Magnetically deflected oscilliscopes

[Tim Shoppa]

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.

 

[Rich Grise]

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?

Inductance you can manage. I'd think your constraints would be limits on
the driver. With a 10 KV supply, you could get some pretty respectable
dI/dt! ;-)

And of course, since it's inductive, you'll get that famous inductive
kick at retrace.

Go ahead and slap together a couple of oscillators, and see what she'll do!

Have Fun!
Rich

 

[Dbowey]

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

What is the shortest period (fastest sweep) shown on the horizontal time-base
switch of your scope?

Don

 

[petrus bitbyter]

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

 

[Vlad]

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

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

 

[Tim Shoppa]

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

What is the shortest period (fastest sweep) shown on the horizontal time-base
switch of your scope?

There is no time-base; it's a pure X-Y scope. Bandwidth was roughly
measured by feeding sine-waves into X and Y amps until they started
distorting (weird distortion at zero-crossing, where slew rate was
fastest).

In fact, the real parameter limiting performance seems not to be frequency,
but slew rate. Which I why I suspect that driver V = dI/dt is the limiting
factor.

Tim.

 

[Tim Shoppa]

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.

 

[N. Thornton]

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.

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

 

[Tim Shoppa]

whatever the deal, you wont get far with it.

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.

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.

 

[Rich Grise]

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?

Just a wild-ass guess, but I'd surmise that the self-resonant frequency
of a deflection coil is so high that it's irrelevant at any frequency
where you could get enough current through it to actually deflect the
beam usefully.

Have Fun!
Rich

 

[N. Thornton]

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.

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