Most oscilloscopes are equipped with a trigger circuit to start measuring when a certain condition occurs in the input signal. Triggering is used both for capturing unique signal events and to stabilize the display of repetitive signals. Without triggering, signals are measured and displayed at random times. The trigger circuit has several settings which are divided into instrument trigger settings and channel trigger settings.
Most trigger settings are combined in the Trigger properties dialog. To open the trigger properties dialog, click the button on the instrument toolbar. The trigger properties dialog allows to view and control instrument and channel trigger settings. Additionally, it gives an explanation on the selected trigger type and examples that do cause a trigger (left column) and do not cause a trigger (right column). There is a dialog available for each opened instrument.
This section treats the instrument trigger settings. These settings affect the whole instrument, as opposed to the channel trigger settings, which affect only one channel of the instrument.
Several different trigger settings are available which determine how the system will trigger on a signal. The trigger source setting of the instrument determines which trigger signals are used to trigger the instrument.
The trigger source can be set to a single channel or to any combination of channels or other trigger sources. The sources can be logically combined using an OR function. In the image below, the trigger source is set to Ch1 OR EXT1.
When no trigger source is selected, the trigger system is disabled and the instrument is free-running: it will start measuring the post samples directly.
Changing the trigger source of an instrument in the Multi Channel oscilloscope software can be done in various different ways:
Besides the normal input channel triggers, most TiePie engineering instruments have an external trigger input, which can be used to connect a external digital trigger signal. Trigger level and hysteresis can not be set for this trigger input, but the edge (rising or falling) the system should react to can be set.
With digital storage oscilloscopes, the record length determines the number of samples that are measured. All these samples can be measured after the trigger has occurred. It is however possible to measure (a part of) the record before the trigger occurs, by selecting pre samples.
The total record will then be divided in a pre trigger part and a post trigger part, respectively containing pre samples and post samples. This way it is possible to "look back in time" since the pre samples were captured before the trigger moment.
With the TiePie engineering instruments it's possible to define the trigger moment at any position in the record.
Changing the trigger moment in the Multi Channel oscilloscope software can be done in various different ways:
When setting the trigger moment through a popup menu, the option User defined... also allows to set the value as a number of samples or as a time.
When pre samples are selected, the trigger system is not activated yet until a specified amount of samples are measured after starting the measurement. This amount of samples is called the trigger hold-off. When the input signal(s) meet the trigger requirements during the trigger hold-off period, this will not generate a trigger and the system will remain sampling pre samples. After the trigger hold-off has passed, at the first occasion that the trigger conditions are met, the system will start measuring post samples. This will ensure that at least that specified amount of pre samples will be measured.
The trigger hold-off can is set as a number of samples. Two special settings are also available: Presamples valid or Off.
In the setting Presamples valid, the trigger hold-off value will always be equal to the number of pre samples that are selected. This will ensure that the complete pre sample buffer is always filled with measured samples.
When trigger hold-off is switched off, the trigger system is activated immediately after starting the measurement. The system will not first fill the pre sample buffer before the trigger system is activated. Depending on the measured signal and the moment the measurement was started, this may result in a situation where not all pre samples are measured, but the first pre samples remain 0 V.
Trigger hold-off = off is useful when measuring once-only events, because in this way never a trigger will be missed. If the system would first fill the pre samples buffer before activating the trigger system, it might happen that the event that should cause a trigger would take place while still filling the pre samples buffer and therefore not generate a trigger. No measurement would take place.
Not all TiePie engineering instruments support trigger hold-off. When trigger hold-off is not supported, the trigger system functions as if trigger hold-off was switched off.
Trigger delay allows to start measuring a specified time after the trigger occurred. This allows to capture events that are more than one full record length past the trigger moment. Changing the trigger delay in the Multi Channel oscilloscope software can be done in several ways:
Once the trigger conditions are set and the measurement is started, the instrument will wait until the trigger conditions are met before the post samples are measured and the measurement is finalized.
If the trigger conditions are set in such a way that the input signal(s) will never meet the trigger settings, the instrument will wait forever. When no measurement is performed, no signals will be displayed.
To avoid that the system will wait infinitely, a trigger time-out is added to the trigger system. When after a user defined amount of time after starting the measurement still no trigger has occurred, the trigger time-out will force a trigger. This will ensure a minimum number of measurements per second. On conventional desktop oscilloscopes, this is called Trigger mode AUTO and the used value is approximately 20 ms.
The trigger time-out is entered as a number, representing the delay in seconds. There are two special values for the trigger time-out setting:
Changing the trigger time-out of a channel in the Multi Channel oscilloscope software can be done in various different ways:
Besides using the available trigger sources and the trigger time-out to start a measurement, it is also possible to stop waiting for the trigger time out and to start measuring the post samples right away by forcing the trigger of the instrument.
Forcing a trigger of an instrument in the Multi Channel oscilloscope software can be done in different ways:
Channel trigger circuits monitor the channels continuously and generate a trigger signal when the input signals meet some predefined trigger condition. To change this condition, several channel trigger settings are available which can be adjusted for all channels individually.
There are several different trigger types:
|edge trigger||trigger on an rising, falling or any edge in the signal|
|window trigger||trigger when the signal enters or leaves a certain window or range|
|pulse width trigger||trigger on a positive or negative pulse in the signal wider/narrower than a specified width|
Changing the trigger type of a channel in the Multi Channel oscilloscope software can be done in various different ways:
|Without Ctrl||With Ctrl|
All oscilloscope channel trigger types use one or two trigger levels. Trigger levels can be set for each channel individually. Changing a trigger level of a channel in the Multi Channel oscilloscope software can be done in several ways:
All oscilloscope channel trigger types use one or two trigger hystereses. The hysteresis defines the distance between the firing level and the arming level and determines the sensitivity of the trigger system. A small hysteresis means that the arming and firing level are close to each other and a small signal change will be enough to cause a trigger. A large hysteresis means that the signal change must be large before a trigger is generated. This makes the trigger system less sensitive to noise.
Trigger hysteresis can be set for each channel individually. Changing the trigger hysteresis of a channel in the Multi Channel oscilloscope software can be done in various different ways:
Several oscilloscope channel trigger types can have an additional trigger condition and corresponding trigger condition time. The following trigger conditions are available:
|None||there is no additional trigger condition.|
|Shorter than||the signal requirements defined by the trigger type must last shorter than the specified trigger condition time to cause a trigger.|
|Longer than||the signal requirements defined by the trigger type must last longer than the specified trigger condition time to cause a trigger.|
Setting the trigger condition in the Multi Channel oscilloscope software can be done in several ways:
The trigger condition time specifies the duration of a specific signal condition, in seconds.
Setting the trigger condition time in the Multi Channel oscilloscope software can be done in several ways: