The most sensitive of receiving circuits .
When broadcasting began there were two 'Super' receivers in the running - the Supersonic heterodyne (superhet - to you and me) and the Super-Regenerative. The former enjoyed only moderate popularity until a few years ago, since when it has come right out on top. The latter aroused much interest for a time, and then became almost forgotten. Now the pendulum is swinging the other way once more, and we are having a super-regenerative revival. I see no likelihood at all of this being on anything like the scale of the superhet revival, but there exists some interesting scope.
The merit of the scheme is that it enables the simple reaction-type of receiver to be pressed to a condition in which it gives abnormally high amplification. There are the inevitable short-comings - loss of selectivity, for instance - that rule it out for most purposes; but sometimes one wants an extremely small portable set. Take policemen; the majesty of the law would not be augmented by sizeable frame aerials wound around the helmet, or conspicuous bulges in the uniform. Any personal police receiver must be discreetly unobtrusive. In general this is difficult, because either (a) a lilliputian frame aerial necessitates a large multi-stage amplifier to yield anything from it, or (b) a receiver with room for only one or two valves, and midget batteries, needs a full-sized aerial for long-distance work.
That is where the super-regenerator steps in. A cigarette-case type of completely self-contained set can be made sensitive enough to provide a useful range. But the main cause for revived interest is the spot light now being directed upon ultra-short waves, largely in the interests of television. Actually the super-regenerator is of no use for television itself; but, whereas vision looks like being the rich man's portion for some time to come, the accompanying sound, to say nothing of amateur and other efforts, can most simply be groped for with that type of receiver. The loss of selectivity is actually a considerable asset, otherwise it is extraordinarily difficult to tune in a transmission at all; or, when once in, to keep it there.
There still exists a good deal of mystery as to how super-regeneration works. It is not easy to explain it concisely and at the same time, with any justifiable claim to accuracy, but readers who know very little about it indeed may be slightly wiser within the confines of this page.
In non-super regeneration, usually described as reaction, some of the amplified signal is coupled back to the input for the purpose of strengthening it. A moderate amount of this is very effective, and stations can be easily heard with its aid that are quite inaudible without. When the back-coupling is increased beyond a certain point, however, the situation gets out of hand and continuous oscillation is set up quite independently of any stimulation from incoming signals. It is like a machine gun, in which the power released from the firing of one cartridge is applied to send off the next, and so to keep up a continuous action so long as there is any ammunition left. There is no need for anybody to keep on pulling a trigger.
The incoming programme is then usually so much obscured by the local oscillation that it is necessary to reduce reaction below the oscillation point in order to restore useful reception.
Now in the ordinary way the reproduction ot sound is produced by the wave sent out from the station varying its strength at an audible frequency between 30 and 10,000 times a second, one may reckon. Suppose, now, a reaction control is twisted back and forth through the oscillating point 20,000 or 30,000 times a second. This being a tiresome operation to perform by hand, it is usual to arrange for it to be done electrically by means of 'quenching oscillator'. The still higher frequency oscillations of the receiver are chopped into groups at the quenching frequency. If the quenching frequency were lower below about 15,000 these groups would produce a piercing note. But they are inaudible.
Now this is the interesting thing. When a circuit is suddenly put into the condition of being able to oscillate it does not immediately take advantage of it. A needle, perfectly balanced on end, would remain there for a fraction of a second until some vibration or draught, though extremely small, sent it over. In the same way, an oscillator requires something to set it going; and usually it finds it, within a very minute space of time, in the random circulation of electrons in valves and circuits. These are irregular, and so, when nothing else is being received, the starting-off points in a super-regenerative receiver are irregular, and likewise, therefore, the durations of the separate groups of oscillations.
Fig. 1. - In a super-regenerative receiver the oscillations brought about by tightly-coupled reaction are broken up into groups by the quenching oscillator. The start of oscillation in each group is not instantaneous, as might be indicated at (a), but there is a delay until some other impulse - stray or intentional - starts it off, (b). The dotted line shows the low-frequency average in each case.
Suppose (a) in Fig. 1. shows diagrammatically the effect in the receiver of the groups of oscillations assuming they were to respond instantly to the word 'Go!' Each block represents a much larger number of oscillations than can be shown. These being of such high frequency, and all alike, they average out to a dead level, as shown dotted. There is no audible result.
But if the starts are delayed in an irregular manner, groups of these are larger or smaller than others (b), causing ups and downs in the average line, and irregular sounds in the receiver. Actually when a super-regenerator is working one hears a rough hissing sound.
When the carrier wave of a station's programme is tuned in these wholly irregular impulses are swamped by impulses which correspond to intelligible sounds, if one may so flatter the average programme. In view of the fact, which I have just explained, that comparatively large chunks of sound-making power are controlled by the inconceivably minute irregularities of electrons in the circuit, it is perhaps not difficult to realise that such an arrangement is very sensitive to weak signals, and the amplification under favourable conditions is enormous.
Readers who have been successful in following this explanation will see that the action of a super-regenerative receiver is entirely different from that of the ordinary receiver with reaction.