Re: 2445A self-test, how does the scope sense the spot on the graticule

Chuck Harris
 

I guess I should follow up on my flip little comment:

When you look at the spot dance, and the scope is properly
calibrated, you would swear that it is looking for the graticule
lines. It starts somewhere off the line, and then it overshoots
to the other side, then it overshoots again, but about half as much,
and keeps going until it converges on the graticule line.

Here is what I think they are really doing:

First, you have to remember that part of the goal of calibration is
to test out as much of the scope as you can, while calibrating those
parts that drift over time.

So, I believe they are closing a loop between a DAC voltage that is
held in one of the sample and hold registers, the trigger hybrid,
and the DAC voltage used for ADC operation. Note that the trigger
hybrid is used as a comparator in a software driven successive
approximation ADC.

So, as I see it, they set one of the sample and holds with a voltage
that would deflect the beam to a graticule line crossing, and then
with another sample and hold register deflecting the beam, try to
measure the voltage the set in the first sample and hold register
(eg the voltage that would deflect the beam to a graticule line crossing).

It makes for a nifty visual diagnostic of the DAC, S-and-H registers,
the trigger hybrid, and the ADC algorithm.


-Chuck Harris

Chuck Harris wrote:

It uses a very sophisticated photodetector: Your eyeball.

-Chuck Harris

J Hunt via Groups.Io wrote:
I have been curious about the self-test sequence for my 2465A scope, particularly the spot dance sequence.
Recent discussion on 2445A calibration (see snippet below) implies that the scope senses the spot position vs. the graticule lines.
Is that the case?

If so, how does the scope detect the spot position without a camera or photodetector?

John Hunt
Portland, OR

Re: 2445A calibration
Oct 16, 2018 , Chuck Harris wrote:
You can see how well you have done when the scope does its
spot dance in steps 111 and 112... and in the other little
automatic sections. The beam does a successive approximation
sequence where it starts some distance away from the graticule,
and converges onto the graticule intersections.
If you haven't properly related the graticule to the cursor,
it will miss the graticule intersection.



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