Detector (radio)

Lore & Background

Early radio stations transmitted information by radiotelegraphy, switching the transmitter on and off to produce Morse code. Early receivers merely had to detect the presence or absence of the radio wave, and the device that performed this function was called a detector. A variety of detector devices were used during the wireless telegraphy era, including the coherer, electrolytic detector, magnetic detector, and crystal detector, until superseded by vacuum tube technology. After the invention of AM radiotelephony during World War 1, the term evolved to mean a demodulator, usually a vacuum tube, which extracted the audio signal from the carrier wave. In a superheterodyne receiver, the mixer is sometimes called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector. In microwave and millimeter wave technology, the terms detector and crystal detector refer to waveguide or coaxial components used for power or SWR measurement, typically incorporating point contact diodes or Schottky diodes.

Reader's Guide

The detector is a foundational concept in radio, evolving from simple presence-or-absence detection in early radiotelegraphy to sophisticated demodulation in modern communications. Several types of AM detectors, including the envelope detector (such as the diode detector and crystal detector) and the product detector, which is preferred by shortwave listeners and radio amateurs for receiving both AM and SSB signals. The quadrature detector splits the FM signal, shifts one path by 90 degrees, and uses a phase detector to recover modulation. AM detectors cannot demodulate FM and PM signals, though slope detection can provide crude FM reception with inferior quality. The legacy of the detector lies in its central role in all radio receivers, from early crystal sets to modern superheterodyne and microwave systems.

Did You Know?

• In a superheterodyne receiver, the mixer is called the first detector and the demodulator is called the second detector.
• The ratio detector has wider bandwidth but more distortion than the Foster–Seeley discriminator.