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On the Superheterodyne Radio

The earliest radio recievers were crystal radios, which used a very long antenna to recieve as much of a signal as possible, a capacitor to create a tuned circuit, and a crystal of galena, to which a "cat's whisker" would make a rectifying contact, creating a semiconductor diode. This allowed the modulating signal to be separated from a radio signal, and heard in sensitive earphones.

Short-wave radios built from simple kits that included only a single vacuum tube or transistor for amplification used one of the earliest designs for a radio with amplification, and they were regenerative receivers. These amplified the weak radio signal by a very large ratio through the use of positive feedback.

Crystal radios and regenerative receivers belonged to the early days of radio. Modern radio receivers, in the sense of the first radio receivers that one could simply plug in and turn on and listen to, without necessarily being up-to-date by current standards, were normally based on the superheterodyne principle, invented by Edwin Armstrong (who also invented the regenerative receiver, and FM radio).

The paradigmatic example of a superheterodyne receiver is the five-tube AM radio, but this principle continues to be used today in all analogue radio receivers.

The sum of two sine waves having frequencies

f  and f
 1      2
is a signal which is the product of a sine wave with the frequency

f  + f
 1    2

the average of those two frequencies, and one with the frequency

f  - f
 2    1

half of the difference between them, or the difference between either one and their average.

An AM radio signal is a sine wave at the carrier frequency multiplied by a number which might be, for example, between 1.2 and 1.8, representing the level of an audio signal. Thus, if a radio station with a carrier frequency of 880 MHz is broadcasting a pure 440 Hz musical tone, the result will be a carrier of 880 MHz plus signals of 880,000,440 Hz and 879,999,560 Hz within the sidebands of the radio signal.

Such a signal is normally produced by allowing the audio signal to control the amplification of the carrier frequency, and such a circuit performs multiplication directly.

In a superheterodyne radio, after the antenna and the tuned circuit (a capacitor and a coil) that selects the frequency of the radio station to which one is listening, the first stage is an RF (radio frequency) amplifier which amplifies the selected radio signal directly.

After the first stage, the signal is strong enough to work with, but is still relatively weak.

The word Heterodyne comes from two Greek words meaning 'other' and 'force'. In a superheterodyne radio, the output of the RF amplifier has added to it a signal about equal in strength, having a frequency that is a small fixed amount (455 kHz in a standard AM radio) below (or possibly above) its carrier frequency. The same mathematics which explain how modulation lets one place the information from a low-frequency audio signal within the high-frequency sidebands of a carrier mean that this will create beat frequencies which produce a replica of the incoming modulated signal, but one whose carrier frequency is the constant difference in frequencies.

Just adding a signal of the offset frequency is not enough to create a new frequency. But if the offset frequency signal is stronger even than the carrier of the (amplified) incoming radio signal, then the combination will match a single-sideband signal in which the offset frequency signal is modulated by a signal which consists of the desired lower-frequency carrier modulated in its own turn by the audio signal.

A single-sideband signal with a strong carrier can be demodulated by simple rectification followed by low-pass filtering, just as an AM signal can be demodulated. A little distortion will be produced by this, but it is the IF carrier, not the audio signal, that is distorted, leading to the presence of signals at multiples of the IF frequency which normally will not require anything to be added to the radio to filter them out.

This modified radio signal can then be amplified by several IF (intermediate frequency) amplifier stages in the radio; these stages are normally coupled by tuned IF transformers. Because the IF stages amplify a signal in a fixed and narrow frequency range, it is easier to make effective and good-quality amplifiers, and the repeated tuned IF transformers between them improve the selectivity of the receiver. This means that another radio station on a nearby frequency will not interfere with listening, and this is what makes the superheterodyne radio so convenient to use, and the obvious choice for a consumer radio receiver.

Then, after multiple IF stages, the internal radio signal is demodulated in a detector stage, and the result then undergoes audio amplification before going to the loudspeaker and being heard.

Copyright (c) 2008 John J. G. Savard

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