Keysight Technologies 3458A Guide D'utilisation page 211

Digitizing 5
The sync source event
In the preceding sub-sampling example, it was assumed that the multimeter
could somehow synchronize itself to the periods of the input waveform. This is the
function of the sync source event. You can use either the EXT event or the LEVEL
event as the sync source event. The EXT sync source event (specified by the SSRC
EXT command) occurs on the negative-edge transition on the multimeter's Ext
Trig connector. This requires an external pulse that is synchronous with the input
signal. Figure 5-12 shows a typical input signal and the required synchronizing
signal. Notice in Figure 5-12 that the synchronizing signal does not necessarily
have to occur once for every period of the input signal. It does, however, have to
be synchronized in time with the input signal.
Figure 5-12 Typical synchronizing signal for EXT sync source
The LEVEL sync source event (which is the power-on/default sync source event)
occurs when the input signal reaches a specified voltage level on the specified
slope (level triggering). Figure 5-10 shows the operation of the LEVEL sync source
event (for this example, the LEVEL is specified as 0%, positive slope,
AC-coupling). The first sync source event occurs when the input signal crosses 0 V
with a positive slope. The multimeter then takes a burst of samples (5 samples in
this case). Following the next occurrence of the sync source event (period 2 of the
Keysight 3458A User's Guide 211