Vaisala RVP900

USER’S MANUAL__________________________________________________________________
122 _________________________________________________________________ M211322EN-D
The Tx and Rx intermediate frequencies are allowed to be different from
each other, so that the RF up-conversion chain for transmission could be
different from the down-conversion chain for reception.
Limits: 6 MHz to 72 MHz
FIR–Filter impulse response length: 1.33 usec
The RVP900 computes "I" and "Q" using a digital FIR matched filter. The
length of that filter (in microseconds) is chosen here.
The filter length should be based on several considerations:
- It should be at least as long as the transmitted pulse width. If it were
shorter, then some of the returned energy would be thrown away when
"I" and "Q" are computed at each bin. The SNR would be reduced as
a result.
- It should be at least as long as the range bin spacing. The goal here is
to choose the longest filter that retains statistical independence among
successive bins. If the filter length is less than the bin spacing, then no
IF samples would be shared among successive bins, and those bins
would certainly not be correlated.
- It should be "slightly longer" than either of the above bounds would
imply, so that the filter can do a better job of rejecting out–of–band
noise and spurious signals. The SNR of weak signals will be improved
by doing this.
In practice, a small amount of bin-to-bin correlation is acceptable in
exchange for the filter improvements that become possible with a longer
impulse response. The FIR coefficients taper off toward zero on each end;
hence, the power contributed by the overlapping edge samples is minimal.
Vaisala recommends beginning with an impulse response length of 1.2–1.5
times the pulse width or bin spacing, whichever is greater.
The maximum FIR filter length is bounded according to the range
resolution that has been chosen; a finer bin spacing leaves less time for
computing a long filter. The filter length can be as long as 110 μsec at
125 m resolution in single-polarization mode, and up to 20 μsec at 16-
meter resolution. In dual-polarization mode, the limits are 80 μsec at 125
m and 20 μsec at 32 m.
More precisely, in single-polarization mode the maximum filter length (in
meters) is 190 times the range bin spacing (also in meters), subject to the
added constraints that the number of coefficient taps cannot exceed 8000
total and 62 per range step. For example, if we want a filter that is 3.8 km
(25.3 μsec) long, then the range resolution can be no less than 3.8 km/190
= 20 m. At the RVP900 maximum 100 MHz AQ clock this filter will