Scope bandwidth limitation - amp or crt?

[Joerg]

cs_posting@hotmail.com wrote:

Just curious, in cheap analog scopes (a mid-90's vintage 20 mhz unit
labeled by JDR micro) what is the main source of bandwidth restriction
- the vertical amplifier or the crt itself?

As Robert said, the limit is usually in the electronics and in
particular in the last stage. Driving the deflection plates at increased
speed requires increased slew rate of the output stage. Because of the
large voltage swings that leads to costly transistors and a more
intricate design.

Regards, Joerg

http://www.analogconsultants.com

 

w2aew wrote:

I've only played with a couple of older low-BW scopes deflection plates
directly - and as I recall, they were in the 10-20pF range. Deflection
voltages were similar to what's been suggested already.

Thanks - that gives enough for initial contemplation.

I just realized my question should have been where does the load
_admittance_ come from (the capacitance) since the source of the
impedance is rather obvious.

 

[Joerg]

Hello Jim,

FYI,the CRT is a definite limit on bandwidth. The CRT electron gun
structure is VERY different between the 100Mhz TEK 465 and the 200-250Mhz
TEK 475 models. The 475 uses a distributed deflection plate assembly that
is terminated by a separate ceramic termination resistor network. Same goes
for the 500Mhz 7904 CRT.

True, but now you are talking about a whole different market, the
high-end. The OP just wanted to get past 5MHz and on those scopes it's
usually the amp that limits the BW. They often just couldn't afford
another buck or two in production for hotter final amp transistors when
the whole scope has to be under $250.

Case in point: I have used the plates of a scope CRT directly for
transmitter measurements. I could easily go to 30MHz with the CRT of a
scope that was otherwise tapering off at 10MHz (3db). With direct drive
there wasn't even one dB of slump in amplitude.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

cs_posting@hotmail.com wrote:

It seems that in a cheap scope, the amplifier is the first limit, but
there's also a limit to what a basic CRT can do - to get really high
bandwidth you also need this transmission line plate structure.

Not if you just want to push your 20MHz scope a tad higher but certainly
when you want to reach VHF with it. I guess that is one of the reasons
why many older Tek scopes aren't repaired once the CRT goes kaputt. A
new CRT would often cost more than the residual market value of the
whole scope.

If you need a fast analog scope the best bet is to hunt for business
closure auctions (that's where I got a 1GHz HP) or other used equipment
sources.

One very tough cookie would be the speed upgrade of the input
attenuator. This thing is rather complicated with all the little trim
caps and the fact that it is all jammed into very tight shielded spaces
doesn't make that job easier.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello Michael,

How about using the video driver ICs and output transistors from a
dead VGA monitor? Use a good op amp to invert the signal to one
channel.

Sure. I think I mentioned video transistors before, they can be had for
even less than those from VGA monitors. However, on a scope that has to
retail for $250 even 10c more might not be in the cards because you need
two of them, or four if it's a 2-channel.

Personally I am cautious with TV chips such as video drivers. They
rpoduction lifetime may not be long and I have seen people get burned
big time by sudden parts obsolescence. Basically when they asked for
leadtime they were told "infinity".

Regards, Joerg

http://www.analogconsultants.com

 

[Jim Yanik]

Joerg <notthisjoergsch@removethispacbell.net> wrote in
news:wfJOf.44881$H71.2482@newssvr13.news.prodigy.com:

cs_posting@hotmail.com wrote:


It seems that in a cheap scope, the amplifier is the first limit, but
there's also a limit to what a basic CRT can do - to get really high
bandwidth you also need this transmission line plate structure.


Not if you just want to push your 20MHz scope a tad higher but certainly
when you want to reach VHF with it. I guess that is one of the reasons
why many older Tek scopes aren't repaired once the CRT goes kaputt. A
new CRT would often cost more than the residual market value of the
whole scope.

No,it's because those CRTs (all TEK-made;single-source) are no longer in
production,and no longer available. IOW,there are no "new CRT's".

IMO,if new CRTs were available,people would buy them despite costing more
than the present market value of the scope.
(assuming the rest of the scope was working or repairable.)


If you are VERY lucky,you can find an unused CRT that was stocked by the US
military and sold off as surplus.
(and TEK sold off their CRT unit,they no longer make -any- CRTs.)


--
Jim Yanik
jyanik
at
kua.net