The Superheterodyne Receiver

Radio Record, Volume IV, Issue 45, 22 May 1931, Page 17

The Superheterodyne Receiver

---- ps the special trade forecasts last week mention was made of the. superheterodyne receiver, In order to make elear the meaning of the term -we"reproduce below an excerpt from the "1981 Radio Guide" entitled "The Superheterodyne Receiver." IN the design of any receiver -the three most important factors are selectivity, sensitivity and fidelity.. A high order of selectivity makes it possible to tune in one station without interference from another operating on an adjacent frequency, the sensitivity determines the ability of ‘the reeeiver to pick up distant stations or to operate on a small aerial and yet deliver the volume desired. The fidelity indicates the quality of the speech and music as delivered by the set in analysing the superhet (as it is usually called) we shall see how these faetors are obtained. / There .are eight essential parts, quite often nine. It will be remembered that in the broadeast set there are. six, In comparing both sets it will be seen that the aerial, the powersupply, the loud-speaker, and the audio amplifier are common to both, so we shall leave them out of the discussion. In the superhet there are two detectors, an intermediate amplifier and an oscillator. The engineer can design a set that will amplify best at one frequency only, at the others it is more or less of a compromise. If the stations in the world could operate on one frequency and share the ether, we should have good reproduction, but since they cannot, we have either to make the set variable or to reduce them: to one frequency in the first stages. In other words, we use a_ frequency

changer. Let us now see how this" is done. Beat Frequencies. When two high-frequency currents : are combined in a detector circuit a third. or beat frequency results. There are really two beat frequencies, the sum and the difference of the two. Thus if a 1000 kilocycle. carrier is combined with another frequency of 100 we should have two new ones at 100 and the other at 1100. Now it is very much more simple to amplify a low frequency than a high one. Those who have built sets and have been troubled with oscillation and feedback on the lower wavelengths will agree on this point. Our plan, then, in designing the best set possible will be to make it amplify at the.100 kilocycles, and we can now design our amplifier to work at this low frequency. Every incoming wave must be combined with a suitable frequency to give the required 100 kilocyeles. Thus a frequency of 720 (that of

2YA) will be combined with one of 620 and that of 2¥YB (1090 kes.) with 990. In practice these two currents are mixed in the first detector of the superheterodyne, and the station’s frequency may or may not have been amplified by previous radio stages. The source of the second frequency is the oscillator, which, as it has been explained, must provide a note 100 kilocycles below or above the station’s carrier, The 100 kilocycle note is now amplified by the intermediate stages, passed to the second detector,

which strips away the high-frequency element and passes the audio signal on to the low-frequency amplifier which adds power and sends the signal on to the speaker, In the present discussion we shall not stop to ‘consider just how selectivity is more pronounced with this type of set; it would involve certain elementary mathematics and the reader has been inflicted with enough of them already. Sufficient to say that as the intermediate frequency is raised the distance between adjacent stations is increased, but it cannot be raised . too high ‘or oscillation troubles will be encountered. The superheterodyne, then, is sensitive, selective and true toned.

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