Oscilloscope Delayed Video Trigger
This circuit is intended for those people who want to work with analog video signals but aren’t fortunate enough to own an oscilloscope with a video trigger. It provides a trigger synchronized the video signal’s vertical sync pulse that can be used with any oscilloscope that has the capability of accepting an external trigger. As a bonus, the trigger delay is adjustable delay so you can focus in on any portion of the video signal you want to see.
The video signal is initial fed to U1, an LM1881 video sync separator. The horizontal sync pulses are extracted from the video signal and appear at pin 2.
U2 and U3 are CMOS dual ‘D’-type flip-flops. The first flip-flop, labeled as U2a, is configured as a one-shot timer that is triggered by the positive-going edges of the horizontal sync pulses and produces a negative-going pulse at pin 2. The length of the pulse is determined by R2 and C3, and set so the output at pin 12 will be low during the video signal’s vertical sync interval and a hi at all other times.
Video signals as composed of a succession of frames. Each frame is composed of two fields, each having its own vertical sync interval. Therefore, at this point, we have two pulses for each frame of video, one for the even field and one for the odd. Since we want only one pulse for each frame of video, U3a is configured as a divide-by-2 counter. The output from the counter is fed to U3b, which is configured as a one-shot timer, but with a variable pulse width controlled by V1. The end of the pulse from U3b is what is used as the oscilloscope trigger.
To use the delayed video trigger, configure your oscilloscope to use a positive-going external trigger. Connect the signal from pin 2 of U3 to the oscilloscope’s external trigger input and feed the video signal simultaneously to the oscilloscope’s input jack and pin 2 of U1. Adjust the oscilloscope’s timebase so you see about 3 or 4 lines of video on the screen. As you adjust V1, the video signal should shift across the screen, giving access to whatever portion of it you want.
For those people who don’t want to put together their own, a circuit board for this project is available from FAR Circuits. Look for CONSUGAR DELAY VIDEO TRIGGER FOR OSCILLOSCOPE under TEST EQUIPMENT & GENERAL MEASURING INSTRUMENTS.
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