Installation Instructions

Hardware Installation
RVP8 User’s Manual
September 2005
2–25
J15-18: TRIG1-4- Output triggers
The waveforms appearing on the four trigger outputs are programmed by the user to meet the
radar’s exact timing needs. These correspond to the trigger generators TGEN1, TGEN2,
TGEN3 and TGEN4. More triggers can be configured on the “SPARE” connectors if they are
required. All lines may be setup and used independently and can contain, for example,
pre-trigger pulses, calibration gates, range strobes, scope triggers, etc. The triggers are driven at
+12V into 75W and can be independently-timed at rates between 50Hz and 20000Hz with better
than 0.02% accuracy. For dual-PRF velocity unfolding applications, the RVP8 trigger generator
must be used as opposed to an externally supplied pre-trigger (see next section).
The timing of the triggers is phase-locked to the sample clock in the IFD, which can be phase
locked to the COHO of a coherent system. For coherent systems that do not sample the actual
transmit pulse (for phase correction), this is recommended.
The trigger waveforms are configurable in software using the “mt” commands. This sets the
trigger timing, trigger sense (active high or active low pulse) and the minimum and maximum
PRF for each pulse width. See sections 3.2.4.
It is sometimes useful to dedicate one of the TRIG outputs to trigger an
oscilloscope.
See Section XXX for a description of how to configure an input pre–trigger from an external
source such as an existing radar trigger system.
Selectable input pre-trigger
Users may supply the RVP8 with their own CMOS-Level pre-trigger for installations in which
adequate trigger control already exists. The trigger input is provided directly on the Rx Card
(bottom BNC connector on the card panel). The trigger input uses CMOS levels (1.5V max low,
2.5V min high) for improved noise immunity. The trigger input may also be driven as high as
+100V or as low as –100V without damage. This makes it easier to connect to existing
high-voltage trigger distribution systems. The rising or falling edge of this external
“TRIGIN”signal is interpreted by the RVP8 as the pretrigger point; the actual pulsewidth of the
signal does not matter. The delay to range zero is configured via the TTY Setups. The other
trigger outputs are then synchronized to the input trigger. The synchronization jitter between the
user pretrigger and the other trigger outputs is less than 0.014 microseconds.
Trigger jitter can be improved in the case of coherent systems, by phase locking the IFD to the
same reference clock used to generate the external triggers (typically the COHO). This provides
approximately 10 dB of additional phase stability.
The RVP8’s response to a missing external trigger is that the processor will insert fake (software)
”triggers” at a rate of 250Hz whenever the trigger input is missing for more than 0.100 seconds.
These fake triggers will keep the RVP8’s internal code and external outputs running in spite of
the missing input (the data values will all be zero, and the ”no trigger” bit will be set in GPARM
immediate status word #1). Normal operation automatically resumes as soon as the external
trigger is restored.