Vaisala RVP900

USER’S MANUAL__________________________________________________________________
186 _________________________________________________________________ M211322EN-D
the FIR filter will typically be greater than 66dBc (Rev.A/B) and 84dBc
(Rev.C).
The reference phase for each transmitted pulse is computed using the same
two FIR sums, except with b
n
substituted for the p
n
. For a magnetron
system the N b
n
samples are centered on the transmitted burst; for a
Klystron system they may be obtained from the burst pulse
(recommended) or from the CW COHO. If the Klystron is phase
modulated by an external phase shifter (as opposed to the IFDR digital
transmitter board), then the samples should be from the modulated COHO.
The coefficients are computed as:
where f
IF
is the radar intermediate frequency, f
SAMP
is the IFDR crystal
sampling frequency, and l
n
are the coefficients of an N-point symmetric
low-pass FIR filter that is matched to the bandwidth of the transmitted
pulse. The multiplication of the l
n
terms by the sin() terms effectively
converts to the low-pass filter to a band-pass filter centered at the radar IF.
The formula for the coefficients is identical except that sin() is
replaced with cos().
The phase of the sinusoid terms, and the symmetry of the l
n
terms, has been
carefully chosen to have a valuable overall symmetry property when n is
replaced with (N-1)-n, that is, the sequence is reversed:
Thus, the coefficients needed to compute I are merely the reversal of the
coefficients needed to compute Q; if you know , then you also
know .
f
n
i
f
i
n
l
n

4
---2
f
IF
f
SAMP
--------------- n
N 1–
2
-------------–


+ , sin n 0N 1–==
f
q
n
f
N 1–n–
i
l
N 1–n–

4
---
2
f
IF
f
SAMP
--------------- N 1–n–
N 1–
2
-------------
–+sin=
f
N 1–n–
i
l
n

4
---
2
f
IF
f
SAMP
--------------- n
N 1–
2
-------------
–


+cos=
f
N 1–n–
i
f
q
n
=
f
n
i
f
n
q