This type of servo loop is required for tuned magnetron systems in which the tuning
actuator is moved back and forth by a motor, but remains fixed in place when motor drive is
removed. These systems require that the AFC output voltage (motor drive) be 0 when the
loop is locked and that the voltage be proportional to frequency error while tracking. See
5.2.1.1 AFC Motor and Integrator Option (page 101).
AFC Servo– 0:DC Coupled, 1:Motor/Integrator : 0
AFC Loop
After turning on a magnetron transmitter, it can take several seconds or minutes until the
output frequency is stable. Because there is nothing gained by chasing the startup transient,
the AFC loop does not need to run during this time. Use the following parameter to set a
holdo delay from the time that valid burst pulses are detected to the time that the AFC
loop begins running.
In general, the AFC feedback loop is active only when RVP900 is not processing data rays.
This is because the Doppler phase measurements seriously degrades when the AFC control
voltage makes a change. To avoid this, the AFC loop can only run between intervals of
sustained data processing. This is
fine as long as the host computer allows a few seconds of
idle time every few minutes. If the RVP900 were constantly busy, the AFC loop would never
have a chance to run.
The loop applies active feedback when the outer frequency limit is exceeded, but holds a
fixed level once the inner limit has been achieved. The hysteresis zone minimizes the amount
of thrashing done by the feedback loop. The AFC control voltage remains constant most of
the time; making small and brief adjustments only occasionally as the need arises.
The outer tolerance
defines the frequency error tolerances for the AFC loop by placing an
upper bound on the frequency error that is tolerated during sustained data processing. AFC
is applied when this limit is exceeded.
Wait time before applying AFC: 10.0 sec
AFC hysteresis –– Inner: 5.0 KHz, Outer: 15.0 KHz
Limits: 0 ... 300 s
AFC outer tolerance during data processing: 50.0 KHz
Limits: 15 ... 4000 KHz
AFC Loop Feedback Computations
The control applied to the AFC is
specified in D-Units, that is, arbitrary units ranging from
–100 ... +100 corresponding to the complete span.
Since the D-Units correspond to a percentage scale, the shorter % symbol is sometimes
used.
RVP900 User Guide M211322EN-J
96
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