Vaisala RVP900

Chapter 4 _____________________________________________________ TTY Nonvolatile Setups
VAISALA______________________________________________________________________ 127
freedom to adjust. Thus, these three questions do not appear in the
CWPulse case.
The linear FM class is also entirely specified by just the bandwidth and
pulsewidth values, and does not reference any of the tuning parameters.
However, the non–linear FM class is the most flexible of all, and
references all three tuning parameters as follows:
- Parameters #1 and #2 are the (X,Y) location of the non–linear
"breakpoint" for the FM curve. Referring to the white plot line in
Figure 33 on page 170, the Time/Frequency behavior of the pulse can
be drawn in a coordinate system whose abscissa ranges from -1 to +1
over the complete time duration of the pulse, and whose ordinate
ranges from -1 to +1 over the complete frequency span of the pulse.
- The class of non–linear FM curves always pass through the points (-
1,-,1), (0,0), and (1,1), that is, they begin at the lowest frequency at the
start of the pulse, end at the highest frequency when the pulse
completes, and pass through the origin (to maintain symmetry across
both halves of the pulse). Between the points (0,0) and (1,1) the curves
also pass through the tunable (X,Y) "breakpoint" defined by the first
two parameters. In other words, the positive–,time portion of the FM
curve consists of two linear segments; one from (0,0) to (X,Y), and the
other from (X,Y) to (1,1). By tuning the breakpoint we create a
diverse class of FM modulations, but all of them adhere to the
physical bandwidth and pulsewidth limits imposed by the earlier setup
questions. Note that to maintain symmetry, the breakpoint is also
mirrored on the negative–,time side as line segments from (-1,-1) to (–
X,–Y), and from (–X,–Y) to (0,0).
- Parameter #3 specifies the X location of the start of the amplitude
taper of the non–linear FM waveform. For example, setting X to 0.95
will result in a pulse having full amplitude over the middle 95% of its
duration, but then having raised cosine amplitude weighting applied
to the leading and trailing 5% of its edges.
Some examples may be helpful:
P1 = 0.0, P2 = 0.0, P3 = 1.0
P1 and P2 place the FM breakpoint at the origin. But the FM curve passes
through that point anyway, so the response reverts to linear FM. P3
indicates that amplitude modulation should not be applied until the very
end of the pulse, and thus will not occur at all. The resulting waveform is
therefore linear FM having abrupt On/Off transitions.
P1 = 0.9, P2 = 0.7, P3 = 1.0