USER’S MANUAL__________________________________________________________________
246 _________________________________________________________________ M211322EN-D
error in the output. However, large previous-ray errors would lead to
incorrect unfolding.
A more subtle side effect of Dual PRF processing arises from clutter
filtering because clutter notches now appear at several locations in the
unfolded velocity span, rather than just at zero velocity. These additional
rejection points come about because the original velocity intervals are
mapped some integer number of times to create the unfolded interval.
Since each original interval has a clutter notch at DC, it follows that the
final expanded velocity interval will have several such notches. For
example, in the 3:2 case, in addition to removing DC the clutter filter
removes velocities at - 2V
u
/3, + 2V
u
/3, and V
u
.
Unfortunately, these clutter filter "images" are a fundamental consequence
of the Dual PRF processing technique and are not easily removed. They
can cause trouble not only for the velocity unfolding itself, but because the
computed clutter corrections to be wrong at the image points. However,
there is a useful work-around in the RVP900 to minimize their impact —
turning the clutter filter off at far ranges where little clutter is expected and
using a narrow clutter filter minimizes the effects of the clutter filter on
weather targets.
The 4:3 and 5:4 PRF unfolding ratios are more susceptible to unfolding
errors in cases where the spectrum width is large and/or the SNR is low.
The user should experiment with these ratios to determine which provides
the best results for their particular application. Although the RVP900
trigger generator can produce any trigger frequency, only the 3:2, 4:3, and
5:4 ratios can be used with the built-in unfolding algorithms. The RVP900
still permits other PRT ratios to be explored, but the unfolding technique
must then be manually programmed on the user's host computer.
Oscilloscope observations of Dual PRF triggers can sometimes be
confusing. Figure 52 on page 245 shows seven possible scope traces (and
their associated probabilities) for the RVP900 trigger during Dual PRF
operation. The PRF ratio is 4:3, and the sample size is 50 pulses at the high
PRF, and 37 pulses at the low PRF. The signal labelled "SCOPE" is the
composite of these traces, and is what would actually be seen on an
oscilloscope. Notice that there are a number of low probability pulses. The
exact details of the sample sizes and the trigger hold off time can make the
low probability pulses appear to come and go randomly. This is normal,
and is no cause for alarm.
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