Rev B
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subject to similar constraints! Thus the first line of defense against noise is to reduce the receiver
gain.
Some forms of noise are naturally "bursty", like mobile voiceband messages or satellite
transmissions. Here the best remedy is for the receiver to attempt to reject signals with
inappropriate time "signatures". This is the function of the pulse interval window (see below).
Setting the window boundaries tightly around the expected pulse interval of the transmitter will
help prevent bursts of noise from being reported as signals. It will also help relieve congestion in
the processor, since invalid events take less time to process than legitimate ones.
Both gain reduction and time interval filtering have limited usefulness if the dominant noise
source is "impulsive". Engine noise of all kinds falls into this category. Impulsive noise is
characterized by repetitive, but typically very narrow pulses, each with sufficient peak power to
be recognized by the receiver even though the average noise power may be well below the level
of the desired signal. In such cases the time interval window must be opened (to include
intervals on the order of the noise period) and signals and noise distinguished on the basis of
pulse duration. In the SRX_400 this is accomplished by delaying the measurement of signal
strength long enough for a typical impulse to have decayed completely before the measurement
occurs.
Different software (and hardware) versions use different strategies, but all SRX_400 data
collection programs employ automatic gain reduction, noise blanking (or delayed power
measurements) and time window discrimination in one form or another. In the examples below,
we use two versions of the data acquisition program
Event_Log
, which is designed to recognize
transmitters coded by pulse rate and by pulse repetition.
The following record illustrates the behaviour of the adaptive gain control (AGC) mechanism
currently available in Event_Log version W21 and, with slightly different convergence
properties, in Code_Log (W16).
ADAPTIVE GAIN CONTROL
(A Real Example)
This experiment looked at two frequencies, each of which exhibited a different kind
of noise problem.
Frequencies
Signal / Noise Environment
149.660 MHz
Strong trans computer noise + strong intermittent
interference from a voice communication channel.
149.600 MHz
A weak transmitter at or below the level of several coded
transmitters (seen as burst noise by Event_Log) + computer noise .
Two groups of switched antennas were used, as follows:
Antenna Groups
Auxiliaries in Group
Programmed Gains
M0 (=A3)
A1, A2
M=50, A1=50, A2=30
M1 (=A6)
A4, A5
M=30, A4=50, A5=30
With adaptive gain control enabled, a test sequence was run as illustrated in the
following table. Each row of the table represents one complete scan cycle (2
frequencies and 2 antenna groups, each with two auxiliary antennas. Table entries
represent the values of the receiver gain set by the AGC algorithm, and reported on
