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A block diagram of the direct conversion receiver. The conversion of the modulated signal to baseband is done in a single frequency conversion. This avoids the complexity of the superheterodyne's two (or more) frequency conversions, IF stage(s), and image rejection issues. The received radio frequency signal is fed directly into a frequency ...
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.
In the Direct conversion receiver, the signals from the antenna are only tuned by a single tuned circuit before entering a mixer where they are mixed with a signal from a local oscillator which is tuned to the carrier wave frequency of the transmitted signal. This is unlike the superheterodyne design, where the local oscillator is at an offset ...
An important and widely used application of the heterodyne technique is in the superheterodyne receiver (superhet). In the typical superhet, the incoming radio frequency signal from the antenna is mixed (heterodyned) with a signal from a local oscillator (LO) to produce a lower fixed frequency signal called the intermediate frequency (IF
In double-conversion superheterodyne receivers, a first intermediate frequency of 10.7 MHz is often used, followed by a second intermediate frequency of 470 kHz (or 700 kHz with DYNAS [15]). There are triple conversion designs used in police scanner receivers, high-end communications receivers, and many point-to-point microwave systems.
In direct modulation these stages must be developed separately for each output RF (so called channel). On the other hand, in superheterodyne transmitters since a single intermediate frequency signal is used, only one type of stage for IF is developed. Thus the said stages are more reliable in superheterodyne. Also R&D is much easier for the ...
IQ imbalance results from these two imperfections, and is one of the two major drawbacks of direct-conversion receivers compared to traditional superheterodyne receivers. (The other is DC offset.) Their design must include measures to control IQ imbalance, so as to limit errors in the demodulated signal.
The heterodyne oscillator had to be retuned each time the receiver was tuned to a new station, but in modern superheterodyne receivers the BFO signal beats with the fixed intermediate frequency, so the beat frequency oscillator can be a fixed frequency. Armstrong later used Fessenden's heterodyne principle in his superheterodyne receiver (below).