USER’S MANUAL__________________________________________________________________
136 _________________________________________________________________ M211322EN-D
The DC–Coupled AFC loop questions (changes shown in bold) are:
AFC Servo– 0:DC Coupled, 1:Motor/Integrator : 0
Wait time before applying AFC: 10.0 sec
AFC hysteresis– Inner: 5.0 KHz, Outer: 15.0 KHz
AFC outer tolerance during data processing: 50.0 KHz
AFC feedback slope: 0.0100 D–Units/sec / KHz
AFC minimum slew rate: 0.0000 D–Units/sec
AFC maximum slew rate: 0.5000 D–Units/sec
and the Motor/Integrator loop questions are:
AFC Servo– 0:DC Coupled, 1:Motor/Integrator : 1
Wait time before applying AFC: 10.0 sec
AFC hysteresis– Inner: 5.0 KHz, Outer: 15.0 KHz
AFC outer tolerance during data processing: 50.0 KHz
AFC feedback slope: 1.0000 D–Units / KHz
AFC minimum slew request: 15.0000 D–Units
AFC maximum slew request: 90.0000 D–Units
Notice that the physical units for the feedback slope and slew rate limits
are different in the two cases. In the DC–Coupled case the AFC output
voltage controls the frequency directly, so the units for the feedback and
slew parameters use D–Units/Second. In the Motor/Integrator case, the
AFC output determines the rate of change of frequency; hence D–Units are
used directly.
The above example illustrates typical values that might be used with a
Motor/Integrator servo loop. The feedback slope of 1.0 D–Units/KHz
means that a frequency error of 100KHz would produce the full–scale (100
D–Units) AFC output. But this is modified by the minimum and maximum
slew requests as follows:
-A zero D–Unit output will always be produced whenever AFC is
locked.
- When AFC is tracking, the output drive will always be at least ±15 D–
Units. This minimum non–zero drive should be set to the sustaining
drive level of the motor actuator, that is, the minimum drive that
actually keeps the motor turning.
- When AFC is tracking, the output drive will never exceed ±90 D–
Units. This parameter can be used to limit the maximum motor speed,
even when the frequency error is very large.
The AFC Motor/Integrator feedback loop works properly even if the motor
has become stuck in a "cold start", that is, after the radar has been turned
off for a period of time. The mechanical starting friction can sometimes be
larger than normal, and additional motor drive is required to break out of
the stuck condition. But once the motor begins to turn at all, then the
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