification appeared in the output of tlio first valve. More than that; this beat frequency could be "rofloxed" and put'through the valvo again. The valve was thus mado to operate at three different frequencies at the samo time—first, that of the recoivod signals, say, ono million cycles per socond; second, the added frequency, in tho neighbourhood of half a million per second, and tho "boat" frequency, equal to tho difference betwoen tho first and twice tho socond—say, 50,000 cycles por second, lho result of this peculiar action is that, as comparod. with the super-hetero-dyne, the second harmonic recoivor requires two valves less, and causes less radiation trouble. It is true that it contains an' oscillating circuit which is tuned (in the supposed caso) to 600 metres or thereabouts; but tho risk of interfering radiation is practically negligible because tho aerial circuit has an entirely different tuning, and has no tendency to oscillate at that frequency.