A Cathode Ray Oscilloscope (CRO) is a versatile electronic instrument used to display and analyze electrical waveforms. It works by deflecting an electron beam across a fluorescent screen, creating a visible trace that represents the input signal's voltage over time.

Understanding CRO Components for Waveform Display

To understand the role of trigger pulses, let's quickly review the relevant parts of a CRO:

• Vertical Amplifier: This circuit amplifies the input signal voltage. The amplified signal is then applied to the vertical deflection plates, causing the electron beam to move up or down according to the instantaneous voltage of the input signal.
• Time Base Generator (Sweep Generator): This circuit produces a voltage that increases linearly with time, usually a saw-tooth waveform. This voltage is applied to the horizontal deflection plates, causing the electron beam to move horizontally across the screen at a constant speed from left to right. This horizontal movement represents the time axis.
• Electron Gun and CRT: Generates and accelerates the electron beam towards the screen.
• Deflection Plates: Vertical and horizontal plates control the position of the electron beam.
• Trigger Circuit: This is where the trigger pulse plays its crucial role.

The Problem: Unstable Waveform Display Without Triggering

If the time base generator's sweep starts randomly without any relation to the input signal, the trace on the screen will not represent a stable, repeating waveform. The sweep might start at different points on the input signal each time, causing the waveform to appear to jitter or move around on the screen, making it difficult or impossible to observe and analyze.

The Solution: Synchronization Using Trigger Pulses

This is precisely where the trigger pulse comes in. The purpose of the trigger circuit is to provide a synchronization signal that tells the time base generator exactly when to start its sweep. The trigger pulse ensures that the horizontal sweep begins at the same point on the input waveform (or another reference signal) every time.

• The trigger circuit monitors the input signal (or an external trigger signal).
• You can set a specific 'trigger level' (a voltage threshold) and a 'trigger slope' (positive-going or negative-going).
• When the input signal crosses the specified trigger level in the specified direction (e.g., crosses 1V while going positive), the trigger circuit generates a pulse.
• This trigger pulse is sent to the time base generator, initiating the sweep.

By starting the sweep consistently at the same point on the repeating input waveform, each subsequent sweep draws the waveform trace in the exact same position on the screen. This results in a stable, stationary display of the waveform, allowing for accurate measurement and analysis.

Analyzing the Options

Let's look at the given options in light of this understanding:

1. to generate high voltage required for CRT

   Incorrect. High voltages for the CRT are generated by separate power supply circuits, not the trigger circuit.

2. to synchronize the input with the time base generator

   Correct. As explained above, the trigger pulse synchronizes the start of the horizontal sweep (generated by the time base) with a specific point on the input signal.

3. to synchronize the input and vertical amplifier

   Incorrect. The vertical amplifier simply processes the input signal. Synchronization in a CRO primarily concerns the relationship between the input signal and the horizontal sweep.

4. to generate low voltage required for the CRT

   Incorrect. Similar to option 1, the voltages for the CRT (both high for acceleration and lower for focusing/intensity) are handled by power supply sections, not the trigger circuit.

Therefore, the primary function of trigger pulses in a CRO is to synchronize the horizontal sweep (from the time base generator) with the input signal, enabling a stable display of the waveform.

Revision Table: CRO Synchronization

Additional Information: CRO Triggering Modes

Modern CROs offer various triggering modes to handle different types of signals:

• Auto Trigger: The sweep runs automatically even without a trigger signal, but synchronizes if a suitable signal is present. Useful for viewing signals when the trigger level is not critical or for having a baseline sweep even with no input.
• Normal Trigger: The sweep only starts when a valid trigger event (crossing the level with the correct slope) occurs. If no trigger event happens, the screen remains blank or shows only the baseline. This mode is essential for viewing non-repeating or slowly changing signals.
• Single Shot Trigger: The sweep starts only once when a trigger event occurs and then stops. This is useful for capturing transient (one-time) events. The CRO typically waits for the next trigger command.
• External Trigger: The trigger signal comes from an external source rather than the input signal being displayed on the vertical axis. This is useful for synchronizing the display to a different part of a system or a master clock.

Understanding trigger settings like level, slope, and mode is crucial for obtaining a stable and meaningful display on a CRO.