How to calibrate your oscilloscope for horizontal accuracy

The X-axis is dedicated almost exclusively to use as the vehicle for the time base(s). As well as multiple vertical channels, there will often be two time bases: Main and Delayed. These may be achieved in DSOs by two independent sampling rates, or via a positioned ‘zoom’ window on a single, but long, acquisition store.

Horizontal deflection accuracy

When determining the accuracy of horizontal deflection, typically with analog oscilloscopes, the geometry of the display must have first been set up. It is assumed that this will be included as part of the initial geometry setup.

Once this has been done, the following adjustments or checks can be attempted:

• X-axis bandwidth
• Horizontal timing
• Timebase delay accuracy
• Time magnification
• Delay time jitter
• Trigger functions
• X-Y phase relationship

X-axis bandwidth

For analog oscilloscopes, the horizontal amplifier’s bandwidth will be checked using a ‘leveled sine wave’, like the checks of vertical channels, but with the time base turned off. This consists first of measuring the displayed length of the horizontal trace, for a sinusoidal wave provided as X input at a reference frequency (usually 50 kHz).

The frequency is then changed, at the same amplitude, to the specified 3 dB point of the horizontal amplifier and the displayed trace length is measured again. The bandwidth is correct if the observed 3 dB point trace length is equal to or greater than 70% of the length at the reference frequency.

DSOs generally employ a vertical channel amplifier as the horizontal amplifier, so having measured the vertical deflection bandwidth, there is generally no need to measure horizontal deflection bandwidth.

Horizontal timing accuracy

Test setup

In this test the time base is switched to the sweep speed (or time/div) to be checked, and the output from a timing marker generator is input via the required vertical channel. With oscilloscope calibrators these are square waves, changing to sine waves at a specific frequency.

Timing calibration accuracy

A timing accuracy of 25 ppm will be sufficient to calibrate most analog oscilloscopes and many DSOs, although a timing accuracy better than 0.3 ppm is required for some higher-performance DSOs.

Why use square waves?

In the past, timing markers have taken the form of a ‘comb’ waveform, consisting of a series of differentiated edges in one direction, with the return edges suppressed. This leads to difficulties in DSOs due to sampling, in which the comb peak can fall between samples, leading to amplitude variations and difficulty in judging the precise edge position. The use of timing markers in the form of square or sine waves significantly reduces the inaccuracies due to this 1-dot jitter.

Measurement

The marker timing is set to provide one cycle per division if the horizontal timing is correct.

By observation, the marker generator’s deviation control is adjusted to align the markers on the screen behind their corresponding vertical graticule lines, and the applied deviation is noted. The applied deviation should not exceed the oscilloscope’s timing specification.

The operation is repeated for all the sweeps and time base time/division settings designated for calibration by the oscilloscope manufacturer.

Time base delay accuracy

For this test, it is assumed that the delayed time base is indicated as an intensification of the main time base and can be switched to show it alone. For all oscilloscopes, you will want the retrigger mode to be switched off.

The output from a timing marker generator is input via the required vertical channel, and the oscilloscope is adjusted to display one cycle per division. The oscilloscope mode switch is set to intensify the delayed portion of the main time base over a selected marker edge as shown (this may require some adjustment of the oscilloscope’s Delay Control).

• The oscilloscope delay mode switch is set to display the delayed sweep alone, and the delay control is adjusted to align the time marker edge to a chosen vertical datum line, i.e., the center graticule line. Note the setting of the oscilloscope’s delay.
• The oscilloscope mode switch is set to intensify the delayed portion of the main time base over a different selected marker edge.
• The oscilloscope delay mode switch is again set to display the delayed sweep alone, and the delay control is adjusted to align the time marker edge to the same vertical datum line. Note the setting of the oscilloscope’s delay again.

Finally, compare the two settings of the oscilloscope delay. You want to check that their difference is the same as the time between the two selected markers, within the specified limits for the oscilloscope.

Horizontal x10 magnification accuracy

The output from a timing marker generator is input via the required vertical channel, and the oscilloscope is switched to display 10 cycles per division. The timing marker generator frequency/period is adjusted to give exactly 10 cycles per division.

The errors are likely to be greatest on the right of the trace (the longest time after the trigger), so the oscilloscope’s horizontal position control is adjusted to place the marker edge at ‘A’ at the center of the screen.

• The oscilloscope is set to display the X10 sweep, and the horizontal position control is adjusted to align the marker edge ‘A’ exactly to the center graticule line.
• The marker generator Frequency/Period deviation control is adjusted to align the marker edges exactly to the graticule lines.
• The marker generator Frequency/Period deviation setting is noted. This setting should be within the specified limits for the oscilloscope.

Similarly, for a DSO, the range of available ‘Zoom’ or X-magnification’ factors are calibrated as designated by the manufacturer.

Delay time jitter

The delay jitter on oscilloscopes is often measured under time magnifications of the order of 20,000:1. This means that the delayed time base must run 20,000 times faster than the main time base (for a main time base running at 20 ms/ div, the delayed time base must run at 1μs/div).

For this test, the intensification of the main time base is adjusted onto the edge at the center graticule line (with such a difference between the speeds of the main and delayed time bases, an exceedingly small part of the main time base is intensified, and adjustment may be difficult).

The 20 ms period output from a timing marker generator is input to the required vertical channel, and the oscilloscope is adjusted to display one cycle per division (20 ms/div).

The delayed time base is set to run at 1 μs/div, and the oscilloscope mode switch is set to intensify the delayed portion of the main time base over the center marker edge as shown using the oscilloscope’s delay time control.

The oscilloscope delay mode switch is set to display the delayed sweep alone, and the delay control is adjusted to align the time marker edge to a chosen vertical datum line, like the center graticule line.

The width of the vertical edge (which displays the jitter) of the displayed portion of the waveform, measured along a horizontal axis, should not exceed the oscilloscope’s specified jitter limits (e.g., for 20,000:1 specification, the oscilloscope’s contribution to the width should be less than 1 division).

Continue calibrating

The next steps in the oscilloscope calibration process continue in these articles:

• How to calibrate trigger operation on an oscilloscope
• How to calibrate an oscilloscope for phasing

Return to previous articles

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