Capture effect
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In a radio receiver, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency or channel will be demodulated.
FM phenomenon
The capture effect is defined as the complete suppression of the weaker signal at the receiver's
The capture effect can occur at the signal limiter, or in the demodulation stage for circuits that do not require a signal limiter.[citation needed] Some types of radio receiver circuits have a stronger capture effect than others. The measurement of how well a receiver rejects a second signal on the same frequency is called its capture ratio. It is measured as the lowest ratio of the power of two signals that will result in the suppression of the weaker signal.
The capture effect phenomenon was first documented in 1938 by General Electric engineers conducting test transmissions. Two experimental FM stations, located 15 miles (24 km) apart in Albany and Schenectady, New York, were configured to transmit on the same frequency, in order to study how this would affect reception. It was determined that, for most of the path between the two stations, only one of the signals could be heard, with the complete elimination of the other. It was concluded that this effect occurred whenever the stronger signal was about twice as strong as the weaker one.[1] This was significantly different than the case with amplitude modulation signals, where the general standard for broadcasting stations was that to avoid objectionable interference the stronger signal had to be about twenty times that of the weaker one. The capture effect thus allowed co-channel FM broadcasting stations to be located somewhat closer to each other than AM ones, without causing mutual interference.
AM immunity
Amplitude modulation, or
The ability to receive multiple signals simultaneously is in some cases considered beneficial and is one reason that the aviation industry, and others, have chosen to use AM rather than FM for communications.
Phenomena similar to the capture effect are described in AM when offset carriers of different strengths are present in the passband of a receiver. For example, the aviation
Digital modulation
For
See also
Notes
- brick-wall filternarrower than the channel spacing, which reduces signals outside the passband to inconsequential levels.
References
- ^ "Armstrong Soon to Start Staticless Radio", Broadcasting, February 1, 1939, page 19.
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).