Triggering the Delayed Time Base

Trigger Modes

The delayed timebase may start immediately after the main timebase has reached the level at the DELAY potentiometer. But now the following may happen. Assume that the signal to be tested is a pulse train and that the time between two successive pulses is not constant, but varies a little around the set repetition rate. The result will be a somewhat unstable display; this is known as jitter. The time between the first and the second pulses varies a little, as does the time between the second and the third pulses. The third pulse varies twice as much with respect to the first one as the second pulse does. The fourth pulse varies three times as much, and so on.

Operating the DTB in the STARTS mode now will show the jitter to be magnified to the same extent as the signal. In the STARTS mode, the main time base is triggered at the first pulse, while the nth pulse is intensified after the delay time. In the DELAYED mode, the nth pulse is displayed. As the variations are too quick for the eye to follow, the transitions of the pulses become a vague band. The width of the band is a measure of the jitter.

However, although the jitter can be measured, the shape of the nth pulse cannot be studied, because the display is too vague. But, by selecting the delayed-time base trigger mode TRIG, the trigger pulse caused by the nth pulse will now start the delayed sweep, resulting in a stable display.

If the DELAY potentiometer is rotated in the STARTS trigger mode, the user will notice the intensified part of the main timebase shifting smoothly through the entire time window. In the TRIG mode, however, rotating the DELAY potentiometer will cause the intensified part to jump from one pulse to another because the delayed time base is triggered each time by the first pulse after the DELAY time.

 

MIXED SWEEP

Another application of the second time base is for the display of “mixed sweep.”

As soon as the amplitude of the delayed sweep exceeds the instantaneous level of the main sweep, the deflection is electronically switched over to the former.  It takes some time after the delay before the amplitude of the delayed sweep will exceed that of the main sweep. At the moment the delayed sweep becomes active, the sweep speed is taken over by it and is thus determined by its setting.

Mixed sweep is most valuable in digital applications where one pulse after the other can be observed and counted simply by varying the delay time. However, the alternate time-base mode is advantageous over the mixed-sweep mode in this case because the pulses can be counted equally well. Moreover, the whole MTB sweep is visible, indicating also the position (intensified) where we are counting.

 

DUAL DELAYED TIME BASE

Further utilization of the delayed-sweep facilities may lead to the measurement of an interval of time. For this purpose, the DC input signal to the comparator may be derived from a circuit.

The DC signal to which the MTB sawtooth voltage is compared comes from an electronic switch. After each MTB sweep, this switch connects voltage A or B, alternating, to the comparator. The controlling voltage ALT may be derived from the SGM of the MTB.

When the switch is in position B, then the DC voltage is derived from the DELAY potentiometer.

References

www.testandmeasurementtips.com/difference-hold-off-control-delayed-sweep/

www.picotech.com/library/oscilloscopes/advanced-digital-triggers

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