Superheterodyne receiver

In electronics, a superheterodyne receiver is a radio wave receiver that uses a frequency mixing or heterodyning process to convert the received signal into a fixed intermediate frequency signal, which can be more suitably elaborated (filtered and amplified) than the radio frequency of the original carrier which provides a fixed level of sensitivity and selectivity. Virtually all modern radio and television receivers use the superheterodyne principle.

History

“Superheterodyne” is a contraction of "supersonic heterodyne", where "supersonic" indicates frequencies above the range of human hearing. The word heterodyne is derived from the Greek roots hetero- "different", and -dina "might". In radio applications the term derives from "heterodyne detector" developed by Canadian inventor Reginald Fessenden in 1907, describing his proposed method of producing an audible signal from Morse code transmissions from an Alexanderson alternator-type transmitter. With the spark gap transmitters then in use, the Morse code signal consisted of short bursts of a highly modulated carrier wave, which could be clearly heard as a series of short chirps or buzzes in the receiver's headphones. However, the signal from an Alexanderson alternator had no such inherent modulation and the Morse Code from one of them could only be heard as a series of clicks or thumbtacks. Fessenden's idea was to use two Alexanderson alternators, one producing a carrier frequency 3 kHz higher than the other. In the receiver's detector, the two carriers could beat together to produce a 3 kHz tone, although in headphones the morse signals would then be heard as a series of 3 kHz beeps. For this, he coined the term & # 34;heterodyne & # 34; which means "generated by a difference" (in frequency).

Features

The superheterodyne receiver performs almost all of the amplification of the constant frequency called intermediate frequency, or IF, using a fixed frequency, thus achieving more precise adjustments in the circuits and taking advantage of everything that the component used can give (thermionic valve, transistor or integrated circuit). It was invented in 1917 by Edwin Howard Armstrong, also the inventor of the regenerative circuit, the super-regenerative receiver, and frequency modulated (FM) radio broadcasting.

In home AM (Amplitude Modulation) receivers, the intermediate frequency is 455 or 470 kHz; in Frequency Modulated (FM) receivers, it is typically 10.7 MHz. Superheterodyne receivers mix or heterodyne a frequency generated in a local oscillator (Floc), contained in the receiver, with the signal incoming on-air (Fant). Two frequencies result from this heterodyning: one higher (Fant + Floc) and one lower (Floc - Fant) than the incoming frequency. One of them, normally the lower one, is chosen as IF (intermediate frequency), filtered with a high Q quality factor filter, amplified and subsequently detected or demodulated to obtain the audio frequency that will be heard, after being suitably amplified, through of a speaker (speaker). The user tunes the receiver by adjusting the local oscillator frequency (Floc) and tuning the incoming signals (Fant).

In most receivers these adjustments are made simultaneously, acting on a variable capacitor with two sections in tandem, that is, coupled on the same axis. One of the sections of this capacitor is part of the local oscillator circuit and the other part of the tuning of the incoming signal, in such a way that when the frequency tuned at the input is varied, the frequency of the local oscillator is also varied, keeping the frequency constant. difference between the two, which is the intermediate frequency (IF). This effect is called "drag".

Currently, almost all receivers use this method. The following diagram shows the basic elements of a single conversion superheterodyne receiver. In practice not all designs will have all the elements of this scheme, nor does it cover the complexity of others, but the essential elements, a local oscillator, a mixer followed by a filter and an IF amplifier , are common to all superheterodyne receivers.

Diagram of a typical superheterodine receiver

• In the superheterodine receiver RF filter/ amplifier (radiofrequency) isolates the signal we wish to receive from the rest of the signals that arrive at the antenna. This filter is generic, so it has little selectivity in frequency.
• The mixer runs the frequency spectrum of the leaked signal, focusing it around the “Intermediate frequency” (fin).
• To move the spectrum, the mixer uses the ascending or descending conversion component (upconverter or downconverter), as appropriate.
• The intermediate frequency filter perfectly isolates the signal to demodular, as it is a high selectivity filter in frequency.
• The detector demodulates the intermediate frequency signal (i.e., recovers the spectrum from the original signal) and the Amplifier gives the exit signal the gain you need.

Advantages and disadvantages of the system

• Most of the radio signal path must be sensitive only to a narrow range of frequencies. Only the part before the conversion stage (the one between the antenna and the mixer) needs to be sensitive to a wide range of frequencies.

As an example, in an AM receiver you might need to be efficient in a range of 1 to 30 MHz, while the rest of the receiver would only need a correct response to the FI, this is to 460 or 470 kHz, depending on the case.

• Another advantage is that improper couplings between steps by parasitic capabilities generated by cables and printed circuit tracks are avoided when using a constant frequency.
• An important disadvantage of these systems is that there is the possibility of demoduleting the image frequency if the rules governing the radio-electric space of a given geopolitical zone are not known.

Double conversion superheterodyne

Sometimes, to overcome obstacles such as the phenomenon called image frequency or image response, more than one IF is used. In such cases, the first part of the receiver should be sensitive to a band from 1 to 30 MHz, as in the previous case, the next stage to 5 MHz (1st IF) and the last one to 50 kHz (2nd IF). Two converters are used and the receiver thus designed is called Double conversion superheterodyne. Frequently, 10.7 MHz is chosen as the first intermediate frequency, and 455 kHz as the second. To obtain 455 kHz from 10.7 MHz, the first IF is mixed with a signal from a fixed local oscillator at 10.245 MHz. This frequency is usually fixed by a quartz crystal.

There are also triple and quadruple conversion superheterodynes.

Advantages over previous systems

Previously used tuned radio frequency receivers suffered from a lack of frequency stability and very poor radio selectivity, since, even using filters with a high Q quality factor, they had a too large bandwidth in the range of radio frequencies. Superheterodyne receivers have superior characteristics, both in selectivity and in frequency stability. It is much easier to stabilize an oscillator than a filter, especially with modern frequency synthesizer technology, and IF filters can have a much narrower passband for the same Q factor than an equivalent RF (radio frequency) filter. A fixed IF allows the use of glass filters in very critical designs such as radiotelephone receivers, which must have extremely high selectivity.

Superheterodyne transmitters

Superheterodyne technology is also applied to radio transmitters. The design of a superheterodyne transmitter is similar to that of the receiver, except that the signal stages are arranged in a reverse path.

Current Designs

Microprocessor technology allows superheterodyne receiver design to be replaced 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 FM radios built into mobile phones, since these systems already have the necessary microprocessor.

Radio transmitters can also use a mixing stage to produce an output frequency, which works more or less like the inverse of a superheterodyne receiver.