SUPERHETERODYNE RADIO RECEIVER:
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio and television receivers use the superheterodyne principle.
DESIGN AND EVOLUTION:
The diagram at right shows the minimum requirements for a single-conversion superheterodyne receiver design. The essential elements are common to all superheterodyne circuits. A signal receiving antenna, a broadband r.f. amplifier, a variable frequency local oscillator, a frequency mixer, a band pass filter to remove unwanted mixer product signals, a demodulator to recover the original audio signal. Cost-optimized designs use one active device for both local oscillator and mixer— called a "converter" stage. One example is the pentagrid converter.
A suitable antenna is required to receive the chosen range of broadcast signals. The signal received is very small, sometimes only a few microvolt‘s. Reception starts with the antenna signal fed to the R.F. stage. The R.F. amplifier stage must be selectively tuned to pass only the desired range of channels required. To allow the receiver to be tuned to a particular broadcast channel a method of changing the frequency of the local oscillator is needed. The tuning circuit in a simple design may use a variable capacitor, or varicap diode. Only one or two tuned stages need to be adjusted to track over the tuning range of the receiver.
The signal is then fed into the mixer stage circuit. The mixer is also fed with a signal from the variable frequency local oscillator (VFO) circuit. The mixer produces both sum and difference beat frequencies signals catch one containing a copy of the desired signal. The four frequencies at the output include the wanted signal fd, the original fLO, and the two new frequencies fd+fLO and fd-fLO. The output signal also contains a number of undesirable frequencies. These are 3rd- and higher-order inter modulation products. These multiple signals are removed by the R.F. bandpass filter, leaving only the desired offset I.F frequency signal fIF which contains the original broadcast information fd.
Intermediate frequency stage:
All the intermediate-frequency stages operate at a fixed frequency which need not be adjusted.  The I.F. amplifier section fIF is tuned to be highly selective. By changing fLO, the resulting fd-fLO (or fd+fLO) signal can be tuned to the amplifier's fIF. the suitably amplified signal includes the frequency the user wishes to tune, fd. The local oscillator is tuned to produce a frequency close to fd, fLO. In typical amplitude modulation ("AM radio" in the U.S., or MW) receivers, that frequency is 455 kHz; for FM receivers, it is usually 10.7 MHz; for television 33.4 to 45.75 MHz.
Other signals from the mixed output of the heterodyne are filtered out by this stage. This depends on the intermediate frequency chosen in the design process. Typically it is 455 kHz for a single stage conversion receiver. The higher the chosen I.F. offset will reduce the effect interference from powerful radio transmissions in adjacent broadcast bands will have on the required signal.
Usually the intermediate frequency is lower than either the carrier or oscillator frequencies, but with some types of receiver (e.g. scanners and spectrum analyzers) it is more convenient to use a higher intermediate frequency. In order to avoid interference to and from signal frequencies close to the intermediate frequency, in many countries IF frequencies are controlled by regulatory authorities. Examples of common IFs are 455 kHz for medium-wave AM radio, 10.7 MHz for FM, 38.9 MHz (Europe) or 45 MHz (US) for television, and 70 MHz for satellite and terrestrial microwave equipment.
The filter must have a band pass range equal to or lesser than the frequency spacing between adjacent broadcast channels. A perfect filter would have high attenuation factor to adjacent channels, but with a broad bandpass response to obtain a better quality of received signal. This may be designed with a dual frequency tuned coil filter design, or a multi pole ceramic crystal filter.
The received signal is now processed by the demodulator stage where the broadcast, (usually audio, but may be data), signal is recovered and amplified. A.M. demodulation requires the simple rectification of the R.F. signal to remove one sideband, and a simple resistor and capacitor low pass RC filter to remove the high frequency R.F. carrier component. Other modes of transmission will require more specialized circuits to recover the broadcast signal. The remaining audio signal is then amplified and fed to a suitable transducer, such as a loudspeaker or headphones.
To overcome obstacles such as image response, multiple IF stages are used, and in some cases multiple stages with two IFs of different values are used. For example, the front end might be sensitive to 1–30 MHz, the first half of the radio to 5 MHz, and the last half to 50 kHz. Two frequency converters would be used, and the radio would be a double conversion superheterodyne; a common example is a television receiver where the audio information is obtained from a second stage of intermediate-frequency conversion. Receivers which are tunable over a wide bandwidth (e.g. scanners) may use an intermediate frequency higher than the signal, in order to improve image rejection.
In the case of modern television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigial sideband reception, first used with the original NTSC system introduced in 1941. This originally involved a complex collection of tunable inductors which needed careful adjustment, but since the 1970s or early 1980s these have been replaced with precision electromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser milling techniques, SAW filters are cheaper to produce, can be made to extremely close tolerances, and are stable in operation. To avoid tooling costs associated with these components most manufacturers then tended to design their receivers around the fixed range of frequencies offered which resulted in de-facto standardization of intermediate frequencies.
Microprocessor technology allows replacing the superheterodyne receiver design by a software defined radio architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in certain designs, such as very low-cost FM radios incorporated into mobile phones, since the system already has the necessary microprocessor. Radio transmitters may also use a mixer stage to produce an output frequency, working more or less as the reverse of a superheterodyne receiver.
Superheterodyne receivers have superior characteristics to simpler receiver types in frequency stability and selectivity. They offer better stability than Tuned radio frequency receivers (TRF) because a tunable oscillator is more easily stabilized than a tunable amplifier, especially with modern frequency synthesizer technology. IF filters can give narrower pass bands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of a crystal filter when exceptionally high selectivity is necessary. Regenerative and super-regenerative receivers offer better sensitivity than a TRF receiver, but suffer from stability and selectivity problems.
Drawbacks of this design:
High-side and low-side injection:
The amount that a signal is down-shifted by the local oscillator depends on whether its frequency f is higher or lower than fLO. That is because its new frequency is |f − fLO| in either case. Therefore, there are potentially two signals that could both shift to the same fIF; one at f = fLO + fIF and another at f = fLO − fIF. One of those signals, called the image frequency, has to be filtered out prior to the mixer to avoid aliasing. When the upper one is filtered out, it is called high-side injection, because fLO is above the frequency of the received signal. The other case is called low-side injection. High-side injection also reverses the order of a signal's frequency components. Whether that actually changes the signal depends on whether it has spectral symmetry. The reversal can be undone later in the receiver, if necessary.'
Image Frequency (fimage):
One major disadvantage to the superheterodyne receiver is the problem of image frequency. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the superheterodyne receiver
Early Autodyne receivers typically used IFs of only 150 kHz or so, as it was difficult to maintain reliable oscillation if higher frequencies were used. As a consequence, most Autodyne receivers needed quite elaborate antenna tuning networks, often involving double-tuned coils, to avoid image interference. Later super heterodynes used tubes especially designed for oscillator/mixer use, which were able to work reliably with much higher IFs, reducing the problem of image interference and so allowing simpler and cheaper aerial tuning circuitry.
For medium-wave AM radio, a variety of IFs have been used, but usually 455 kHz is used
Local oscillator radiation:
It is difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. The receiver's local oscillator can act like a miniature CW transmitter. This means that there can be mutual interference in the operation of two or more superheterodyne receivers in close proximity. In espionage, oscillator radiation gives a means to detect a covert receiver and its operating frequency. One effective way of preventing the local oscillator signal from radiating out from the receiver's antenna is by adding a shielded and power supply decoupled stage of RF amplification between the receiver's antenna and its mixer stage.
Local oscillator sideband noise:
Local oscillators typically generate a single frequency signal that has negligible amplitude modulation but some random phase modulation. Either of these impurities spreads some of the signal's energy intosideband frequencies. That causes a corresponding widening of the receiver's frequency response, which would defeat the aim to make a very narrow bandwidth receiver such as to receive low-rate digital signals. Care needs to be taken to minimize oscillator phase noise, usually by ensuring that the oscillator never enters a non-linear mode.