Capture effect

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In telecommunications, 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 will be demodulated.

The capture effect is defined as the complete suppression of the weaker signal at the receiver limiter (if it has one) where the weaker signal is not amplified, but attenuated. When both signals are nearly equal in strength, or are fading independently, the receiver may switch from one to the other 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.[citation needed] Some types of radio receiver circuits have a stronger capture effect than others. The measurement of how well a receiver can reject a second signal on the same frequency is called the capture ratio for a specific receiver. It is measured as the lowest ratio of the power of two signals that will result in the suppression of the smaller signal.

Amplitude modulation, or AM radio, transmission is not subject to this effect. This is one reason that the aviation industry, and others, have chosen to use AM for communications rather than FM, allowing multiple signals to be broadcast on the same channel. Similar phenomena 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.[citation needed]

Amplitude modulation immunity to capture effect[edit]

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. In AM, the receiver tracks the signal strength of the AM signal as the basis for demodulation. This allows any other signal 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.

If the signals are close but not exactly on the same frequency the mix will not only include the audio from both carriers but depending on the carrier separation a whistle might be heard as well representing the difference in the carrier frequencies. This mix can also occur when an AM carrier is received on a channel that is adjacent to the desired channel. The resulting overlap forms the high pitched whistle (about 10 Kilohertz) that can often be heard behind an AM station at night when other carriers from adjacent channels are traveling long distances due to atmospheric bounce.

Since 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. FM radios suppress short term changes in amplitude and are therefore much less prone to noise during storms and during reception of electrical noise impulses.

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.

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