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 ratiofor a specific receiver . It is measured as the lowest ratio of the power of two signals that will result in the suppression of the weaker signal. 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 during storms and during the reception of electrical noise 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. 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. 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. If  signals are close but not exactly on the same frequency, the  mix will not only have the audio from both carriers, but depending on the carrier separation also include an audible heterodyne "beat note" tone equal to the difference between the carrier frequencies. For instance, if one carrier is at 1000.000 kHz, and the other 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 an adjacent frequency, if the receiver's ultimate bandwidth is wide enough to include the carriers 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 commonly used spacing in the rest of the world. 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 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  .
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. 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 .
A
rapidly
R
the other
another
D
,
R
can reject
rejects
R
the capture ratiofor a specific receiver
its capture ratio
A
,
R
during storms and during the reception of electrical noise
from thunderstorms and electrical
A
,
R
In AM
For AM reception
R
any other signal
signals
R
. So
, so
R
This is
The ability to receive multiple signals simultaneously is in some cases considered beneficial, and is
D
for communications
R
, allowing multiple signals transmitted on the same channel to be heard
for communications
A
AM
A
reception
R
also
will
R
is
transmits
A
mix
R
the carriers
reception
R
commonly used spacing
AM band frequency spacing commonly used
D
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.
A
. 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