[sdiy] Old 20Mhz scope high on LSD!!

[harrybissell]

Hi Karl (inline)


> > Of course you are correct... there are a few problems
> > 1) The phosphor of the scope tube will have been selected for the sweep
> > speed.  Higher sweep rates will need a shorter persistance phosphor, and
> > a lot higher acceleration voltage for the tube.
>
> My friend says that he had looked at two old Hitachi scopes, one 20Mhz
> the other was a 100Mhz, both used the same type of CRT tube, so that
> contradicts you sugestion of shorter persistance phosphor and higher
> acc voltage! Im not saying that you are wrong, my frind might have
> missed some issues.But explain the effect of "persistance phosphor"?

A faster scope must have a shorter persistance phosphor, or it will display
a big fuzzy blur from sucessive traces still on the screen.

Writing that short persistance phosphor takes more energy (ie higher voltage)
to get the picture on the screen in the shorter time allowed...

Hitachi might have used the faster tube on a slower scope... but you can't take
the slower tube up to faster speeds (at least not with guaranteed success...)

> Btw, I did a quick search on the net and found:
> http://members.tripod.com/michaelgellis/scope.html
>
> Wich also speaks about BW's about 100Mhz!!
>
> > 2)  The high voltage deflection amps need to be beefed up to handle the
> > faster signals, at high voltages.
>
> My schemos says that they uses high voltage but almost no current at +150V.
> Why would i need to beef up them for higher voltage when it is the Bw thats
> going to be expanded. Explain please!!

not higher voltages. AT high voltages.  The voltages will be the same (and high)

but what seemed to be an acceptable waveshape at 20MHz will be very sloppy
at 100MHz.  The problem is in the rise times and overshoot control.  The 20MHz
signals will be almost like sine waves by comparison...

> If the BW are allready high for the defelections amps and it is the
> input amp who limits BW, wich seams resonably build whise, why not try!
> I dont see why the voltage should be higher?
>
> > 3) Damping everything properly will be tough, stray capacitances that
> > were  not designed out of the chassis will come to haunt you in spooky
> > LSD way.
>
> Damping, well exactly where would damping cause problems and why would
> anyone inwolve the chassis otherwhise then using it as usual gnd!
> I men if one looks at this scope update as RF design (with a little
> higher voltages used) then i think its quite possibly!

as another post pointed out... the parasitic components are part of the circuit
at 100MHz
and were probably ignored at 20MHz, That was my point...

> > One possible solution would be
> > to build a digital sampling front end and use the slow scope to read out
> > the results.
>
> Thas much more expencive and cumbersome!

That would be my first choice though... you can get good fast A/D converters
etc...
of course it will be tough unless you already HAVE a good fast scope  :^P

> > I would recommend finding a broken scope of high bandwidth and restoring
> > it if you > really want to play.
>
> I want to play, with my own scope, where im located i would
> have to hunt for ages to find a faulty scope to play with.
>
> Aaarrrgghhhhh!!!
>
> You bastards,telling me its to difficult..... i wont give up !.......notyet!
>
> I will put pubes in your cilliconcarne!

That will be one spicy chili.  BTW I get these e-mails promising they will make
me hard but thats no problem for me. Where can I get something to make me soft
???  <bfg>

H^) harry