Have you ever heard of an oscilloscope trigger?
Sure, the oscilloscope trigger may sound like a remote autonomous device coming out of some sci-fi movie to most of you.
It’s far from that.
It is, in fact, your go-to instrument if you need more information than a simple multimeter provides while troubleshooting circuits.
It gives you many more important details that may change with time, like noise, amplitude, and frequency. Most electrical engineers have and use an o-scope to check varying signals.
You may not know this or be aware of its many uses, but modern oscilloscopes perform far more functions than their predecessors. The earlier models used to display graphs on their screen. These were graphs of the sine wave of an AC voltage source by calculating the frequency of a signal.
Nowadays, oscilloscope manufacturers produce more refined models.
These models come with many new features, including a very advanced oscilloscope trigger.
Like other devices that have transformed with technological advancements, the oscilloscope has also journeyed the test of time and updated its status to today’s latest requirements.
These advanced trigger settings allow a user to detect a narrower-than-usual pulse quickly. This was not possible earlier because scopes could not see using just a voltage threshold oscilloscope trigger.
Modern o-scopes come with more advanced type A and B oscilloscope triggers when compared to the older and more traditional oscilloscope triggers.
Some other advanced features include:
· The trigger resetting sequence after a particular state, time, or transition
· A logic qualification to view a specific event and to capture events of complex signals
What is the function of an Oscilloscope?
It is easier to understand the functioning of an oscilloscope trigger after understanding what an oscilloscope does.
Why would an engineer require this device?
Engineers need o-scopes mainly because it displays electrical signals in a graphical pattern, revealing how these signals change over time.
As it is very well known, most consumer products today are embedded with electronic components and electrical circuits. These noble components are critical in the product design and testing process of products in factories worldwide.
The oscilloscope tests these essential electronic elements located in the everyday home and office appliances to perfection. It ensures the appliance is in prime working condition, and there’s no risk of ending up with a faulty product.
The primary function of an oscilloscope lies in measuring the voltage. In addition to this, it also measures current, sound, capacitance, DC voltage, frequency, and inductance.
However, some elements may require an additional device to measure and get an even more accurate reading.
Oscilloscope trigger system
The trigger function plays an essential part in the scope. A person who knows how to use the scope trigger properly will make maximum use of the o-scope.
It is the oscilloscope trigger that permits the steady and stable display of repetitive waveforms on the screen. It does it by enabling the time base to start scanning at the same point on every waveform repetition.
The trigger can otherwise be easily manipulated to start at random points and thus maintain an unflinching static display. The absence of control in maintaining a steady display can lead to seizures like sweeping waves and a poorly triggered wave.
The trigger system comprises a level knob and an array of buttons used to function as the source to trigger. The rest of the trigger system consists of a series of buttons and screen menus.
What is the oscilloscope trigger function?
This is a control function that helps in the detection and stabilization of repeated unwanted waveforms. Oscilloscope trigger controls help achieve explicit signal characterization.
They serve two purposes- They help stabilize any repeating waveforms and capture single-shot waveforms. They work by making all repeating waveforms look static on the o-scope screen.
Simply put, it enables the user of the device to stabilize these waveforms and capture manageable single-shot waveforms.
A trigger level indicates where the trigger point occurs on edge. The slope indicates whether the trigger point is on the rising or the falling edge of a signal.
The trigger level and slope direction can be adjusted to capture the required view on the scope. This feature exists in both analog and digital oscilloscopes. It is this trigger voltage level control that sets the trigger voltage fire.
If changed, this voltage ends up changing the point where the time base started on the waveform.
So it is more like the trigger circuit functions more like a comparator. Now here are some of the types of triggers generated due to this control function.
Types of oscilloscope triggers
The oscilloscope has several types of triggers, namely:
1. Edge trigger
2. Pulse width trigger
3. Threshold trigger
4. Serial pattern trigger
5. A&B trigger
6. Search & mark trigger
7. Slope trigger
8. Timeout triggering
9. Window triggering
10. Delayed triggering
11. Trigger correction
Out of the above-mentioned oscilloscope triggers, the edge and threshold triggers exist in analog and digital oscilloscopes. A noteworthy fundamental fact is that the edge trigger is the most common and essential oscilloscope trigger function.
i. Edge trigger
Edge triggering is a common oscilloscope trigger model. It is extensively used for simple debugging and testing functions. However, many a time, it is even used for more complex trigger situations like isolating a particular signal shape.
The other triggers are all advanced trigger functions that isolate events of interest to enhance the oscilloscope’s sample rate and record length. They offer additional flexibility for capturing serial protocols, advanced events, and signal characteristics.
More advanced triggering functions include glitch, pulse width, setup-and-hold, slew rate, and time out. They allow users to trigger pulses influenced by time, runt pulses influenced by amplitude, and logic triggering influenced by logic state or delineated by pattern.
ii. Pulse width trigger
The Pulse width triggering allows a pulse polarity which can either be a positive or negative pulse with a Rigor range from 8–10 ns as its width. This sort of pulse triggering is particularly resourceful when you get to investigate serial data streams coupled with delicate horizontal control.
iii. iii. Runt or threshold trigger
The Runt trigger allows the user to bypass signals that cross from one voltage signal threshold without crossing the second threshold. The signals instead return to the initial point before hitting the second threshold.
There is a specific period qualifier that determines the typical threshold levels. This way, the user can select the polarity condition, which can be positive, negative, or either of the two.
iv. Serial pattern triggering
This triggering system triggers upon seeing a serial pattern in a serial data stream. It proves especially useful while testing and debugging digital and microprocessor-based circuits.
v. A & B triggering
Most oscilloscopes offer triggering from A and B channels. However, there are also a few digital models offering more complex triggers for the channels. For example, some may offer logic control qualifications while looking for various events.
Others may come with a delayed triggering system that is triggered by a previous trigger event.
vi. Search & mark triggering
This trigger triggers after scanning for multiple event types. Individual marks added to some portions of the sweep prove helpful in highlighting areas.
vii. Slope trigger
Trigger conditions tend to fluctuate from lower and higher voltage thresholds in a slope trigger. This is called slope triggering, which is also particularly useful for capturing triangular and sawtooth-type waves.
The scope latches to that point in a certain period whenever the term delta signifies a voltage change. They are handy for triggering the area on a positive or negative slope for a specific period.
viii. Timeout triggering
When a signal remains unaffected for a certain amount of time, it could mean that the system is faulty. This is the timeout trigger which indicates a sort of ‘dead-time detected within your system.
If this happens, it shows that there are issues in the design and requires a thorough diagnostic check.
The trigger helps narrow down the system’s lead times. This type of trigger is defined by two conditions which are a timeout period and a slope inclination. An upward and downward voltage fluctuation, in turn, determines the slope indication.
ix. Window triggering
A window trigger is a type that allows the user to set a ‘window’ period using voltage levels to set up a position to trigger at. It will enable users to focus and hone in on a particular signal with multiple logic levels.
This mechanism lets users set up a time domain to frame their window by positioning it to enter or exit the window. It thereby allows a certain level of flexibility. It also permits the setting of a time domain to frame the window.
x. Delayed triggering
Then, the delayed trigger allows users to set two sources (Source A & B) by raising or dropping the logic for each reference to the function. The user then has to decide and select a delay type from the <, >, <>, >< options along with a period. It is because of the decisions and selection made that the possibilities end up working just like expected.
There is a corresponding time setting to each of the delay types, which is possible to select. The < type only requires an upper limit, whereas the <> type will require both an upper and lower limit.
xi. Trigger correction
Very fast systems require accuracy in trigger delays. However, there is usually an inherent difference between the trigger position and acquired data. It exists because of different time delays in the trigger and signal paths.
This difference can, in turn, lead to jitters in the skew or display. This is where a trigger correction system is employed to overcome the difference. It works by compensating for any delay in differences between the trigger and data acquisition paths.
The trigger point can be used for reference reasons when used in this mode.
Factors that determine the scope trigger performance
Two common aspects determine the trigger performance of an oscilloscope:
a) Trigger flexibility
An oscilloscope’s trigger flexibility indicates how easily a trigger threshold can improve the o-scope trigger’s efficiency. It works at adapting to fulfill the signal tests conducted under a test.
Most of the o-scopes offer various vendor-defined trigger functions. They, however, come with a restricted few settings like level and width, and it is not possible to optimize them.
b) Trigger dead time
Trigger dead time is an inherent feature of a trigger mechanism. It shows how long the o-scope fails to detect triggers in between acquisitions. When interest falls into a slow time, it leads to missing trigger conditions in the oscilloscope.
It, in turn, triggers dead time. Though trigger dead time is an important feature, various techniques and methods help reduce the dead time.
Many oscilloscope vendors are offering software-based triggers for added flexibility. However, the software consumes lots of dead time because of its post-processing requirements. It is therefore not an ideal choice for use in sporadic and infrequent events.
Trigger modes
The trigger mode indicates whether or not the oscilloscope draws a waveform whenever it doesn’t detect a waveform. The standard trigger modes are normal and auto modes.
Normal mode
When in normal mode, the o-scope sweeps only when the input signal reaches a set trigger point. If not, the screen remains blank in the case of analog oscilloscopes.
It, however, remains frozen on the latest acquired waveform in the case of digital versions. This is a complicated mode because there is a chance of missing the signal if the level control isn’t properly adjusted.
Auto mode
When in auto mode, the oscilloscope tends to sweep even in the absence of a signal. It is triggered by a timer in the scope whenever there isn’t any signal. The mechanism ensures the display does not disappear in case the signal drops to low voltages. It is thus the best mode to use while looking at multiple signals because there is no need to set triggers every time.
It is, however, a practical and wise option to use a combination of both modes. Each mode offers its individual advantages. In the normal mode, it provides for more versatility, while the auto mode requires minimal adjustments.
Some scopes have special or unique modes for the single sweep, a video signal or triggering modes, or an automatic trigger level setting.
Trigger coupling
It is possible to select between the vertical system’s AC or DC coupling and select the ideal trigger coupling. An oscilloscope can possess high-frequency rejection, low-frequency rejection, and noise rejection trigger coupling other than AC/DC coupling.
These unique settings eliminate all trigger sounds, thereby preventing false signals.
Trigger Hold off control
The trigger hold-off control is a unique and useful trigger in scopes. It proves useful while triggering complicated waveforms when users need to trigger the right part of a signal. This is something that requires great skill. This is why many oscilloscopes offer this special trigger hold-off feature to make the task easier.
It is better to understand the trigger hold-off controls working in analog oscilloscopes.
The control lets the scope user add delay to the re-arming of the trigger circuit. The control provided extends beyond the sweep and retrace period, giving users complete control over how quickly the scope is triggered.
Some waveforms have multiple points for signaling the scope.
The trigger hold-off feature helps add clarity to the scope’s displayed image. The scope triggers on the first pulse, and the trigger circuit ignores any further pulses arriving once the scope sweep completes.
The scope returns the sweep voltage to the start point, and as the beam or trace is blanked, the flyback isn’t visible on the screen. All further arriving trigger pulses are ‘held off until the completion of the sweep and retrace.
It’s when the trace is at the start point of an appearing waveform that it’s ready to trigger again. Though the working was explained in analog scopes, it doesn’t mean that the feature exists only in analog scopes. Digital oscilloscopes also have the feature.
Oscilloscope auto-trigger
The trigger facility is fine when a signal is present, and the scope is triggering. However, in the absence of a signal, it is always better to see where the trace is.
For example, while setting the trace to a particular place on the screen before applying the signal and making a measurement.
An auto-trigger feature is included in oscilloscopes to overcome the lack of trace during small or no-signal conditions. It helps by starting the sweep in the absence of a signal.
There is a timer in the scope that starts the sweep. Upon detecting that the scope hasn’t been triggered for a while, the auto-trigger sets the sweep in motion.
Most oscilloscopes even have a feature that lets users set the delay.
The oscilloscope is usually set to the auto-trigger mode when in general use. It is then set to ‘normal’ mode in case of more exacting waveforms and measurements.
Oscilloscope trigger sources
Along with knowing the benefits of an oscilloscope trigger, identifying its sources helps understand how to tackle disruptive waveforms. External trigger sources sometimes help stabilize the waveform to make it look much more stable.
Several sources that can trigger the sweep:
1. Video oscilloscope trigger source:
The video oscilloscope trigger source is widely incorporated in television and analog video applications. The source is extracted and used the sync pulses embedded in the analog video signals.
This proves helpful in stabilizing the waveform.
2. Signal channel trigger source:
This is the most common waveform source because the signal channel is the trigger. When compared to the other sources, this is a primary source. The triggering is marked A/B trigger source channel or its equivalent.
In multiple channel scopes, the trigger automatically defaults to channel A. However, it triggers in other channels.
3. Line signal trigger source:
A line signal trigger source is prominently helpful for diagnostic purposes to detect problems associated with the line. This is because the scope triggers the line voltage waveform or power input.
4. External trigger source:
As the name suggests, these scopes depend on an external source. The system is synchronized by connecting to an external signal. It is usually used to manifest similar controls on an identical pattern of trigger voltage and slope.
Uses of an Oscilloscope
Oscilloscopes are most commonly used by engineers and in the field of research and troubleshooting. It is used for:
- Quantifying the phase difference between a set of different signals. Identifying if the waveform shape is triangle, sawtooth, sine, square, complex, etc.
- Determining the noise level in the circuit
- Identifying a malfunctioned component in a given circuit. It is done by estimating the amplitude and frequency of a signal. This information helps in debugging a circuit’s internal systems, its input, and output circuitry.
How to determine the right oscilloscope to use
Before investing in an oscilloscope, it is better to define and hone how well the oscilloscope suits the task. This is very important because not all oscilloscopes are created the same.
Apart from the upgraded oscilloscope trigger available in it, there are some other essential factors to consider. They are the bandwidth, rise time, channel density, probe compatibility, and sample rate.
Conclusion
It is worth knowing how to use an oscilloscope trigger as it plays an important part in configuration the scope to view waveforms. It is thus an important function all users should know and understand to use an oscilloscope properly.
With practice, users will become comfortable using oscilloscope triggers, and soon become a professional. A new device is always exciting but confusing to understand and use in the beginning.
It is only after mastering its functions it is possible to make it work more efficiently. This applies to oscilloscopes and oscilloscope triggers too.
