A Citizen's Band transceiver equipped with a noise blanker for the receiver, controlled by a button on the front panel (2nd from right)
In the
design of radio receivers, a noise blanker is a circuit intended to reduce the effect of certain kinds of radio noise on a received signal. It is often used on broadcast
shortwave receiver
A shortwave radio receiver is a radio receiver that can receive one or more shortwave bands, between 1.6 and 30 MHz. A shortwave radio receiver often receives other broadcast bands, such as FM radio, Longwave and Mediumwave. Shortwave radio recei ...
s or
communications receiver
A communications receiver is a type of radio receiver used as a component of a radio communication link. This is in contrast to a ''broadcast receiver'' which is used to receive radio broadcasts. A communication receiver receives parts of the r ...
s and some types of two-way radio
transceiver
In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. Thes ...
s. The noise blanker is only effective on impulse-type noise such as from lightning or from automotive ignition systems, and cannot improve performance on wideband continuous background noise, or interfering signals on the same frequency. In cases where there are strong signals on frequencies near to the desired frequency, a noise blanker circuit may be ineffective and may reduce the quality of the received signal.
Implementation
Typically this is a network in the
intermediate frequency section of the receiver; when a pulse of noise passes through the
IF amplifiers, it is usually of greater amplitude than the desired signal. The noise blanker circuit momentarily reduces the gain of the IF stage during the impulse.
More complex noise blankers may use a secondary IF stage and have adjustable threshold and timing characteristics so as to reduce the noise passed through to the audio stages of the receiver.
A noise blanker is best applied before any narrow-bandwidth filters in the signal path, so as not to introduce "ringing" and distortion in the filtered signal. Noise blankers are most useful with
amplitude modulation or
single sideband
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude m ...
signals.
Frequency modulation
Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing.
In analog fre ...
receivers generally include a signal limiter stage which tends to reject noise pulses.
References
Radio electronics
Receiver (radio)
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