Calibrating an oscilloscope for trigger operation

For most oscilloscopes, a wide variety of trigger modes exist, being sourced either via a nominated Y-input channel, or from a separate external trigger input.

The functionality of the trigger modes allows for AC or DC coupling, repetitive or single sweep, and trigger-level control operations.

These standard function tests check the operation of:

• Internal trigger sensitivity in both polarities, from each of the available Y-input channels
• Operation of the trigger level control for a sinewave external trigger input
• Effect of vertical position on trigger sensitivity.
• Minimum trigger levels for normal and ‘trigger view’ modes
• Bandwidth of trigger circuits, and effect of HF rejection filters
• LF and DC performance of the trigger circuits
• Single-sweep performance and response to position controls

Note: Tests which are performed using a Y-channel input are also carried out on all the other available Y-channels.

How to calibrate internal triggers— trigger level operation

a. Initial setup

A standard 4 Vp-p (50 Ω) reference sinusoidal signal is input via AC coupling into the Y-input channels in turn. Using internal triggers and DC trigger coupling (not ‘AUTO’), the +ve and -ve slopes are selected in turn. The sweep speed setting is 10 μs/division; the Y-channel sensitivity is 0.5 V/division so that the input signal occupies 8 divisions.

b. Trigger level adjustment

Over almost all its range of adjustment, the trigger level control must be shown to produce a stable trace, moving the starting point over a range of levels up and down the selected slope of the displayed sine wave.

c. Trigger sensitivity

With the input signal reduced to 10 % of its amplitude, adjustment of the trigger level control must be shown to reacquire stable triggering. When using AC trigger coupling, it's essential to ensure stable triggering even when the trace is vertically positioned at both the extreme upper and lower boundaries of the display.

d. ‘Display triggers’ feature

If the oscilloscope has a ‘Display Triggers’ or ‘Trigger View’ feature, this is selected to display the trigger region of the waveform. Using a 200 mV sinusoidal signal input to the channel, the trigger region is checked for correct amplitude on the display.

Note: In the following trigger operations, during tests on a DSO, the trace will not disappear because of the interruption of the trigger (or reduction of its amplitude below the threshold). Instead, the trace will remain but not be refreshed, and this is the condition to be detected.

How to calibrate internal triggers for DC-coupled operation

a. Initial setup

With the Ext Trig input disconnected, and the Y-channel input externally grounded, the oscilloscope Y-channel is set to ‘DC-coupling’ and trigger mode for ‘internal triggers’ from the Y-channel. There should be no trace on the display.

b. DC triggering

By adjusting the Vertical Positioning control to pass through a point in its range corresponding to the Trigger Level setting and selected slope direction, a single trace should appear then disappear.

c. ACLF rejection trigger mode

With the input signal set as for the Trigger Bandwidth check, the ACLF Reject feature (if available) is activated and paragraph (b) is repeated. The single trace action should not occur.

External triggers— Single-sweep operation

a. Initial setup

(Applies only to those scopes with single sweep capability). With the Ext Trig input connected, the oscilloscope is set to ‘Single Sweep’, and trigger mode for ‘Internal Triggers’ from the Y-channel. There should be no trace on the display.

b. Single sweep triggering

Pressing the ‘Reset’ or ‘Rearm’ switch should produce a single trace. This action should not produce a trace when the Ext Trig input is disconnected.

How to calibrate external triggers

a. Initial setup

These tests start with the 200 mV signal, described in the previous paragraph ‘Display Triggers’ feature, applied to the external trigger input of the oscilloscope.

b. Presence of a trace

Adjustment of the oscilloscope’s trigger level control should be able to produce a trace. The Ext Trig input is disconnected and reconnected again, while checking that the trace disappears and is then reinstated.

c. Trigger sensitivity

With the input signal reduced to the minimum amplitude specified by the manufacturer, adjustment of the Trigger Level control must be shown to regain stable triggering.

d. Trigger bandwidth

With the input signal set to the minimum amplitude and maximum frequency specified by the manufacturer, adjustment of the trigger level control must be shown to acquire stable triggering. The Ext Trig input is disconnected and reconnected again, while checking that the trace disappears and is then reinstated.

e. ACHF rejection trigger mode

With the input signal set as for the trigger bandwidth check, the ACHF Reject feature is activated then deactivated again, while checking that the trace disappears and is then reinstated.

How to calibrate low frequency triggers

a. Initial setup

A 30 mV, 30 Hz sinewave signal is input simultaneously to Channel 1, Channel 2, Ext Trig Sweep A (main time base) and Ext Trig Sweep B (delayed time base). The oscilloscope is set for: trigger mode to ‘Internal Triggers’, Channels 1 and2, sensitivity to 10 mV/div, and sweep speed to 5 ms/div. Both main and delayed time bases should be displayed when selected, for both channels.

b. Channel 2 grounded

With Channel 2 input grounded, and Channel 1 set for 0.1V/div with the trigger selector set to Channel 1, stable displays should appear as expected.

c. Channel 1 grounded

With Channel 1 input grounded, and Channel 2 set for 0.1V/div with the trigger selector set to Channel 2, stable displays should appear as expected.

d. ACHF reject

With Channel 2 input grounded, and Channel 1 set for 50 mV/ div with the trigger selector set to Channel 1, the ACHF Reject feature is activated for both Sweeps A and B. Adjusting the trigger level control should acquire a stable display.

e. Positive and negative slope operation

With Channel 2 input grounded, and Channel 1 set for 10 mV/ div with the trigger selector set to Channel 1, adjusting the trigger level control should acquire a stable display for both + and -slope selections.

f. ACLF reject

With Channels 1 and 2 set for 10 mV/div with the trigger selector set to either channel, the ACLF Reject feature is activated for both Sweeps A and B. Adjusting the Trigger Level control should not be able to acquire a stable display for either + or slope selection.

How to calibrate the Z-axis

Z-axis input

If provided, the Z-axis input is usually positioned on the rear panel, but sometimes can be found near the display screen controls on the front panel. Digital Storage Oscilloscopes (DSOs) typically lack a Z-input feature.

Z-axis bandwidth

a. Initial setup

A 3.5 Vp-p, 50 kHz sinewave is applied to both Channel 1 and Ext Trig inputs. The sweep speed, trigger slope and trigger level controls are set to provide a stable display of 1 cycle per division.

b. Signal transfer to Z input

The signal input to Channel 1 is disconnected and transferred to the Z-axis input. the trace should collapse to a series of bright and dim sections. Using the oscilloscope brightness control, the trace is dimmed so that the brightened portions just disappear.

c. Bandwidth check

The frequency of the input sinewave is increased to the exact specified Z-axis bandwidth point. The amplitude of the sinewave is increased to 5 Vp-p. Adjustment of the sweep speed and trigger level controls should acquire a dotted, or intermittently brightened trace.

Continue calibrating

The next steps in the oscilloscope calibration process continue in this article:

• How to calibrate an oscilloscope for phasing

Return to previous articles

• How to calibrate an oscilloscope
• How to calibrate your oscilloscope for vertical accuracy
• How to calibrate your oscilloscope for horizontal accuracy