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I'm a novice and have used my Rigol scope to look at small circuits from kits and attempt to diagnose issues on my vintage computers. I'm at a point where I want to check out composite video (for learning purposes) and I keep reading that a video signal expects a 75 ohm load.

This makes sense, but given my scope only has 50 ohm and 1M ohm input resistance, and also taking into account my probe, do I need to impedance match to 75 ohms, and if so how do I do it? In my case I would be looking at the composite output coming out of the device.

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  • \$\begingroup\$ At 5MHz bandwidth for NTSC, a 2 meter cable ( 6nS if air , 12nSec if Er=4) is 12nS/200nS or 6% of the highest period. You can either terminate in 50 or infinity, unless the video source needs the precise 75. For computer displays, with 5 nanosecond pixels, you'd better terminate accurately. \$\endgroup\$ – analogsystemsrf May 6 '17 at 3:54
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If you are looking for accuracy and (more probably) avoiding reflections then the \$50\:\Omega\$ oscilloscope inputs will need a \$75\:\Omega\$ feed-thru terminator of some kind. You can buy them or make them. The T connector I'm sure you are familiar with. But you can see the T, the termination, and both hooked together here:

enter image description here

Also, you will really wish you had triggering that recognized parts of the signal you will look at. Assuming AC coupling, hopefully you can find (under the triggering menu) something that talks about "video" and allows you to trigger on odd or even fields and even specified horizontal line numbers.

Note: Image borrowed from the page located here

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  • \$\begingroup\$ Thank you for the answer. I had purchased these in anticipation of possibly needing them, so this works out well. And yes, my scope has the option to trigger on particular scanlines, etc. \$\endgroup\$ – Brian Sturk May 6 '17 at 0:27
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Yes, you should terminate the video signal in 75 Ohms at the scope input.

You can get in-line 75 Ohm BNC terminators, or use a BNC T and 75 Ohm termination at the scope input.

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It depends if you want to terminate the signal with 50 coax into scope, then add 25 Ohm series R.

It is far better to use 10:1scope probe calibrated to square wave test pattern then remove clip and ground wire and only test between a terminated signal ( either by LCD VGA cable, TV SCART or on board)

enter image description here

This can be home made with spring wire or purchased and is how I prefer to measure video unless I have many signals to compare.

But you can go between two adjacent pins video, gnd. close together using resistor wire as test pins.

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  • \$\begingroup\$ I have a 10x probe and a spring ground clip so this is do-able for me. Can you purchase 50 ohm probes? I have read about 50 ohm coax (and 75 ohm) but I can't say I've seen probes like that. \$\endgroup\$ – Brian Sturk May 6 '17 at 2:28
  • \$\begingroup\$ I have used FET buffered Diff Probes <1pF but RF signals usually need a suitable connector not inductive pins. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 6 '17 at 2:30

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