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A superheterodyne receiver or superhet receiver is a radio frequency (RF) receiver that uses a mixer to produce an intermediate frequency (IF) from the incoming RF and the local oscillator. It is a practical approach to building commercial receivers because even for a receiver with more than six stages, it has only two sets of tuned circuits that need to be adjusted simultaneously. These tuned circuits are in the front-end RF amplifier and the local oscillator. The front-end RF amplifier is the most sensitive stage that takes in the RF input from the air wave and creates a first-stage amplified output. This output signal is fed into a mixer that generates the needed beat frequency or IF.
The tuned radio frequency receiver is an attempt to make a straightforward receiver that works very well for one or more frequencies, but not for an entire band. There are no commercially available tuned radio frequency receivers for amplitude modulation (AM) broadcast because the combinations of inductances and capacitances to make several stages tune to the same frequency make the approach impractical. As a substitute for the tuned radio frequency receiver, the superheterodyne receiver was invented, and it uses a mixer to produce an IF using the amplified version of the incoming RF and the local oscillator output.
Unlike a superheterodyne receiver, the direct conversion receiver deals only with the incoming receiver frequency followed by a demodulator or detector that extracts the message. While the direct conversion receiver has no mixer and no IF, the basic superheterodyne receiver is a single-conversion receiver with one IF. A variation of the superheterodyne receiver is the double-conversion receiver that has two IFs, the first of which is usually several tens of megahertz (MHz) like 45 MHz.
The reflectional receiver was an older version of receiver that uses a single vacuum tube that performed the function of radio frequency and audio frequency amplification. Vacuum tubes were very popular before the 1960s, but these devices were bulky, sensitive to shock, and consumed more power. Before the 1960s, the reflectional receiver proved very advantageous over alternative multi-vacuum tube designs.
Automatic gain control is a feature that is available in any receiver type. When the incoming RF is low, the gain of the front-end amplifier is increased, but when the incoming RF is high, the gain is decreased. There are designs where the gain will only decrease when the incoming RF is above a certain threshold, and this is delayed automatic gain control.
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