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Detectors#

What is a detector?#

In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound (an audio signal) on an uninterrupted carrier wave, early transmitters transmitted information by radiotelegraphy; the transmitter was switched on and off to produce pulses of radio waves which spelled out text messages in Morse code. Therefore early radio receivers only had to detect the presence or absence of the radio wave. The device which did this was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector were used during the wireless telegraphy era until superseded by vacuum tube technology.

After sound (amplitude modulation, AM) transmission began around 1920, the term evolved to mean a demodulator, (usually a vacuum tube) which extracted the audio signal from the radio frequency carrier wave. This is its current meaning, although modern detectors usually consist of semiconductor diodes, transistors, or integrated circuits.

In a superheterodyne receiver the term is also sometimes used to refer to the mixer, the tube or transistor which converts the incoming radio frequency signal to the intermediate frequency. The mixer is called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector.

In microwave and millimetre wave technology the terms detector and crystal detector refer to waveguide or coaxial transmission line components, used for power or SWR measurement, that typically incorporate point contact diodes or surface barrier Schottky diodes.

A Simulation Of A Simple Diode Amplitude Modulation Detector#

AM diode detector - YooFab

The above circuit#

In the above circuit we see the diode recovering the 50 kHz modulated signal from the 1 MHz carrier wave. Essentially, the diode chops one half off the wave which has amplitude modulation in both the positive and negative quadrants. Apart from the fact that 50 kHz is well above human hearing, trying to listen for this 50 kHz signal without the diode is useless because the negative side completely cancels out the positive going side within the electromagnetic AM wave.

In this simulation after the diode there is a simple filter circuit that lets us see an approximation of the recovered signal.

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