Re: Cinematronics vector generator...

On Fri, 3 Apr 1998 09:16:10 -0800 , Clay Cowgill <ClayC@diamondmm.com> wrote:

>Hi everybody.
>
>I was playing with the Cinematronics vector generator last night in
>Electronics Workbench (a spice simulator).
>
>I have a couple questions that maybe someone knows the answers to.
>Otherwise I'm going to try to measure it with a DSO over the weekend...
>;-)
>
> From playing with my (fairly) realistic model, it looks like the time
>for the output to react to a change in voltage at the DAC is about 1us.
>Does this sound realistic? Seemed kinda fast to me. (I think this is
>while in "DRAW" mode-- voltage going through the 10K+5Kpot series
>current limiter. Maybe I screwed up and had it in "INIT" mode... Hmmm.)
>
>Do any of you know what the "clipping" (z blank) timing is? For
>example-- when watching the output on the "virtual scope" in Electronics
>Workbench it looks like the linear portion of each charging curve starts
>about 50ns after the new voltage is applied. From there it's pretty
>linear out to about 150ns or so. Is z-blanking software controlled?

The z-blanking is hardware controlled by placing a value in the Line Length
counter.

>Seems like 50ns is pretty tight timing to keep, but since it's also the
>period of the 20MHz clock it seemed like a strange number to show up
>"accidentally". (I'm wondering if the clipping is done in hardware and
>starts automatically one clock tick after the DAC is loaded and lasts
>for two clock ticks after it.)

Can't help you here, I have no idea what value this time should be.

>Anyway, it just seemed really fast, so I'm hoping for a reality check.
>(This might be explained if I had the thing running with the "INIT"
>switch closed instead of "DRAW"... Up too late-- don't remember.)
>
>I also put in that weird-ass bridge rectifier-resistor *thing* in the
>feedback loop of the output Op-amp. It looks like it kinda flattens out
>the charge curve of the cap, but it also looked like it was screwing up
>the discharge curve shape. Maybe I had it wired wrong (or it confused
>SPICE, which really shouldn't happen)...

The weird-ass bridge is there for the blooming effect of the CRT toward the
edges. As the output of the OP amp get higher, the gain is changed to
compensate for the bloom.

-Zonn

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Received on Fri Apr 3 10:35:15 1998