Why Detuning is Bad Control

Radio Record, Volume I, Issue 37, 30 March 1928, Page 12

Why Detuning is Bad Control

ON different occasions the practice of detuning has been condemned in this column as being very liable to produce distortion. It is impossible to lay down any exact rule in this matter that may be applied to all receivers, but listeners who are troubled with distortion are recommended to give the matter careful attention, and when distortion occurs in reception, a very careful attempt should be made to reduce it by fine tuning on the dials. ‘The dial position giving maximum signals is calied the "resonant point," but when the signals are strong and spread out slightly, it is sometimes difficult to determine very exactly when the resonant point has been reached, and yet the slightest deviation on either side may tend to introduce distortion, as we shall see. ‘The wave sent out by the transmitter is several kilocycles wide, the centre portion carrying the low notes and the two outer fringes the high notes and harmonics. Figure 1 shows the diagram of the resonance peak of a selective receiver, If we tune in to the resonant point correctly, the receiver bands will fall as the shaded portion A, which is sub-

stantially all on the resonance peak, so that the exact centre and the sides of the band are both correctly tuned in, so that both the high and low notes will receive equal treatment. Slightest Detuning May Cause Distor: tion, On slight detuning, the peak of the curve is moved with reference to the transmitter, so that the frequency band falls on the curve in some such way as suggested at B in Tig. 2. Here it will be seen that one-half of the band is received at full strength, while the other half is cut down very considerably by being situated on the steeply falling side of the peak, so that while half of the high notes, and practically all of the low notes, are heard well, the remaining half of the high notes is cut out more or less completely. On further detuning, the centre of the band falls at the beginning of the lower part or "skirt’? or the curve, as show at C in I‘ig 1. In this case one-half of the high notes is accentuated, while the low notes and the remainder of the high notes are badly heard, Iu either of thees two cases, in which the tuned circuit picks out part of the total wave at the expense of the

rest, there is discrimination between low and high notes, which inevitaby, results in distortion. On still further detuning the circuit, so that the peek of the curve is so completely removed from the frequencybend of the transmitter that the whole of the band falls on the skirt of the curve, as at D in Fig. 2, the distortion disappears again, for once again all parts of the band receive equal treatnent from the tuned circuit. DAMPED CIRCUITS. ig follows, then, that if it is desired ° to detune a receiver, and at the same time to avoid distortion, each tuned circuit must be adjusted either exactly in resonance with the station being received, or well away from the resonant point. ‘The only exception that may safely be made is where one of the circuits, owing to damping either by the aerial, or by a grid detecior, tunes flatly, so that it has a resonant curve of the type shown in lig. 8. It will at once be seen from the figure that in such circumstances, no matter what position the frequency band may have with respect to the curve, no appreciable distortion can be introduced.

It must not be imagined that tue distortion described here is purely theoretical and academic; it can be heard only too plainly in the loudspeaker whenever a really low-resist-ance circuit is tuned slowly through the wave-length of the lecal station, the distortion corresponding to B being particularly well marked. This type of dis‘ortion is heard at its worst when receiving on a frame aerial with a nearly oscillaing detector, when the most minutely accurate tuning is mecessary ‘to centre the transmission on the very sharp peak of the resonance curve. USE OF H.F. VALVE RHEOSTAT. (COMPLET I} detuning is particularly useful as a volume control when receiving the local station on a modern neutralised receiver, in which the chief yolume control is by the filament rheostat of the high-frequency valve. It often happens with such a set that the local station continues steadily to overload the output valve until the filament is turned right out, when dead silence supervenes. If the cerial circuit is now detuned at least far enough to ensure freedom from distortion, and the filament of the H.I*. valve relighted, a position of the rheostat, will be found at which the local station is recétvé® at convenient volume, Detuning, then, cannot be used for fine control of volume, except when one cirenit of the receiver is of high resistance, but when there is already a voluine control of insufficient range, it can he made to provide a very valuable auxiliary control. If the aerial tuning of the set is accomplished by a leose gounling, and there is no earth von:ection to any other part of the circuit, then the coudenser in the earth icad may be dispensed with, as therc will then be no direct path to earth for the B current. In the case of the two-valve amplifier recently described in this column, if constructed with circuit as shown, 0 condenser would be required in the earth lead, but the precaution would have to be taken if not plugging in for crystal reception only before turning off the B supply, otherwise a heavy B current would be drawn threugh the last valve. ‘To obviate any chance of this the connection between ctystal socket (1) and centre phone terminal or clip (4) could be dispensed with. In every case care should be taken not to come into direct contact with the aerial-earth circuit whilst making adjustments, It is on account of these limitations of the direct method that, after all, the double-wound transformer type appears to be worth the extra trowble involved, (Continued on Page 13.)

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