Delay (Trigger)

If delay is zero, the measurement hardware takes data immediately after detecting a valid trigger signal. If delay is positive (called post-trigger delay) or negative (called pre-trigger delay), the amount of delay determines when the time record begins with respect to the trigger signal.

For multi-channel hardware, different trigger delay cannot be specified for the two channels. When trigger delay is set for one channel, the VSA sets the delay for the other channel to the same value.

For information on trigger delay with specific measurement hardware, see Available Measurement Hardware for a link to your specific hardware.

Trigger parameters (including this one) affect the outcome of the time record capture from hardware when creating a recording as well as when making a measurement.

Trigger Delay can also be used to control the Frequency Mask trigger style

Pre-Trigger (negative) Delay

Pre-trigger delay indicates that the measurement hardware should capture a certain amount of data (say, X seconds) before the trigger signal arrives. This is accomplished using sample memory (RAM). When a pre-trigger delay is selected, the measurement hardware stores the last X seconds of data in sample memory until it receives a valid trigger signal. The hardware then uses the stored data as part of the time capture to achieve the pre-trigger delay.

Pre-trigger delay is limited by the amount of sample memory available for the measurement and the frequency span.

Post-Trigger (positive) Delay

Post-trigger delay indicates how long after the trigger signal to start capturing data and is limited by the range of the trigger counter (which counts the time after trigger happens). This parameter is not affected by sample memory. Again, the time represented by this count depends on the time each sample represents, which varies with span.

If triggering is not behaving as expected, see Trigger Level and Delay Issues.

Multiple shared measurements

Although other trigger parameters must be the same for measurements that share the same Analyzer Configuration, Trigger can be set separately for each measurement.

The VSA captures a time record from the Analyzer Configuration that is long enough to accomodate the trigger delays for the shared measurements.

The VSA also ensures that the time record defined by the combination of trigger delays can be acquired by the Analyzer Configuration. When a measurement trigger delay is requested such that the combined time record length is not supported by the Analyzer configuration, the other shared measurements' trigger delays are adjusted as necessary.

For example, two measurements share Analyzer1, which is able to capture 1 second of time data for the given measurement span. Both measurements are configured to capture 0.5 sec of time data. If you set Meas01 trigger delay to -0.5 sec and then set Meas02 trigger delay to 1 sec, the VSA will adjust the trigger delay for Meas01 to 0.5 sec so that the overall time capture length is 1 second (0.5 sec to 1.5 sec).