Mixing and detection

Abstract

In a tuned radio frequency receiver the modulated carrier wave is received by a tuned input stage then amplified by further stages before the intelligence is extracted. All r.f. stages handling the signal must be tunable: that is, it must be possible to change the frequency at which maximum gain occurs. This requirement reduces the number of amplifying stages because it is difficult to arrange satisfactory tuning of more than two or three stages. The shortage of stages thus renders the receiver insensitive to small amplitude signals. In addition, a TRF receiver has reduced selectivity (that is, the ability to select a particular carrier frequency among many) because the fewer the number of stages of amplification the wider is the band width of the overall frequency response curve. A supersonic heterodyne receiver (or superhet) overcomes these problems by the use of a mixer stage at the input. The mixer stage combines the incoming frequency and a locally generated oscillator frequency to give an intermediate frequency that carries the same modulation as was carried by the original carrier. When the receiver is tuned, both the input tuned circuit and the oscillator are tuned together, so that the oscillator frequency is separated from the incoming frequency by a constant amount, and the intermediate frequency is thereby constant regardless of the frequency of the incoming carrier. This means that several further stages of amplification to improve sensitivity and selectivity can be employed and the tuning of these stages, when once set, can remain fixed.