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  • EEVblog #524 – Vignetting on a Cathode Ray Tube

    Posted on September 22nd, 2013 EEVblog 13 comments


    Dave explains what causes CRT vignetting problems and how to fix it.
    A follow up to the HP 35660A Dynmaic Signal Anayser video here:
    http://www.youtube.com/watch?v=VNsJ8qSk6CI
    Whiteboard explanation of how a Cathode Ray Tube works, and how it can cause interference to sensitive measurements and the shielding required in a precision instrument like the HP DSA.
    Forum HERE

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    • http://truthspew.wordpress.com kd1s

      Nice job. I finally realized DSA is Digital Spectrum Analyzer. Duh – we use the same kind of thing except at slightly higher frequencies in amateur radio.

      • http://www.eevblog.com EEVblog

        Yeah, DSA is essentially a low frequency spectrum analyser. Not quite sure of the history of the term “Dynamic” though. Anyone?

    • Fred

      Good video Dave would like to add one thing and that is that the front of the screens phosphorus has an aluminum coating to prevent ionize burning without it an X would be burned on the screen. This technology will soon be long gone which in a way is to bad.

    • Steve

      Great video. Back in my TV days I always wondered why pulling the yoke back would cut off the corners. Now I know. I might point out however is that CRT probably uses electrostatic focusing instead of magnetic focusing. Never seen a tube with focus coils built in the inside of the CRT. You will probably find the voltage on that focus wire is >1000VDC — way too high for magnetic focus.

    • Kostas

      Nice video Dave, would like to make a small correction: the pair of coils above and below the tube produce HORIZONTAL deflection; the coils to the left and right of the tube produce VERTICAL deflection.

      • Anatol

        nicely spotted!

      • http://www.eevblog.com EEVblog

        Yes, I instinctively drew electrostatic deflection, but mentioned magnetic.

    • (*steve*)

      As someone with a background in photography, I feel qualified to tell you that the G in vignette is pronounced very much like the G in sign. :-)

      • Simon

        I (non-native-speaker) had no idea that “sign” is spoken with a hard g. “si-gn” – I need to train this. :)

    • Pierre

      I guess with enough sample memory, if you captured that signal you could recreate the CRT picture on a PC.

    • Stuart

      Damn straight “In God We Trust”

    • Bruce

      We do, but all others, pay cash!

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