1. Home
  2. Electronics Equipment
  3. Magnetically deflected oscilliscopes

Magnetically deflected oscilliscopes

Some here may remember my quest for a large-screen X vs Y scope.

I picked up a Telonics 101 10" X-Y scope on E-bay, and was surprised to find that internally it uses magnetic deflection. The CRT is about the same aspect ratio and length as a 10" TV tube. (i.e. it's not long and skinny, like a typical electroscatic scope CRT). There's a yoke, and two smallish PCB's for HV generation and deflection.

Frequency response is limited (seems to not go much above a few tens of kHz) but is good enough for my playing around.

Knowing something about Telonics instrumentation, this was almost certainly intended as a display for a spectrum analyzer. Seems to be maybe mid-70's vintage.

My question: What are the fundamental constraints on frequency response of a magnetically deflected scope? Inductance in the yoke would seem, to me, to be the limiting factor in setting the maximum sweep rate. Bigger currents in the yoke driver will get you faster slews. There's probably some frequency (10's of kHz? 100's of kHz?) at which the yoke becomes self-resonant. Am I missing anything?

Tim.

Reply to
Tim Shoppa
Loading thread data ...

"Tim Shoppa" schreef in bericht news: snipped-for-privacy@posting.google.com...

Tim,

For higher frequency response you do not need more current. The current has to change faster. To achieve a faster changing current you need a higher voltage. You can of course try to reduce the inductance of the yokes but then you need more current. I never did some calculations on it but I guess this power problem practicaly sets the limits, not the selfresonance of the coils. Although you have to count with it when designing such a thing. Even ordinary TVsets sometimes have a ringing problem that is shown as vertical bars at the side of the screen.

petrus bitbyter

--
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.778 / Virus Database: 525 - Release Date: 15-10-2004
Reply to
petrus bitbyter

What type of amplifier thus it use to drive the coils? That may be your first limitation. Getting low impedance high current and high voltage with tubes, without transformers, was not a easy thing to do.

Vlad

Reply to
Vlad

I haven't actually traced out the driver circuitry, but it's transistors in TO-18 cans. Construction technique and parts make me think that it's probably from the mid-70's, maybe earlier. No IC's to read date stamps off of, that's for sure!

Tim.

Reply to
Tim Shoppa

whatever the deal, you wont get far with it. Signal measured can of course be 20x the sweep freq, past that you can use an envelope detector... but you still will never make it too far on mag deflection.

NT

Reply to
N. Thornton

In my case, I can just scale the max frequency down without any pain. After all, I am using it strictly in X-Y mode as a display for an analog computer where I get to choose all the time constants.

*I* control the horizontal, *I* control the vertical :-). It's wonderful to have a big CRT to do this with.

That's my original question: what's the fundamental limit on frequency response with magnetic deflection? We have V=L dI/dt (more of a slew-rate limit than a bandwidth limit, until you start insisting that the picture fill the whole screen!), but is there anything else? When does the self-resonant frequency of yoke coils bite back?

Tim.

Reply to
Tim Shoppa

That I dont know. For some jobs you can also us amplitude modulation for the signal, giving a just usable measurand frequency of upto 1000x timebase. That way if you get say a 20kHz scan, which should be easy enough, that takes you upto 20MHz - just not with much detail.

I've just had a thought: if you could measurand modulate the EHT, using a relatively low frequency timebase the EHT changes will produce deflection changes, since deflection depends on both EHT and the deflection coils.

You can run the EHT as low as poss, old scopes went down as far as

2kV, with the right electrode Vs. Does your analogue computer do 2kV??

NT

Reply to
N. Thornton

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.