Specifications

Measurement Examples
19
Example 3: Externally Triggered Gate Mode
This example is a variation of Example 2, but requires no additional trigger pulses to begin the DWELL
PERIOD. In this example, the SR400 is programmed to make 10 counting measurements of a 10 kHz pulse
train. The counting gates, each 500 ms wide, are delayed 300 ms from external triggers arriving at a 1 Hz rate.
In this case, the SR400 requires only one trigger per COUNT PERIOD, however the trigger configuration is a
bit trickier than the previous example.
The following timing diagram illustrates this example.
To avoid the triggered dwell period, the T counter is logically ANDed with the A gate. This is accomplished
by letting the T counter count the SR400’s internal 10 MHz oscillator signal for a period extending our
beyond the A gate, but not exceeding the trigger repetition period (1 second in this case). Mathematically, this
is expressed:
A DELAY + A WIDTH < TSET < (1/Trigger Rate)
Furthermore, DWELL must be set to EXTERNAL in the MODE menu. This enables the SR400 to reset and
begin each COUNT PERIOD upon receiving a TTL pulse at the START BNC input.