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Guest
Tue Feb 28, 2006 5:08 pm
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...
Robert Baer
Guest
Wed Mar 01, 2006 7:28 am
cs_posting_at_hotmail.com wrote:
Quote:
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
Guest
Thu Mar 02, 2006 2:45 am
cs_posting_at_hotmail.com wrote:
Quote:
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
Guest
Thu Mar 02, 2006 5:33 pm
Joerg wrote:
Quote:
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.
Jim Nagy
Guest
Thu Mar 02, 2006 11:46 pm
In article <1141320811.920129.231950_at_i40g2000cwc.googlegroups.com>,
<cs_posting_at_hotmail.com> wrote:
Quote:
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
Joerg
Guest
Fri Mar 03, 2006 1:27 am
Hello Jim,
Quote:
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
Guest
Fri Mar 03, 2006 4:41 pm
Joerg wrote:
Quote:
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.
Guest
Fri Mar 03, 2006 7:25 pm
w2aew wrote:
Quote:
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.
w2aew
Guest
Fri Mar 03, 2006 8:14 pm
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.
Joerg
Guest
Fri Mar 03, 2006 10:31 pm
cs_posting_at_hotmail.com wrote:
Quote:
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
Jim Yanik
Guest
Sat Mar 04, 2006 2:22 am
Joerg <notthisjoergsch_at_removethispacbell.net> wrote in
news:1YMNf.17504$rL5.16117_at_newssvr27.news.prodigy.net:
Quote:
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
Jim Yanik
Guest
Sat Mar 04, 2006 2:25 am
cs_posting_at_hotmail.com wrote in
news:1141396864.640028.214440_at_p10g2000cwp.googlegroups.com:
Quote:
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
Guest
Sun Mar 05, 2006 2:10 am
Hello Jim,
Quote:
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
Guest
Sun Mar 05, 2006 6:28 pm
Joerg wrote:
Quote:
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
Guest
Sun Mar 05, 2006 11:04 pm
cs_posting_at_hotmail.com wrote:
Quote:
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
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