Grupa: Administrator 
Lokalizacja:: Kłodzko JO80IK
Posty: 39687 #8124780
Od: 2013-10-18
| #42: Composite Video on an Analog Oscilloscope using XYZ - Lab tour...
One more video playing with the Z-axis input on an analog oscilloscope. This time, I turn the scope into a composite video monitor by creating X and Y sawtooth ramp signals from the vertical and horizontal sync pulses. For this hack, I use 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 simply inverting amplifier to invert the video signal so that the resulting image on the scope is not 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 10uF cap in the vertical ramp circuit was changed to 0.1uF, and the corresponding 680ohm resistor in the current source feeding that cap was increased to 68Kohm. And, I changed the 39nF cap that creates the horizontal ramp to 22nf 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 dependent - set 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 accomodate 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. If anyone wants details for these additional changes, please let me know.
Updated schematic can be found here: http://www.qsl.net/w2aew/W2AEW_NTSC_t...
I added a few more improvements to the circuit. I replaced the composite video amplifier with an improved version which gives better gain (contrast) and offset (brightness) adjustments which helps it to work better with more scopes. This is an inverting amplifier since all of my scopes blank the trace as the Z input is raised.
http://www.qsl.net/w2aew/W2AEW_NTSC_2...
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73! Stanisław Kłodzko JO80IK
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