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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.
The capture effect is defined as the complete suppression of the weaker signal at the receiver's limiter (if present) where the weaker signal is not amplified, but attenuated. When both signals are nearly equal in strength, or are fading independently, the receiver may rapidly switch from one to another and exhibit picket fencing.
The capture effect can occur at the signal limiter, or in the demodulation stage for circuits that do not require a signal limiter. 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. 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.
Amplitude modulation (AM) immunity to capture effect
Amplitude modulation, or AM radio, transmission does not exhibit this effect. Because AM assumes short term changes in the amplitude to be information, any electrical impulse will be picked up and demodulated along with the desired carrier. Hence lightning causes crashing noises when picked up by an AM radio near a storm. In contrast, FM suppresses short term changes in amplitude, and is therefore much less prone to noise from thunderstorms and electrical impulses. In FM demodulation the receiver tracks the modulated frequency shift of the desired carrier while discriminating against any other signal, since it can only follow the deviation of one signal at a time. For AM reception, the receiver tracks the signal strength of the AM signal as the basis for demodulation. This allows signals to be tracked as just another change in amplitude, so it is possible for an AM receiver to demodulate several carriers at the same time, resulting in an audio mix.
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 glideslope vertical guidance clearance beam is sometimes described as a "capture effect" system, even though it operates using AM signals.
If AM signals are close but not exactly on the same frequency, the reception mix will not only have the audio from both carriers, but depending on the carrier separation will include an audible heterodyne "beat note" tone equal to the difference between the carrier frequencies. For instance, if one carrier transmits at 1000.000 kHz, and the other at 1000.150 kHz, then a 150 Hz "beat frequency" tone mix will result. This mix can also occur when a second AM carrier is received on an adjacent frequency, if the receiver's ultimate bandwidth is wide enough to include reception of both signals. In ITU Region 2 locations, consisting of the Americas, for the AM broadcast band this occurs at 10 kHz; elsewhere it can occur at 9 kHz, the AM band frequency spacing commonly used in the rest of the world. Where such an overlap within the passband occurs, a high pitched heterodyne whistle at precisely 9 or 10 kHz can be heard. This is particularly common at night when signals from adjacent frequencies travel long distances due to skywave. Modern SDR-based receivers can eliminate this by utilizing a brick-wall filter narrower than the channel spacing, that reduces signals outside the passband to inconsequential levels.
For digital modulation schemes it has been shown that for properly implemented on-off keying/amplitude-shift keying systems, co-channel rejection can be better than for frequency-shift keying systems.
- "Armstrong Soon to Start Staticless Radio", Broadcasting, February 1, 1939, page 19.
- This article incorporates public domain material from the General Services Administration document: "Federal Standard 1037C". (in support of MIL-STD-188)