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 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 slow drifts in the radar system, so response
times of this magnitude are reasonable.
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
Note that the feedback slew is based on a frequency error that is derived from a time
averaging process (see burst frequency estimator Settling Time above).
AFC span– [-100%,+100%] maps into [ -32768 , 32767 ]
AFC format– 0:Bin, 1:BCD, 2:8B4D: 0, ActLow: NO
AFC uplink protocol– 0:Off, 1:Normal, 2:PinMap : 1
The AFC loop becomes unstable if a large feedback slope is used 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.
For information on the slope and slew rate parameters, see 5.2.1.1 AFC Motor and Integrator
Option (page 101) .
AFC Output Process
The RVP900 AFC implementation has been generalized so that there is no
dierence
between configuring an analog loop and a digital loop. The AFC feedback loop parameters
are set the same way in each case.
The dierence is the model for how the AFC information is made available to the outside
world. Many interfaces and protocols are possible according to how the AFC parameters are
defined. This flexibility allows easy hookup to any STALO synthesizer.
Chapter 5 – TTY Non-volatile Setups
97
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