Scope bandwidth limitation - amp or crt?

Guest
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?

If one built a better amp, one presumably able to slew the capacitance
of the plates in the existing CRT more rapidly, would that increase the
useable bandwidth of the scope?

Of course to do more than show squarer fast square waves the timebase
would also need to be upgraded...

And the original probes probably are a limiting factor too...
 
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?

If one built a better amp, one presumably able to slew the capacitance
of the plates in the existing CRT more rapidly, would that increase the
useable bandwidth of the scope?

Of course to do more than show squarer fast square waves the timebase
would also need to be upgraded...

And the original probes probably are a limiting factor too...

The amplifiers are the limiting factor in that case.
Timebase is relatively slow, so faster sweep speeds are more easily
implimented.
But.
The trigger circuitry may need a major upgrade to be able to respond
to the faster input rise/fall signals you want to see.
 
Joerg wrote:

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.
Thank you. Any idea what sort of voltages are typically involved?

Any rough (order of magnitude) guess on the capacitance of the plates?

The chances of actually changing anything are probably low, but it's
interesting to contemplate.
 
In article <1141320811.920129.231950@i40g2000cwc.googlegroups.com>,
<cs_posting@hotmail.com> wrote:

Joerg wrote:

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.

Thank you. Any idea what sort of voltages are typically involved?

Any rough (order of magnitude) guess on the capacitance of the plates?

The chances of actually changing anything are probably low, but it's
interesting to contemplate.

I had an old 5MHz Heathkit scope that I was going to reinvent at one
point. I swore the sweep wasn't linear enough (from an RC waveform),
and the 5MHzbw could certainly be improved on. IIRC I think it needed
something like 170V for a full 10 div deflection!
One cycle of 5MHz across the whole screen was then 200nsec. I needed to
improve on 170/0.2usec=850V/usec to show a higher frequency!
I gave up at that point, and never finished the project! (...and am now
a proud Tektronix owner)

--
Jim Nagy
Elm Electronics
 
Hello Jim,

I had an old 5MHz Heathkit scope that I was going to reinvent at one
point. I swore the sweep wasn't linear enough (from an RC waveform),
and the 5MHzbw could certainly be improved on. IIRC I think it needed
something like 170V for a full 10 div deflection!
One cycle of 5MHz across the whole screen was then 200nsec. I needed to
improve on 170/0.2usec=850V/usec to show a higher frequency!
I gave up at that point, and never finished the project! (...and am now
a proud Tektronix owner)
Mine is an old Hameg 207 and I still have it. Extremely sturdy but drops
off at 8MHz. IIRC it needed less but still above 100Vpp for a full
swing. Capacitance wasn't the issue.

I thought about the same thing but gave up because opening up the BW of
the input attenuator circuitry around that huge rotary switch appeared
too daunting. Actually I had the final stage licked, using rather cheap
"video transistors". They are (or were) used to drive the cathodes of
color TV CRTs.

The reason why mine isn't always linear: It doesn't have a trigger, just
a synchronizer. You have to couple it only enough to lock the waveform.
If you just leave the synchronizer pot pegged it'll distort badly.

Regards, Joerg

http://www.analogconsultants.com
 
Joerg wrote:
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

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.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
Hello Jim,

I don't know.I have not seen any references to high-BW analog scopes for
sale.
OTOH there are still a lot of used one to be had a reasonable cost. Here
in the lab the trusty old 7704A is dutifully continuing its job.

It's FAR cheaper to use a LCD video display and digitizing circuitry than
to make a high bandwidth CRT for an analog scope;that's one problem.
However, there are things you can't properly diagnose with an LCD
screen. Also, I found that even on EMI analyzers (!) there are LCDs that
produce an incredible racket above 30MHz. It's pathetic but once at a
client I had to place aluminum foil over the display, hit "store" and
then remove the foil to look at the spectrum. The only other option
would have been to find a larger space for 10m measurements and look at
the screen with binoculars (seriously, I have done that). This was never
the case with the older HP gear.


Beware of used TEK TDS scopes,as they may be past the LTPS(long term
product support) period,and thus have NO parts or assemblies available,and
no schematics for troubleshooting.
Regards, Joerg

http://www.analogconsultants.com
 
Joerg <notthisjoergsch@removethispacbell.net> wrote in
news:REmPf.57075$dW3.8653@newssvr21.news.prodigy.com:

Hello Jim,


I don't know.I have not seen any references to high-BW analog scopes
for sale.


OTOH there are still a lot of used one to be had a reasonable cost.
Here in the lab the trusty old 7704A is dutifully continuing its job.

Yes,I was saddened to see the 7000 series disappear.
(except for the 7500 models!)
They were potent,flexible scope platforms.
I too would much rather use an analog scope than a digital scope.





--
Jim Yanik
jyanik
at
kua.net
 
Hello Jim,

Yes,I was saddened to see the 7000 series disappear.
(except for the 7500 models!)
They were potent,flexible scope platforms.
I too would much rather use an analog scope than a digital scope.
I even bought a cheap digital camera for the lab, to be able to document
the screen contents since there is no GPIB except for the really slow
7D20 digital module. Well, at least that beats the messy fumbling with
that instant film back in the 80's.

Regards, Joerg

http://www.analogconsultants.com
 
Jim Yanik wrote:

Always thought glass blowing would be an interesting hobby...

Actually,most TEK-made CRTs had a ceramic cast bell,gllued-on
faceplate(nice and FLAT),and just the neck tube was blown glass.
Only the early CRTs were all-glass.
I wonder if that might actually make a hobbyist attempt at a custom CRT
more reasonable?

I would certainly be nice to be able to get in there and change things,
rather than have each attempt be a one-shot try.

Idle contemplation though... no space for such a project even if there
were time and funds.
 
cs_posting@hotmail.com wrote in
news:1143138772.558485.42070@u72g2000cwu.googlegroups.com:

Jim Yanik wrote:

Always thought glass blowing would be an interesting hobby...

Actually,most TEK-made CRTs had a ceramic cast bell,glued-on
faceplate(nice and FLAT),and just the neck tube was blown glass.
The flat glass,glued-on faceplate also gave us the internal graticule.
Earlier CRTs had external faceplates,or bonded-on faceplates that sometimes
debonded or got bubbles underneath.

Only the early CRTs were all-glass.

I wonder if that might actually make a hobbyist attempt at a custom CRT
more reasonable?
I dunno;it wasn't an easy task even for TEK,they had to trash batches of
CRTs,some lines had very high reject rates.The 2465 CRT was quite an
engineering feat with it's quadrupole lens that eliminated the mesh lens
and enabled a shorter CRT and still retain high BW capability,also
achieving low deflection sensitivity.

I would certainly be nice to be able to get in there and change things,
rather than have each attempt be a one-shot try.
Uh,even TEK did not repair or alter ceramic-belled CRTs after they were
finished,evacuated and tested. They did break them up,and reuse the
electron gun/deflection structures,and recycled the faceplate glass.
(or they used "seconds" for internal TEK use;that's how I got a FREE new
CRT for my 2213!)
We had to return old CRTs to Beaverton for recycling.

Idle contemplation though... no space for such a project even if there
were time and funds.


--
Jim Yanik
jyanik
at
kua.net
 
Joerg wrote:

Mine is an old Hameg 207 and I still have it. Extremely sturdy but drops
off at 8MHz. IIRC it needed less but still above 100Vpp for a full
swing. Capacitance wasn't the issue.
Where's the load impedance come from if not capacitance?

Voltage with next to no load is easy (well, I suppose not trivial when
the bandwidth needs to go down to DC)

Basically it seems the vertical amp is an RF linear amplifier...
question is what kind of load it needs to be designed to drive.
 
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.

As has been suggested - vertical bw is primarily limited by the
vertical preamp and driver stages. But, once you upgrade those, you'll
want to upgrade the horizontal sweep and drive, and the trigger
circuits.
 
cs_posting@hotmail.com wrote:

Joerg wrote:


Mine is an old Hameg 207 and I still have it. Extremely sturdy but drops
off at 8MHz. IIRC it needed less but still above 100Vpp for a full
swing. Capacitance wasn't the issue.


Where's the load impedance come from if not capacitance?

Voltage with next to no load is easy (well, I suppose not trivial when
the bandwidth needs to go down to DC)

Basically it seems the vertical amp is an RF linear amplifier...
question is what kind of load it needs to be designed to drive.
IIRC it was a little under 10pF. Not much of a load unless you want to
push it into the VHF range. The stray capacitance of your circuit and
the parts is often higher than that of the plates.

Regards, Joerg

http://www.analogconsultants.com
 
Joerg <notthisjoergsch@removethispacbell.net> wrote in
news:1YMNf.17504$rL5.16117@newssvr27.news.prodigy.net:

Hello Jim,


I had an old 5MHz Heathkit scope that I was going to reinvent at one
point. I swore the sweep wasn't linear enough (from an RC waveform),
and the 5MHzbw could certainly be improved on. IIRC I think it needed
something like 170V for a full 10 div deflection!
One cycle of 5MHz across the whole screen was then 200nsec. I needed to
improve on 170/0.2usec=850V/usec to show a higher frequency!
I gave up at that point, and never finished the project! (...and am now
a proud Tektronix owner)


Mine is an old Hameg 207 and I still have it. Extremely sturdy but drops
off at 8MHz. IIRC it needed less but still above 100Vpp for a full
swing. Capacitance wasn't the issue.

I thought about the same thing but gave up because opening up the BW of
the input attenuator circuitry around that huge rotary switch appeared
too daunting. Actually I had the final stage licked, using rather cheap
"video transistors". They are (or were) used to drive the cathodes of
color TV CRTs.

The reason why mine isn't always linear: It doesn't have a trigger, just
a synchronizer. You have to couple it only enough to lock the waveform.
If you just leave the synchronizer pot pegged it'll distort badly.

Regards, Joerg

http://www.analogconsultants.com
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.


--
Jim Yanik
jyanik
at
kua.net
 
cs_posting@hotmail.com wrote in
news:1141396864.640028.214440@p10g2000cwp.googlegroups.com:

Joerg wrote:

Mine is an old Hameg 207 and I still have it. Extremely sturdy but drops
off at 8MHz. IIRC it needed less but still above 100Vpp for a full
swing. Capacitance wasn't the issue.

Where's the load impedance come from if not capacitance?

Voltage with next to no load is easy (well, I suppose not trivial when
the bandwidth needs to go down to DC)

Basically it seems the vertical amp is an RF linear amplifier...
question is what kind of load it needs to be designed to drive.
You neglect the inductance of the CRT leads,and inductance of the CRT
deflection plates,along with their capacitance.Higher BW CRTs(TEK) use a
distributed deflection plate electron gun structure,actually a transmission
line with an end termination.(a differential termination,too.)

--
Jim Yanik
jyanik
at
kua.net
 
Joerg wrote:

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.
Actually you've both been very helpful.

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.
 
Joerg wrote:

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.
Actually I regularly use high bandwidth analog & digital scopes at
work, so what I can do with my home gear aquired during my teenage
gears (and just reclaimed from parent's basement) is more curiosity
than necessity.

Speaking of deflection, I noticed there was a spot of something on the
scope screen, and grabbed a paper tissue to clean it. Talk about
electrostatic deflection!

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.
Yeah, I have that to look forward if I go ahead with the sampling scope
project I've been thinking about...
 
Jim Yanik wrote:

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.)
Always thought glass blowing would be an interesting hobby...

More seriously, either someone in asia is making something comparable,
or there are no new high bandwidth analog scopes to be had at all?

My guess though is that affordable real time sampling either has, or
soon will surpass the bandwidth of affordable CRT's - at least on the
new market, and on the used market eventually.
 
cs_posting@hotmail.com wrote in
news:1141705507.107568.255990@z34g2000cwc.googlegroups.com:

Jim Yanik wrote:

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.)

Always thought glass blowing would be an interesting hobby...
Actually,most TEK-made CRTs had a ceramic cast bell,gllued-on
faceplate(nice and FLAT),and just the neck tube was blown glass.
Only the early CRTs were all-glass.


More seriously, either someone in asia is making something comparable,
or there are no new high bandwidth analog scopes to be had at all?
I don't know.I have not seen any references to high-BW analog scopes for
sale.

My guess though is that affordable real time sampling either has, or
soon will surpass the bandwidth of affordable CRT's - at least on the
new market, and on the used market eventually.
It's FAR cheaper to use a LCD video display and digitizing circuitry than
to make a high bandwidth CRT for an analog scope;that's one problem.

Beware of used TEK TDS scopes,as they may be past the LTPS(long term
product support) period,and thus have NO parts or assemblies available,and
no schematics for troubleshooting.

--
Jim Yanik
jyanik
at
kua.net
 

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