Stanford Research Systems SR620, Руководство пользователя

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12 Sample Arming
SR620 Universal Time Interval Counter
TIME EXTERNAL with HOLDOFF is similar
except that Stops are not enabled until the trailing
edge of the EXT input. A particular Stop pulse
may be selected to end the time interval by using
the EXT input to inhibit, or holdoff, the Stop input.
An example of this might be measuring the time
from the index mark on a hard disk drive to a
particular data bit. By adjusting the holdoff time
one could measure the time to any data bit instead
of just the first one.
The trigger may be delayed or scanned from an
EXT trigger input by setting SCAN parameters in
the CONFIG menu. A blinking EXT LED indicates
that the EXT is used as a trigger for the delayed
gate.
±TIME MEASUREMENTS
(-1000s<t< 1000s)
In all of the ± TIME modes Starts and Stops are
armed simultaneously and so either a positive or
negative time interval may be measured. There is,
unfortunately, some ambiguity to this method of
arming. For periodic inputs there is no way for the
instrument to know if the desired time interval
should be measured from the Start to the previous
Stop or to the next Stop. For example, if the Start
and Stop are both 1 KHz square waves, with the
Stop edge following the Start edge by 1 uS, then
the unit will report a Time interval of 1 us or
-999 us. In the
±TIME and ±TIME COMPLEMENT
modes the start and stop inputs are armed by
parity, that is, the reception of either a Start/Stop
pair or a Stop/Start pair of pulses. By changing
between these two modes one may choose to
measure either the time from Stop to the next Start
or Start to the next Stop. Since the parity of the
input signals is randomly determined at power-up
there is no way to specify which arming mode will
correspond to which measurement. But by
changing between these modes both
measurements may always be obtained.
Alternately, one may arm a ±Time measurement
with a signal applied to the Ext input in which case
the measurement that is made (Start to Stop or
Stop to Start) is determined only by the
relationship of the Ext input to the Start and Stop
signals.
The EXT input requires about 10 ns setup prior to
the Start or Stop inputs.
Time intervals as a function of delay from an EXT
trigger may be measured by using the internal
delay generator. The delay generator is triggered
by the EXT input. The trigger delay may be set or
scanned via the CONFIG Menu.