Feed-through null
Encyclopedia
Feed-through null follows the duplexer and is commonly used with continuous-wave radar
to improve performance.
The signal that leaks from the transmitter to the receiver is called bleed-through. Electronic circuits have limited dynamic range, and this reduces receiver sensitivity.
Feed-through nulling significantly reduces the bleed-through signal, which can be used to increase receiver sensitivity in continuous wave radar.
The feed-through nuller samples the bleed-through signal arriving at the receiver. This bleed-through sample is amplified, filtered (low-pass), and this is used to drive the attenuator.
Attenuator output is phase shifted 180 degrees added to the receive signal. This cancels the bleed-through signal.
Total gain determines the amount of cancellation.
to reduce low-frequency signals before digital signal sampling, and this includes the bleed-through signal.
The band-pass filter used with a microwave receiver is typically 1 MHz. The band-stop filter used as a feed-through nuller is typically 1KHz, which attenuates signals in the center of the band-pass filter.
The band-stop filter is generally wide enough to reduce clutter, in addition to the bleed-through signal.
Passive feed-through nulling is not used with CW radar altimeters because this eliminates the ground-bounce signal.
Continuous-wave radar
Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects.Continuous wave radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving...
to improve performance.
Definition
The transmitter is constantly issuing high power RF with continuous wave radar. Some of the transmit signal will leak into the receiver when the receive antenna is near the transmit antenna.The signal that leaks from the transmitter to the receiver is called bleed-through. Electronic circuits have limited dynamic range, and this reduces receiver sensitivity.
Feed-through nulling significantly reduces the bleed-through signal, which can be used to increase receiver sensitivity in continuous wave radar.
Active
A sample of the transmit signal is fed to an attenuator.The feed-through nuller samples the bleed-through signal arriving at the receiver. This bleed-through sample is amplified, filtered (low-pass), and this is used to drive the attenuator.
Attenuator output is phase shifted 180 degrees added to the receive signal. This cancels the bleed-through signal.
Total gain determines the amount of cancellation.
Passive
A passive feed-through nuller uses a band-stop filterBand-stop filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter...
to reduce low-frequency signals before digital signal sampling, and this includes the bleed-through signal.
The band-pass filter used with a microwave receiver is typically 1 MHz. The band-stop filter used as a feed-through nuller is typically 1KHz, which attenuates signals in the center of the band-pass filter.
The band-stop filter is generally wide enough to reduce clutter, in addition to the bleed-through signal.
Passive feed-through nulling is not used with CW radar altimeters because this eliminates the ground-bounce signal.