Receivers: sensitivity, selectivity and the superhet

A good receiver must hear weak signals, reject strong ones nearby, and handle every mode. In this lesson: the simple receivers, sensitivity and selectivity, the superhet, detectors for each mode, and automatic gain control.

The simplest receiver is the crystal set: a tuned circuit, a diode detector and headphones, powered by the signal itself. A direct conversion receiver adds a radio frequency amplifier, then mixes the signal with a local oscillator on almost the same frequency, so the output is audio straight away, ready for an audio amplifier. In any analogue receiver, the R F amplifier boosts weak signals, the demodulator, or detector, recovers the audio, and the audio amplifier drives the speaker.

Sensitivity is a receiver's ability to detect weak signals. But very strong signals can overload a receiver, causing distortion in the audio. Selectivity is its ability to reject frequencies outside the wanted signal's bandwidth. Tuned circuits provide selectivity, and a higher Q gives a narrower response. But at high frequencies, even a good tuned circuit's bandwidth is too wide to separate stations a few kilohertz apart, and it's hard to keep several tuned circuits in step as you tune.

The superheterodyne receiver solves the problem. A mixer combines the incoming signal with a local oscillator, producing an intermediate frequency that is either the sum of, or the difference between, the two. Because the intermediate frequency is fixed, its filters and tuned amplifiers can be designed once for excellent selectivity. To tune, or to change band, you just change the local oscillator, so one receiver can cover many bands. Tuned circuits in the R F and I F amplifiers select the wanted signal. For example, a fourteen point two megahertz signal mixed with a local oscillator on twenty three point two megahertz gives an intermediate frequency of nine megahertz.

Each mode needs its own detector. For A M, a diode detector rectifies the signal, and smoothing leaves the audio envelope. For C W, a beat frequency oscillator beats with the carrier to make an audible tone. For S S B, a carrier insertion oscillator restores the missing carrier, and a product detector recovers the audio. For F M, a discriminator turns frequency changes back into audio. Finally, automatic gain control senses the signal strength at the detector and turns down the gain of the I F, and sometimes the R F, amplifiers on strong signals, keeping the audio fairly constant. The same A G C voltage drives the S meter.