Vaisala RVP900, User Manual, Page: 121 / 484

The ranges that are selected by the bit mask in the
LRMSK
command are spaced according
to the range resolution you define here. Also, the upper limit on the impulse response length
of the matched FIR filter (see below) is constrained by the range resolution. If you choose a
range resolution that cannot be computed at the present filter length, then a message of the
form
Warning: Impulse response shortened from 72 to 42 taps
appears.
Limits: 10 m ... 1000 m (exact limits depend on other Rx setup values)
Range mask spacing: 125.00 meters
Intermediate Frequency
The radar intermediate frequency can be selected separately for each pulse width, so that
different width pulses could occupy different frequency bands if desired.
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 ... 72 MHz
Tx Intermediate Frequency: 30.0000 MHz Rx Intermediate Frequency: 30.0000 MHz
Filter Length
RVP900 computes I and Q using a digital FIR matched filter. Define the length of that filter
(in microseconds) here.
FIR–Filter impulse response length: 1.33 usec
Table 32 Filter Length Considerations
Consideration Description
Transmitted pulse
width The filter length should be at least as long as the transmitted pulse width.
If it were shorter, 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.
Range bin spacing The filter length should be at least as long as the range bin spacing.
The goal 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.
Out-of-band noise The filter length 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.
This improves the
SNR of weak signals.
Chapter 5 – TTY Non-volatile Setups
119