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The key feature of the superheterodyne approach is the mixing of the incoming radio signal with a locally generated, different frequency signal within a radio set. That circuit is called the mixer. The result is a fixed, unchanging intermediate frequency, or I.F. signal which is easily amplified and detected by following circuit stages.
In the Armstrong method, the audio signal and the radio frequency carrier signal are applied to the balanced modulator to generate a double sideband suppressed carrier signal. The phase of this output signal is then shifted 90 degrees with respect to the original carrier. The balanced modulator output can either lead or lag the carrier's phase.
The result is a narrow-band radio-frequency filter and amplifier. The non-linear characteristic of the transistor or tube also demodulated the RF signal to produce the audio signal. The circuit diagram shown is a modern implementation, using a field-effect transistor as the amplifying element. Armstrong's original design used a triode vacuum tube.
Schematic of an early autodyne receiver. The autodyne circuit was an improvement to radio signal amplification using the De Forest Audion vacuum tube amplifier. By allowing the tube to oscillate at a frequency slightly different from the desired signal, the sensitivity over other receivers was greatly improved. [1]
A 5-tube superheterodyne receiver manufactured by Toshiba circa 1955 Superheterodyne transistor radio circuit circa 1975. A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency.
The type 36 screen-grid tube (obsolete since the mid-1930s) had a non-regenerative detection gain (audio frequency plate voltage divided by radio frequency input voltage) of only 9.2 at 7.2 MHz, but in a regenerative detector, had detection gain as high as 7,900 at critical regeneration (non-oscillating) and as high as 15,800 with regeneration ...