Composite Video on an Analog Oscilloscope Using XYZ

April 27, 2022
Alan Wolke, call sign W2AEW, turns an oscilloscope into a TV monitor.

This video is part of the TechXchangeOscilloscope Techniques

You always hear about them, but they are hard to find: Tech talks on YouTube that are fun and interesting. Well, stop searching for test and measurement titles and check out Alan Wolke's channel

Alan is a ham in more ways than one. His call sign is W2AEW. He also has a knack for presenting technical topics in an entertaining way. The video above is just one example. I actually remember building my first monitor using logic similar to this. Of course, turning an oscilloscope into a TV is more of an engineering exercise than a practical application, but it's one of many videos he has on his channel that's subscription-worthy. 

This particular video uses the Z-axis input on an analog oscilloscope. The input is composite video that's used to create X and Y sawtooth ramp signals from the vertical and horizontal sync pulses.

For this hack, he used an LM1881 Composite Video Sync separator to easily extract the sync pulses from the composite video, then a couple of transistors to create the sawtooth waveforms, and finally a simple inverting amplifier to invert the video signal so that the resulting image on the scope isn't a negative image.

"Then, I take a little lab equipment tour, using the scope video monitor to show the equipment (just for fun!). Note: I made a little tweak to the circuit to enhance the vertical linearity and video contrast. The 10-µF cap in the vertical ramp circuit was changed to 0.1 µF, and the corresponding 680-Ω resistor in the current source feeding that cap was increased to 68 kΩ. And, I changed the 39-nF cap that creates the horizontal ramp to 22 nF to increase the amplitude of it a bit. More careful power supply decoupling got rid of a lot of the noise on the ramps, and sharpened the video.

"Also, I lowered the value of the collector resistor in the video inverter to lessen the contrast on my scope. This 'gain' change will be scope-dependentset the gain as needed to get the contrast you want on your scope based on your video amplitude and scope z-axis sensitivity.

"A few more tweaks... I added an adjustable gain stage for the composite video amplifier so that it can accommodate the Z-axis gain differences on different scopes. I also added an inverter for the X-axis ramp signal, so that I didn't have to invert it on the scope. My old 485 uses CH2 for X, and CH2 has an invert switch. However, many other scopes use CH1 for the X input, and thus typically would not have the invert function."


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