Detector (radio)
The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code.
After the invention of amplitude modulation (AM) enabled the development of AM radiotelephony, the transmission of sound (audio), during World War 1, 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 microwave and millimeter 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.
The limited frequency response of the headset eliminates the RF component, making the low pass filter unnecessary.
They can be implemented using something as simple as ring of diodes or a single dual-gate Field Effect Transistor to anything as sophisticated as an Integrated Circuit containing a Gilbert cell.
Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.
The detector consists of a special center-tapped transformer feeding two diodes in a full wave DC rectifier circuit.
A ratio detector is a variant of the Foster–Seeley discriminator, but one diode conducts in an opposite direction, and using a tertiary winding in the preceding transformer.
Now the output from the phase detector will differ from zero, and in this way, one recovers the original signal that was used to modulate the FM carrier.
The XOR gate produces a stream of output pulses the duty cycle of which corresponds to the phase difference between the two signals.
Due to the varying phase difference between the two inputs, a pulse-width modulated (PWM) signal is produced.
With a fixed-frequency carrier, the result is a phase demodulation, which, in this case is an integral of the original modulating signal.
