User's Manual Part 2

RVP8 User’s Manual
April 2003
TTY Nonvolatile Setups (draft)
3–22
between every pair of transmitted pulses, and remains correctly positioned regardless
of changes in the PRF Enter this multiplier as “0” if you do not wish to use this term,
and it will be omitted entirely from the printout..
In the above example, Trigger #2 is a 10.0 msec active-high pulse whose leading edge
occurs precisely halfway between the zero-range of every pair of pulses. Likewise,
Trigger #6 is a 2.0 msec active-low pulse whose falling edge is nominally 5.0 msec
prior to range zero, but which is advanced by 1.0 msec for every millisecond of
trigger period. All other triggers behave normally, and have fixed starting times that
do not vary with trigger period.
Some subtleties of these variable start times are:
S The PRT multipliers can only be used in conjunction with the RVP8’s internal
trigger generator. The PRT-relative start times are completely disabled whenever
an external trigger source is chosen from the Mt menu.
S When PRT-relative triggers are plotted by the Pb command, the active portion of
the trigger will be drawn cross-hatched and at a location computed according to
the current PRF. The cross-hatching serves as a reminder that the actual location
of that trigger may vary from it’s presently plotted position.
S The PRT multiplier for a given pulse is applied to the interval of time between
that pulse and the next one. This distinction is important whenever the RVP8 is
generating multiple-PRT triggers, e.g., during DPRT mode, or during Dual-PRF
processing. Multipliers from 0.0 to +1.0 are generally safe to use because they
shift the trigger into the same pulse period that originally defined it. For
example, a start time of (0.0 msec + (0.98 * PRT)) would position a trigger 98%
of the way up to the next range zero. But, if –0.98 were used, and if the period of
the previous pulse was shorter than the current one, then that shorter period would
become incorrect (longer) as a result of having to fit in the very early trigger.
A small but important detail is built into the algorithm for producing the six user
trigger waveforms. It applies whenever a) the trigger period is internally determined,
i.e., the external pretrigger input is not being used, and b) the overall span of the six
trigger definitions combined does not fit into that period. What happens in this case
is that any waveforms that do not fit will be zeroed (not output) so that the desired
period is preserved. This means that you can define triggers with large positive start
times, and they will pop into existence only when the PRF is low enough to
accommodate them.
For example, if Trigger #2 is defined as a 200.0msec pulse starting at +400.0msec,
then that trigger would be suppressed if the PRF were 2000Hz, but it would be
present at a PRF of 1000Hz. Whenever a trigger does not completely fit within the
overall period it is suppressed entirely. Thus, even though the +400.0msec start time
is still valid at 2000Hz, the entire 200.0msec pulse would not fit, and so the pulse is
eliminated altogether.
Start limits: –5000 to 5000 msec. Width limits: 0 to 5000 msec.