USER’S MANUAL__________________________________________________________________
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several kiloHertz. Thus, the AFC feedback loop will typically have a time
constant of several seconds or more.
Most of the burst pulse analysis routines, including the AFC feedback
loop, are inhibited from running immediately after making a pulsewidth
change. The center-of-mass calculations are held off according to the value
of Settling time (to 1%) of burst frequency estimator, and the AFC loop is
held off by the Wait time before applying AFC (see Section 4.2.6 Mb —
Burst Pulse and AFC on page 126). This prevents the introduction of
transients into the burst analysis algorithms each time the pulse width
changes.
Additional information about using AFC can be found in Section 3.4
Digital AFC Module (DAFC) on page 84 and Section 4.2.6 Mb — Burst
Pulse and AFC on page 126.
6.1.4 Burst Pulse Tracking
The RVP900 has the ability to track the power-weighted center-of-mass of
the burst pulse, and to 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, etc.) will be tracked and nulled out during
normal operation; so that fixed targets will remain fixed in range, and clean
Tx phase measurements will always be available on every pulse.
The Burst Pulse Tracker feedback loop makes changes to the trigger timing
in response to the measured position of the burst. Timing changes will
generally be made only when the 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 will be made right
away. Also, there will be an approximately 5ms interruption in the normal
trigger sequence whenever any timing changes are made.
The Burst Pulse Tracker and AFC feedback loop are each fine-tuning
servos that keep the burst pulse "centered" in time and frequency. These
servos have been expanded to include a combined "Hunt Mode" that will
track down a missing burst pulse when we are uncertain of both its time
and frequency. This coarse-tuning mode is especially valuable for
initializing the two 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 either of the following have happened:
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