User's Manual

RVP8 User’s Manual
October 2005
TTY Nonvolatile Setups
3–23
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.
S The class of non-linear FM curves always pass through the points (–1,–1),
(0,0), and (1,1), i.e., 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).
S 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
During the middle 90% of the waveform’s duration the frequency chirp uses 70% of
its available bandwidth. Then, within the 10% pulse tails, the remaining 30% of the
bandwidth suddenly gets covered. No amplitude modulation is applied. Pulses of
this type have been studied theoretically, but do not perform very well for a given
total bandwidth that includes the leading/trailing “ears”.
P1 = 0.9, P2 = 1.0, P3 = 0.8
The entire frequency band is chirped within the middle 90% of the pulse duration, so
that the frequency remains constant in the 10% pulse tails. An amplitude modulation
is also applied over 20% of the pulse tails, i.e., encompassing both the ends of the
chirp and the entire constant frequency intervals. Pulses of this type have superior
sidelobe behavior and fit very neatly within their prescribed bandwidth limits. We
recommend using non-linear FM waveforms that combine chirp limits and amplitude
modulation in this manner.