Vaisala RVP900

USER’S MANUAL__________________________________________________________________
166 _________________________________________________________________ M211322EN-D
baseline at higher zoom levels. For example, the converter's worst case DC
offset of 10 mv would appear as a several-hundred-count offset in the 16-
bit A/D range. At the x32 or higher zoom scales, this offset would peg the
sample plot off scale. Typically the DC offset will be much less than this
worst case value; but the RVP900 preserves the DC term in the Pr sample
plot so that its presence is not forgotten.
The "AC" amplitude of the IF samples increases wherever targets are
present. On top of these samples is drawn the detected power on a
logarithmic scale. Each horizontal line represents a 10 dB change in
power. The graph is scaled so that the LOG power reaches the top display
line when the samples occupy the full amplitude span. Using Figure 30 on
page 163 as an example, the two equal-power targets just to the left of
center are approximately 18 dB down from the top. The amplitude of the
samples is thus 10(
-18/20
) = 0.13, that is, 13% of full scale. This
correspondence between the LOG scale and the amplitude scale applies
regardless of the plot's zoom level. As the IF samples are zoomed up and
down by factors of two, the LOG plot will shift up and down in 6 dB steps.
The LOG plot is obtained by convoluting the FIR filter coefficients with
the raw IF data samples, and then plotting log(I
2
+ Q
2
) at each possible
offset along the sampling interval. This convolution produces only (1 + N
- I) output points, where N is the number of sample points and "I" is the
length of the FIR filter. For this reason the LOG plot begins approximately
I/2 samples from left side and ends approximately I/2 samples from the
right.
The LOG points are computed at each possible offset within the raw IF
samples. At the nominal 72 MHz sampling rate the spacing between LOG
samples are a mere 4.17 m. Thus, the LOG plot gives a very detailed view
of received power versus range. Of course, successive LOG points will be
highly correlated because successive input data intervals differ by only one
sample point. This is why the LOG plots appear smooth compared to the
instantaneous variation of the raw IF samples.
As the starting offset of the Pr plot is decreased to range zero you will
begin to see part of the burst pulse (the second half of it) appear at the left
edge of the plot. This is because the burst data samples are multiplexed
onto the same fiber cable that carries the IF data samples. Zero range is
defined to occur at the center of the burst window; hence, the later half of
the burst pulse will be visible when the plot begins at range zero.
A second type of Pr display is shown in Figure 31 on page 165. This plot
shows a frequency spectrum of the received data samples in a format that
is nearly identical to the Ps display. The horizontal axis represents the
same band of frequencies (half the sampling rate), and the vertical axis
represents power in 10 dB steps. The entire vertical axis is used so that an