Vaisala RVP900

Chapter 4 _____________________________________________________ TTY Nonvolatile Setups
VAISALA______________________________________________________________________ 131
make sense for the AFC loop to be running during this time since there is
nothing gained by chasing the startup transient. This question allows you
to set a holdoff delay from the time that valid burst pulses are detected to
the time that the AFC loop actually begins running.
Limits: 0 s to 300 s
AFC hysteresis –– Inner: 5.0 KHz, Outer: 15.0 KHz
These are the frequency error tolerances for the AFC loop. The loop will
apply active feedback whenever the outer frequency limit is exceeded, but
will hold 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 will remain constant most of the time;
making small and brief adjustments only occasionally as the need arises.
AFC outer tolerance during data processing: 50.0 KHz
In general, the AFC feedback loop is active only when the RVP900 is not
processing data rays. This is because the Doppler phase measurements are
seriously degraded whenever the AFC control voltage makes a change. To
avoid this, the AFC loop is only allowed to run in 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; but if the RVP900 were
constantly busy, the AFC loop would never have a chance to run. This
question allows you to place an upper bound on the frequency error that is
tolerated during sustained data processing. AFC is guaranteed to be
applied whenever this limit is exceeded.
Limits: 15 KHz to 4000 KHz
AFC feedback slope: 0.0100 D-Units/sec / KHz
AFC minimum slew rate: 0.0000 DUnits/sec
AFC maximum slew rate: 0.5000 D-Units/sec
These questions control the actual feedback computations of the AFC loop.
The control that is applied to the AFC is specified here in “D-Units”, i.e.,
arbitrary units ranging from –100 to +100 corresponding to the complete
span. Since the D–Unit corresponds in a natural way to a percentage scale,
the shorter “%” symbol is sometimes used.
AFC feedback is applied in proportion to the frequency error that the
algorithm is attempting to correct. The feedback slope determines the
sensitivity and time constant of the loop by establishing the AFC rate of
change in (D–Units/sec) per thousand Hertz of frequency error. For
example, a slope of 0.01 and a frequency error of 30 KHz results in a
control voltage slew of 0.3 D–Units per second. At that rate it would take