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From: "Phil Allison"
Subject: Re: scope tubes
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 9 Sep 2002 22:05:14 +1000
NNTP-Posting-Date: Mon, 09 Sep 2002 21:54:45 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Hugo Coolens" wrote in message
> Does anyone on this list know why the CRT in oscilloscopes is of the
> electrostatic type and in televsions of the electromagnetic type?
** Electrostatic deflection of the electron beam in an oscilloscope CRT
enables very much faster sweep speeds and wider vertical bandwidth than
could be practically achieved with a magnetic system. It also means much
simpler drive circuitry with voltage proportional deflection angles for both
vertical and horizontal. The information is conveyed in the vertical
deflection and must have a wide and flat response - hard to do if inductive
coils are involved.
Magnetic deflection is preferable for a TV picture tube, despite
the speed limitations, since the larger magnetic forces easily deflect an
electron beam highly accelerated by the 20 KV plus HT voltage. The high HT
voltage produces a beam resistant to the effects of practical electric
deflection fields especially at the very wide angles (110 degrees or more)
needed for a typical TV picture tube. The fixed vertical and horizontal
sweep speeds needed for TV set make magnetic deflection viable despite the
inductance of the drive coils. The visual information is intensity
modulated onto the beam/s unlike with a 'scope.
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