User's Manual

RVP8 User’s Manual
October 2005
TTY Nonvolatile Setups
3–27
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 RVP8 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 RVP8 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 to 4000 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
These questions control the actual feedback computations of the AFC loop.
The overall span of the AFC output voltage is set by Gain and Offset potentiometers
on the RVP8/IFD module (See Section 2.2.11). The control level that is applied to
the AFC’s 16-bit Digital-to-Analog converter is specified here in “D-Units”, i.e.,
arbitrary units ranging from –100 to +100 corresponding to the complete span of the
D/A converter. Since the D–Unit corresponds in a natural way to a percentage scale,
the shorter “%” symbol is sometimes used.
AFC feedback will be 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’s 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 30KHz would result in a control voltage slew of 0.3 D-Units per second. At
that rate it would take approximately 67 seconds for the output voltage to slew one
tenth of its total span (20 D-Units / (0.3 D-Units / sec) = 67 sec). AFC is intended to
track very slow drifts in the radar system, so response times of this magnitude are
reasonable.
Keep in mind that the feedback slew is based on a frequency error which itself is
derived from a time averaging process (see Burst Frequency Estimator Settling Time
described above) . The AFC loop will become unstable if a large feedback slope is
used together with a long settling time constant, due to the phase lag introduced by
the averaging process. Keep the loop stable by choosing a small enough slope that
the loop easily comes to a stop within the inner hysteresis zone.
See Section 3.2.6.1 for more information about these slope and slew rate parameters.