Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 189
gain and high gain channels), we can use this information to determine the
signal values of the components in these two channels.
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 need to 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. Thus an amplifier with an output saturation value of between
+10 dBm and +15 dBm should be used.
6.1.3 Automatic Frequency Control (AFC)
AFC is used on magnetron systems to tune the STALO to compensate for
magnetron frequency drift. It is not required for Klystron systems. The
STALO is typically tuned 30 MHz 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. It is important that the system's IF frequency is at least
4 MHz away from any multiple of half the digital sampling frequency, that
is, 18 MHz, 36 MHz, 54 MHz, or 72 MHz.
The 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 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 MHz to 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
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