Vaisala RVP900, User Manual

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1) Environment Canada – Aldergrove BC
Figure 30 Example of Combined IF Sample and LOG Plot
The IF samples are plotted on a linear scale as signed quantities, with 0 appearing at the
center line of the scope. Any DC offset present in the A/D converter is not removed, and is
seen as a shift in the 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 is much less than this worst case value; but 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 when 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 the previous figure 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
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 shifts 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.
RVP900 User Guide M211322EN-J
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