Chapter 5 _______________________________________________________ Plot-Assisted Setups
VAISALA______________________________________________________________________ 161
the details of the Tx waveform are being captured. If the entire Tx envelope
does not fit within the FIR filter, then the filter loss will be underestimated
because the Tx spectrum will appear to be narrower than it really is.
The RVP900s calculation of digital filter loss is very similar to how the
loss of an analog filter would be measured on a test bench. Suppose we are
given an analog band pass filter and are asked to determine its spectral loss
when a given waveform is presented. We could use a power meter to
measure the waveform power before and after the filter is inserted, and
compute the ratio of these two numbers. This corresponds to the first
integral ratio in the above equation. However, this is not by itself an
accurate measure of filter loss because it does not take into account the
bandwidth-independent insertion loss. Put another way, a flat 3 dB pad
would seem to produce a 3 dB filter loss in the above measurement, but
that is certainly not the result that we desire. The remedy is to make a
second pair of power measurements of the filter's response to a CW tone at
the passband center. This serves to calibrate the gain of the filter, and
allows us to compute a filter loss that captures the effects of spectral shape
independent of overall gain. This normalization step corresponds to the
second integral ratio in the above equation.
If your radar calibration was performed using CW waveforms, then the
reported filter loss should either be added to the receiver calibration losses,
or subtracted from the effective transmit power; the net result being that
dBZ
0
increases slightly.
In dual-receiver systems the filter loss is computed for the primary and
secondary channels using only the portion of bandwidth that is allocated to
that channel. For example, if the two IFs are 24 MHz and 30 MHz, then the
filter losses for each channel would use the frequency intervals 21 MHz to
27MHz and 27 MHz to 33 MHz, respectively. This is necessary to avoid
picking up energy from the other receiver and interpreting it as out-of-band
input power. A consequence, however, is that the real out-of-band power
is underestimated, that is, the filter loss itself is underestimated. We
recommend temporarily switching dual-receiver systems back to
single-receiver mode when the filter loss is being measured. This is easily
done by changing the Mc setup question back to "single", and
disconnecting the secondary burst input to the IFDR.
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