This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (April 2012) (Learn how and when to remove this template message)
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 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. 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 transmitted on the same channel to be heard. 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.
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 an audible tone (a beat signal) may be heard at a frequency equal to the difference in the carrier frequencies involved. For instance, if one carrier is at 1000.000 kHz, and the other is at 1000.150 kHz, then a 150 Hz "beat frequency" tone will result.
This mix can also occur when a second AM carrier is received on a channel that is adjacent to the desired channel if the receiver's ultimate bandwidth is wide enough to include the carriers of both signals. In the US AM broadcast bands this occurs at 10 kHz, which is the US channel spacing for the AM broadcast band. Elsewhere it can occur at 9 kHz, a commonly used channel spacing in many locales.
Modern SDR-based receivers can completely eliminate this by utilizing a brick-wall filter narrower than the channel spacing that reduces signals outside the passband to inconsequential levels. Where such an overlap within the passband occurs, a high pitched whistle at precisely 9 or 10 kHz can be heard. This is particularly common at night when other carriers from adjacent channels are traveling long distances due to skywave.
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 during storms and during the 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.