Vaisala RVP900

Chapter 5 _______________________________________________________ Plot-Assisted Setups
VAISALA______________________________________________________________________ 177
5.6.4 Bench Testing of Compressed
Waveforms
Working with compressed pulse waveforms can be tricky, so it is
reassuring to run some simple bench tests to verify that things are working
properly. Once the Tx waveform has been designed it can be injected into
the IFDR for testing with the Pr command. This not only verifies that the
analog waveform is generated properly, but also that the matched filtering
on the RVP900/Rx card is able to deconvolve the compressed information.
RxLoss The RxLoss is a measure of how much information is
"thrown away" by the receiving filter in order to achieve
the desired level of sidelobe suppression. These two
quantities often trade off against each other in receiver
systems, so that optimum range/time sidelobes can only be
achieved at the expense of a few deciBels of loss of
sensitivity. The receiver filter loss is calculated as:
Where T(t) is the complex-valued transmit waveform, and
R(t) is the complex-valued filter being used to receive it.
When R(t) is designed to be the complex conjugate of T(t),
we have the ideal matched filter case whose receive loss is
0dB. However, this matched filter has rather poor sidelobe
behavior that makes it unsuitable for use directly in the
receiver. Instead, a windowed version (Hamming,
Blackman, etc.) of the ideal matched filter is used to
achieve the desired sidelobe levels. Of course, that
windowing operation also has the effect of discarding some
valid information in the leading and trailing portions of the
pulse. Hence, there is a loss in receive sensitivity whenever
a window is applied.
NOTE
Given a compressed transmit waveform, the RVP900 designs the
appropriate "mismatch" Rx filter automatically, using an optimized
Blackman window in all cases. Code developers can also access the
internal APIs directly to design any desired transmit waveform along with
the associated FIR filter to receive it.
dB
loss
10log
10
TtRt td
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2
Tt
2
tRt
2
td

d

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