User's Manual Part 2

Plot–Assisted Setups
RVP8 User’s Manual
April 2003
4–19
power spectrum estimate that is derived in an identical manner using the same number of
samples, but of a pure sine wave at the radar’s IF. The RVP8 determines B
^
(f) according to its
sampled measurement of the transmitted waveform; however it can calculate C
^
(f) internally
based on an idealized sinusoid. The reported filter loss is then:
dB
loss
+ –10log
10
ȧ
ȧ
ȧ
ȧ
ȡ
Ȣ
ŕ
|H(f)|
2
B
^
(f) df
ŕ
B
^
(f) df
B
ŕ
|H(f)|
2
C
^
(f) df
ŕ
C
^
(f) df
ȧ
ȧ
ȧ
ȧ
ȣ
Ȥ
Where |H(f)|
2
is the spectral response of the RVP8 IF filter, and the integrals are performed over
the Nyquist frequency band that is implied by the RVP8/IFD sampling rate. Note that the two
integrals involving C
^
(f) will have constant value and need only be computed once. They serve
to normalize the B
^
(f) integrals in such a way that the filter loss evaluates to 0dB whenever the
transmit burst is a pure tone at IF.
This normalization is necessary for the filter loss values to be meaningful. Regardless of the
bandwidth and center frequency of H(f), the filter loss should be reported as 0dB whenever the
Tx waveform appears to have zero spectral width, i.e., is indistinguishable from a pure IF
sinusoid. Of course, the real Tx waveform has only finite duration, and thus should never look
like a pure tone as long as the RVP8 is able to “see” the entire Tx envelope. For this reason, it is
important that the filter’s impulse response length be set long enough (using the Pb plot) to
insure that all of 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 RVP8’s 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 bandpass 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 3dB pad would seem
to produce a 3dB 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
will increase 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
24MHz and 30MHz, then the filter losses for each channel would use the frequency intervals