Vaisala RVP900, User Manual, Page: 149 / 484

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 ... 27MHz and 27 MHz ... 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 to
single
, and disconnecting the secondary burst
input to the IFDR.
6.6.5 Adjusting Plot Burst Spectra and AFC
1. Make sure you have successfully captured the burst pulse with the Pb command.
2. Type the Ps command.
3. Use the space bar to display the burst spectrum plot by itself, and use the Z key to shift
the entire graph into view.
The plot shows the frequency content of the transmitted pulse.
a. Check that the plot shows a clean main power lobe centered at the receivers
intermediate frequency.
b. Check the spectrum for spurious harmonics, excessive width, and other out-of-band
noise.
c. Adjustment the transmitter to give a sharper main lobe or reduce spurious noise.
Chapter 6 – Plot-assisted Setups
147