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7.2.4 Burst Pulse Tracking
RVP900 can track the power-weighted center-of-mass of the burst pulse and automatically
shift the trigger timing so that the pulse remains in the center of the burst analysis window
of the
Pb plot.
This means that external sources of drift in the timing of the transmitted pulse (temperature,
aging, and so on) are tracked and nulled out during normal operation so that fixed targets
remain fixed in range and clean Tx phase measurements are always available on every pulse.
The burst pulse tracker feedback loop changes the trigger timing in response to the
measured position of the burst. Timing changes are generally made only when RVP900 is
not actively acquiring data, in the same way that AFC feedback is held off for similar quiet
times.
However, if the center-of-mass has drifted more than 1/3 the width of the burst analysis
window, then the timing adjustment is done immediately. Also, there is an approximately
5 ms interruption in the normal trigger sequence when any timing changes are made.
The burst pulse tracker and AFC feedback loop are fine-tuning servos that keep the burst
pulse centered in time and frequency. These servos include a combined hunt mode that
tracks down a missing burst pulse when we are uncertain of both its time and frequency.
This coarse-tuning mode is valuable for initializing the 2 fine-tuning servos in radar systems
that drift significantly with time and temperature.
When the radar transmitter is on but the burst pulse is missing, it may be because it is either:
• Misplaced in time.
The Tx pulse is outside of the window displayed in the
Pb
plotting command.
The trigger timing must be changed to bring the center of the pulse back to the center
of the window.
• Mistuned in frequency.
The AFC feedback is incorrect and has caused the burst frequency to fall outside of the
passband of the RVP900 anti-alias filters.
The AFC (or DAFC) must be changed to restore the correct tuning.
The hunt mode performs a 2D search in time and frequency to locate the burst by searching
across a ±20 μsec time window, and across the entire AFC span. If a valid Tx pulse (that is,
meeting the minimum power requirement) is found anywhere in those intervals, then the
Burst Pulse Tracker and AFC loops are initialized with the time and frequency values that
were discovered. The fine servos then start running with a good burst signal starting from
those initial points.
Depending on how the hunting process has been configured in the
Mb menu, the procedure
may take several seconds to complete. The RVP902 host computer interface remains
functional during this time, but any acquired data is questionable.
GPARM
status bits in
word
#55
indicate when the hunt procedure is running, and whether it has completed
successfully.
More Information
‣
Hunt for Burst Pulse (BPHUNT) (page 312)
Chapter 7 – Processing Algorithms
177