Difference between revisions of "Superheterodyne Receiver"

Revision as of 13:43, 12 February 2016

The Supersonic Heterodyne receiver, or Superheterodyne receiver 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. At the cost of an extra frequency converter stage, the superheterodyne receiver provides superior selectivity and sensitivity compared with simpler designs. Superheterodyne receivers have better performance because the components can be optimized to work a single intermediate frequency, and can take advantage of arithmetic selectivity.

So, the incoming radio signal is mixed with a local oscillator to produce sum and difference frequency components. The lower frequency difference component called the intermediate frequency (IF), is separated from the other components by fixed tuned amplifier stages set to the intermediate frequency. The tuning of the local oscillator is mechanically ganged to the tuning of the signal circuit or radio frequency (RF) stages so that the difference intermediate frequency is always the same fixed value.

The superheterodyne design is nearly (or may already be depending on when you are reading this) 100 years old. Although it has been around a long time, the design is still the most widely used today. New semiconductor technology and high levels of integration have kept the superheterodyne architecture vitalized and in popular use both in the transmit and receive application.

How it works

The antenna collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity and prevent strong out-of-passband signals from saturating the initial amplifier. A local oscillator provides the mixing frequency. The oscillator is typically a variable frequency oscillator which is used to tune the receiver to different stations. The frequency mixer then changes the incoming radio frequency signal to a higher or lower, fixed, intermediate frequency (IF). The IF band-pass filter and amplifier supply most of the gain and the narrowband filtering for the radio. The demodulator extracts the audio or other modulation from the IF radio frequency. Finally, the extracted signal is then amplified by the audio amplifier.

Superheterodyning

Heterodyning is the mixing of two frequencies together so as to produce a beat frequency. With AM (Amplitude Modulation) the information signal is mixed with the carrier to produce the side-bands. The side-bands occur at precisely the sum and difference frequencies of the carrier and information.

Superheterodyning is creating a beat frequency that is lower than the original signal. The lower side band and the difference between the other of the mixed frequencies is superheterodyning. What superheterodying does is to purposely mix in another frequency in the receiver, so as to reduce the signal frequency prior to processing.

History

Major Edwin Howard Armstrong is credited with developing the first practical superheterodyne receiver design. While conducting military work in France during WWI Armstrong developed an approach that first converted the signal to a fixed intermediate frequency, which was then processed and amplified. His work began in 1918 and by 1920 the superheterodyne design started to replace Tuned Radio Frequency receivers. His approach first converted the signal to a fixed intermediate frequency, which was then processed and amplified which allowed reception of a much broader frequency range, in particular providing superior reception of signals higher than 1500 kHz.

The development of high quality vacuum-tubes during WW1 made possible great improvements to the superheterodyne receiver.

Alternatives

TRF

Before radios implemented the superheterodyne design most of the available radios on the market were Tuned Radio Frequency (TRF) receivers. In an early TRF receiver there were tuned circuits separated by the radio frequency (RF) amplifier stages and the last tuned circuit feeded the AM detector stage. The individual tuning capacitors were attached to separate tuning dials. Each had to be reset each time a different station was selected.

Homodyne

The homodyne is a direct conversion front-end design that eliminates 1/3 of the components necessary for the superheterodyne receiver design. ZIF, or Zero-IF is the homodyne attracting industry attention as a means to greatly reduce cost. However, due to problems with ZIF, it is typically not considered a beneficial tradeoff.

Direct conversion receivers perform the RF to baseband frequency translation in a single step. The RF signal is mixed with a local oscillator at the carrier frequency thus eliminating any image components to cause signal corruption.

Today these homodyne or direct conversion receivers are typically only used where modulation methods do not put any significant signal energy near direct current. The cellular phone industry in particular has taken interest in direct conversion both for transmit and receive. Currently, direct conversion is found in user terminals for cellular communications.

Super-regenerative

An innovative receiver design that looked promising when introduced by superheterdyne innovator Major Edwin Howard Armstrong. It consisted of an amplifying vacuum tube with its output connected to its input through a feedback loop, providing positive feedback. It had a bad tendency to emit radio interference and turned out to be too complicated and limited to be broadly adopted.

Some sources have this invention as being in 1912, while others state introduced it in 1922 after returning to the Institute of Radio Engineers in New York. Wikipedia states he patented the design in 1914 white he was an undergraduate at Columbia University. What is clear is that his superheterodyne reciver is far more practical then was the super-regenerative.

Neutrodyne

The neutrodyne design was inferrior to superheterdyne immediately upon its inception. Even though inferror, companies in the early 1920s used the neutrodyne design to produce radios because RCA radio company held a patent on the superheterodyne receiver and were too greedy to license the design to other manufacturers.

Neutrodyne receivers solved some problems associated with earlier TRF receivers including solving the well known squealing problem during tuning. Consumer neutrodyne receivers had a recognizable three dial tuning arrangement. They were less expensive to produce than the superheterodyne receiver. However due to inferrior performance and the eventual licensing of the superheterodyne design by RCA to other manufacturers, the neutrodyne receiver quickly disappeared.

Crystodyne

This is a term for a crystal radio, which is a radio receiver powered only by the incoming signal. It uses a crystal detector and is considered the most simple radio receiver design. The term crystodyne has sometimes been used mistakenly as an alternative to the superheterodyne receiver. In reality, a crystodyne radio can use the superheterodyne receiver design.

Russian inventor Oleg Losev is credited for the crystal radio and used a zinc oxide crystal as a detector to produce the first crystal radios as both regenerative and superheterodyne receivers. In the most simple form, the crystodyne receiver does not use a superheterodyne circuit. The fewer the components the more efficient the crystal radio can operate. With so little power to work with, the elimination of components can be necessary to produce audio loud enough for the human ear to detect.

Simple crystal radio design.

What about SDR

It is sometimes mistakenly stated that Software Defined Radio is something different from the superheterodyne receiver design. This is not true. Today's SDRs still use superheterodyne. This is due to the consistent performance a superheterodyne receiver offers across a large range of frequencies while maintaining good sensitivity and selectivity.

Modern SDR receivers are using variations in traditional superheterodyne. They do not "sample" the antenna directly. Despite some appeal, the data converters are not directly on the antenna. It is simply not practical to do it this way. An analog front-end remains necessary before the ADC in the receive path and after the DAC in the transmit path that does the appropriate frequency translation, this is where superheterdyning is being used.