Vaisala RVP900

USER’S MANUAL__________________________________________________________________
132 _________________________________________________________________ M211322EN-D
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 becomes
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 4.2.6.1 AFC
Motor/Integrator Option on page 134 for more information about these
slope and slew rate parameters.
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 RVP900 implementation of AFC has been generalized so that there is
no difference between configuring an analog loop and a digital loop. The
AFC feedback loop parameters are setup the same way in each case; the
only difference being the model for how the AFC information is made
available to the outside world. Many types of interfaces and protocols thus
become possible according to how these three questions are answered.
AFC output follows these steps:
- The internal feedback loop uses a conceptual [-100%,+100%] range
of values. However, this range may be mapped into an arbitrary
numeric span for eventual output. For example, choosing the span
from -32768 to +32767 would result in 16–bit AFC, and 0 to 999
might be appropriate for 3–digit BCD; but any other span could also
be selected from the full 32–bit integer range.
- Next, an encoding format is chosen for the specified numeric span.
The result of the encoding step is another 32–bit pattern which
represents the above numeric value. Vaisala will make an effort to
include in the list of supported formats all custom encodings that our
customers encounter from their vendors.
- Available formats include straight binary, BCD, and mixed–radix
formats that might be required by a specialized piece of equipment.
The "8B4D" format encodes the low four decimal digits as four BCD
digits, and the remaining upper bits in binary. For example, 659999
base–10 would encode into 0x00419999 Hex.
- An output protocol is selected for the bit pattern that was produced by
encoding the numeric value. The bits may be sent to the IFDR and
converted to an analog voltage, or they may be retransmitted by the