Vaisala RVP900, User Manual, Page: 178 / 484

For example, if the LNA has a gain of 17 dB, the mixer has a conversion loss of 7 dB, there is 1
dB miscellaneous losses and 3 dB loss in the power splitter, then the signal level at the
output of the power splitter is ( -113 + 18 - 7 - 1 - 3 ) = -106 dBm for the minimum signal, and
-1 dBm for the maximum signal. In the low gain channel, we must bring the - 1 dBm up to the
maximum input value of the IFDR (+ 6 dBm). To do this we need about 8 dB of amplification
(7 dB plus one more deciBel to account for the anti-alias filter loss of the IFDR).
If we assume 25 dB of channel separation, on the high gain channel we require about +33 dB
of amplification. Finally, this tells us that on the low gain channel, the minimum and
maximum signals presented to the IFDR are ( -106 + 8 ) = -98 dBm and ( -1 + 8 ) = 7 dBm. For
the high gain channel, the signal levels are ( -106 + 33 ) = -73 dBm and ( -1 + 33 ) = +32 dBm.
As +32 dBm is above the maximum input level tolerated by the IFDR, the amplifier on the
high gain channel must limit its output to less than +16 dBm. An amplifier with an output
saturation value of +10 dBm ... +15 dBm should be used.
7.2.3 Automatic Frequency Control (AFC)
AFC is used to tune the STALO to compensate for magnetron frequency drift.
RVP900 analyzes the burst pulse samples from each pulse, and produces a running estimate
of the power-weighted center frequency of the transmitted waveform. This frequency
estimate is the basis of the RVP900 AFC feedback loop, whose purpose is to maintain a
fixed intermediate frequency from the radar receiver.
The STALO is typically tuned 30 or 60 MHz away from the magnetron frequency. The
maximum tuning range of the AFC feedback is approximately 7 MHz on each side of the
center frequency. This is limited by the analog filters that are installed just before the signal
and burst IF inputs on the IFDR. The system's IF frequency must be at least 4 MHz away
from any multiple of half the digital sampling frequency, that is, 18, 36, 54, or 72 MHz.
The instantaneous frequency estimate is computed using four autocorrelation lags from
each set of
N b
n
samples. This estimate is valid over the entire Nyquist interval (for example,
18 ... 36 MHz), but becomes noisy within 10% of each end. Since the span of the burst pulse
samples is only approximately one microsecond, several hundred estimates must be
averaged together to get an estimate that is accurate to several kHz. The AFC feedback loop
typically has a time constant of several seconds or more.
Most burst pulse analysis routines, including the AFC feedback loop, are inhibited from
running immediately after making a pulsewidth change. The center-of-mass calculations are
held off according to the value of Settling time (to 1%) of burst frequency estimator, and the
AFC loop is held off by the wait time before applying AFC. This prevents the introduction of
transients into the burst analysis algorithms each time the pulse width changes.
AFC is not required for Klystron systems.
More Information
‣
Mb — Burst Pulse and AFC (page 93)
‣
Digital AFC (DAFC) (page 50)
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