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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.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.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 fed 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. Consumer multistage TRF radios suffered from poor selectivity and required several tuning knobs in a row. [[File:twostagetrfreceiver.png]]<BR>''Two Stage TRF Receiver''Today these homodyne or direct conversion receivers are typically only used where modulation methods do not put any significant signal energy near direct current.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.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.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.[[File:crystalradiosimple02.png]]<BR>''Simple crystal radio design.''Crystodyne was not introduced as an alternative to superheterodyne, but rather an alternative to the vacuum tube being used in a radio design. The crystal radio operated without the vacuum tune, using a solid state crystal which has led to the innovation of the transistor which eventually replaced vacuum tunes in radios.== What about SDR ==It is sometimes mistakenly stated that Software Defined Radio is something different from the superheterodyne receiver design or does not require superheterodyning. 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 (Analog to Digital Converter) in the receive path and after the DAC (Digital to Analog Converter) in the transmit path that does the appropriate frequency translation, this is where superheterdyning is being used.[[Category:Electronics]][[Category:Radio]]