Vaisala RVP900 - Page 122

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 coecients
taper o toward 0 on each end. 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
coecient 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 requires (25.3
μsec)(100 MHz) = 2530 taps to compute, which fits within the 8000- tap limit.
In dual-polarization mode the 190 in the above formula is replaced with 95.
Burst Frequency Estimator
This estimator is mostly used with the Pb (plotting commands). See 6.5.2 Pb Subcommands
(page 134).
Burst Freq Estimator– Length: 1.33 usec, Start: 0.00 usec
FIR–Filter Prototype Passband Width
The passband width of the ideal lowpass
filter is used to design the matched FIR band pass
filter.
The bandwidth of the final FIR filter depends on the filter's impulse response length and the
design window used in the process. The 3 dB bandwidth is:
• Larger than the ideal bandwidth if that bandwidth is narrow and the FIR length is too
short to realize that degree of frequency discrimination. In these cases it may be
reasonable to increase the
filter length.
• Smaller than the ideal bandwidth if the FIR length easily resolves the frequency band.
This is because of the interaction within the
filter's transition band of the ideal filter and
the particular design window being used. For example, for a Hamming window and
suciently long filter length, the ideal bandwidth is an approximation of the 6 dB (not
3 dB) attenuation point. Hence, the 3 dB width is narrower than the ideal prototype
width.
You must tune this parameter using the TTY output and interactive visual plot from the Ps
command. The 3 dB bandwidth is shown in the plot for comparison with the ideal prototype
bandwidth.
RVP900 User Guide M211322EN-J
120