Magnetically deflected oscilliscopes

T

Tim Shoppa

Guest
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.
 
"Tim Shoppa" <shoppa@trailing-edge.com> schreef in bericht
news:bec993c8.0410260927.7148111e@posting.google.com...
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.
Click to expand...
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


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On Tue, 26 Oct 2004 20:28:44 GMT, "petrus bitbyter"
<p.kralt@reducespamforchello.nl> wrote:

"Tim Shoppa" <shoppa@trailing-edge.com> schreef in bericht
news:bec993c8.0410260927.7148111e@posting.google.com...
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.

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
Click to expand...

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
 
Vlad <Bla@dot.com> wrote in message news:<2tk0o0tc9igmep71tcs1nujp6fkq3smhhh@4ax.com>...
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.
Click to expand...
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.
 
shoppa@trailing-edge.com (Tim Shoppa) wrote in message news:<bec993c8.0410280901.42ad4726@posting.google.com>...
Vlad <Bla@dot.com> wrote in message news:<2tk0o0tc9igmep71tcs1nujp6fkq3smhhh@4ax.com>...
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.

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.
Click to expand...

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
 
bigcat@meeow.co.uk (N. Thornton) wrote in message news:<a7076635.0410281656.49a9debb@posting.google.com>...
whatever the deal, you wont get far with it.
Click to expand...
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.

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.
Click to expand...
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.
 
shoppa@trailing-edge.com (Tim Shoppa) wrote in message news:<bec993c8.0410290205.76eb4200@posting.google.com>...
bigcat@meeow.co.uk (N. Thornton) wrote in message news:<a7076635.0410281656.49a9debb@posting.google.com>...
whatever the deal, you wont get far with it.

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.
Click to expand...
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
 
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