Triggering - Classroom - TiePie engineering

In most situations it is not desired that capturing and displaying the input signals starts at random moments, like e.g. immediately after the previous measurement, or when the user presses start. Most of the time it is preferred that capturing and displaying starts when one or more input signals meet some predefined conditions. We call this triggering. This page contains basic information about triggering measurements.

Contents

1 Introduction
2 Channel trigger settings
2.1 Trigger type
2.1.1 Edge trigger
2.1.2 Window trigger
2.2 Trigger level
2.3 Trigger hysteresis
3 Instrument trigger settings
3.1 Trigger source
3.1.1 Digital external
3.1.2 Analog external
3.1.3 Generator-trigger
3.2 Pre trigger / Pre samples
3.3 Trigger time out

Introduction

To let the instrument trigger at the desired conditions, the instruments are equipped with a trigger circuit. The trigger circuit has several settings that can be changed. These settings are divided into a group of channel trigger settings and a group of instrument trigger settings.

Channel trigger settings

In this section, we will treat the channel trigger settings.

The channel trigger circuits monitor the channels continuously. When the input signals meet some predefined condition, a trigger signal is generated.

There are several channel trigger settings. For most instruments, these settings can be adjusted for all channels individually, but for some instruments, the channel trigger settings apply to all channels.

Trigger type

There are several different types of trigger conditions:

edge trigger : trigger on an edge in a signal
window trigger : trigger when the signal enters or leaves a certain window or range
TV trigger : trigger on a line sync pulse or frame sync pulse of a TV signal

Edge trigger

Edge trigger can be used to trigger on a rising edge (or slope) or a falling edge in the signal. Which edge is being triggered on, can be selected.

To trigger on an edge, the system has two adjustable levels, the arming level and the firing level. The trigger system will constantly compare the input signal with the two levels.

As soon as the input signal passes the arming level in the specified direction, the trigger system is "armed" and ready to detect a passing of the firing level. When triggering on a rising edge, the arming level must be passed in an upward direction, so the signal must first be smaller than the arming level and then become larger than the arming level. When triggering on a falling edge, the arming level must be passed in a downward direction, so the signal must first be larger than the arming level and then become smaller than the arming level.

When the input signal passes the firing level in the specified direction and the the trigger system is armed, the required edge is found, the trigger condition is met and a trigger signal is generated. When the input signal passes the firing level but is not armed, nothing happens and the system will continue monitoring the input signals.

In the image below, rising edge triggering is shown.

Window trigger

Window trigger can be used to trigger when the input signal enters a window or leaves a window.

To trigger based on a window, the system has an adjustable window of which the upper and lower level can be adjusted individually. The trigger system will constantly compare the input signal with the upper and lower level of the window..

When using Outside window triggering, the system will generate a trigger as soon as the signal becomes larger than the upper level of the window or smaller than the lower level of the window, in other words: is outside the window.

When using Inside window triggering, the system will generate a trigger as soon as the signal becomes larger than the lower level of the window and smaller than the upper level of the window, in other words: is inside the window.

In the image below, Outside window triggering is shown.

[s3;id=tvtrigger;title=TV trigger]

An image on a TV screen consists of a number of horizontal lines. All these lines are transmitted to the TV one after another. To reduce flickering, first all even numbered lines are transmitted, and then all odd numbered lines. This is called interlacing. So one image is built of two frames with lines.

To make sure that the TV receiver reconstructs the image in the correct way, there are line synchronization pulses between all the lines and frame synchronization pulses between the different frames. The frame sync pulse after an odd numbered frame is different from the frame sync pulse after an even numbered frame.

Some of the TiePie engineering instruments have special sync pulse separator circuits, that will detect the TV line and frame pulses in a TV signal. These pulses can be used to trigger the measurement, so that a measurement will always start at the beginning of a line, or at the beginning of the even frame or at the beginning of the odd frame.

The options to choose from in the software are:

TV Line : trigger on a line sync pulse
TV Frame odd : trigger on an odd frame sync pulse
TV Frame even : trigger on an even frame sync pulse

Trigger level

In Edge Trigger, the level at which the actual trigger takes place, is called the trigger level. This level corresponds with the firing level.

Trigger hysteresis

In edge trigger, the distance between the firing level and the arming level is called hysteresis. The hysteresis determines the sensitivity of the trigger system. A small hysteresis means that the arming and firing level are close to each other. 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.

Instrument trigger settings

In this section, we will treat 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.

Trigger source

As described in the Channel trigger settings section, the channels have several different trigger settings which determine how the system will trigger on a signal. The trigger source setting of the instrument determines which trigger signal(s) is/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 OR and AND functions. In the image below, the trigger source is set to Ch1.

Digital external

Besides the normal input channel triggers, most TiePie engineering instruments have an external trigger input, which can be used to connect a trigger signal. This is usually a digital (TTL) input. 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.

Analog external

Some TiePie engineering instruments also have an analog external trigger input. This input behaves like a normal input channel, with sensitivity settings and signal coupling. For this trigger input, level, hysteresis and type can be set, just as for a normal input channel.

Generator-trigger

The Handyscope HS3 is equipped with an Arbitrary Waveform Generator. This generator has internal trigger signals that can be used as trigger source:

Generator Start: trigger at the moment that the generator starts generating the signal
Generator New period: trigger at the moment that the generator starts generating a new period of the signal
Generator Stop: trigger at the moment that the generator stops generating the signal

Pre trigger / Pre samples

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's 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.

When pre samples are selected, the trigger system is immediately activated after starting the measurement. At the first occasion that the trigger conditions are met, the system will start measuring post samples. 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 Volt.

The reason this technique is used is that 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.

Trigger time out

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 timeout is entered as a number, representing the delay in seconds. There are two special values for the trigger timeout setting:

trigger-timeout = 0	immediately after starting a measurement a trigger is forced. Basically this bypasses the trigger system and the instrument measures always immediately. No pre samples are recorded.
trigger-timeout = infinite	The system will wait infinitely for a trigger. The software will never force a trigger, only when the trigger conditions are met, a trigger will occur and a measurement will take place. This setting is particularly useful for single shot measurements. On conventional desktop oscilloscopes, this is called Trigger mode NORM.

Pressing the W key on the keyboard will toggle the trigger timeout between the set value and infinite. Pressing the 0 (zero) key on the keyboard will toggle the trigger timeout between the set value and zero.