WHICH CIRCUIT?

Press, Volume LXVIII, Issue 20731, 16 December 1932, Page 27

WHICH CIRCUIT?

SUPERHETERODYNE OR STRAIGHT. RADIO ENGINEER'S ADVICE. Buy era of radio sets who find themselves in doubt as to the most suitabletype of receiver for their requirements, or as to the respective merits of the many instruments brought to their notice, will find the following discussion of the advantages of the supcyhoteroydr.o and the straight tuned radio ficquenoy circuits of-help to them tn making their selection. "The essential difference between the two receiver types lies in the method of tuning from one .station to another, states ix prominent radio engineer. "In the straight set, every tuned circuit throughout tho receiver is tuned to tho frequency of the incoming signal, and when changing from 0110 station to another, it is necessary to change tho resonance frequency of each circuit by adiustiug the variable condensers. Now the amplilier portion of a superheterodyne is very similar, and consists ol valves coupled by coils and condensers which form resonant circuits tuned all to tho sumo frequency. No provision, however, is made in this amplifier lor varying the tuning, tho amplifier is fixed, and designed to function at only one frequency. In order to make use of it, therefore, it is preceded by a piece of apparatus whose purpose it is to change die frequency of tho desired station to that of the amplifier. Instead of tuning by adjusting the amplifier to operate upon tho signal liequency, as in a straight set, we work the other way round, and alter the frequency of the signal to conform to tho requirements of the amplifier. "The fact that the amplifier or as it is usually termed, the intermediate lrcquency amplifier, is required to uoi.v only upon ono frequency lends to a number of important points. Iroviued that the frequency-changing process oo carried out with constant eihciencv, neither the sensitivity nor the selectivity of the receiver will vary over its tuning raniro. mid the quality oi■ reproduction will also be unaffected by the frequency of tho signal. Furthermore, we. have the opportunity of i>s |n A special types of.circuit in the amplifier. Perhaps tho chief point, however is that within limits we can choose the particular frequency upon which tjc amnlifier is to operate. . "With ordinary tuning circuits, 1 m lower the frequency the higher the selectivity and the greater the a nip miration which can be obtained. the intermediate frequency, therefore, is nenrlv always chosen to he considerably lower than that of the incoming signal."