brian whatcott wrote:
I agree with the equation for summing bandwidth determining
components, and I agree with many of your other comments.

But concluding that a 60 MHz scope with 100MHz probes provides a 51
MHz bandwidth combination is (in my view) mistaken.

This is not the Tektronix way.
And the Tektronix way is the ONLY way with scopes! :-)

Brian W

Dave M wrote:
brian whatcott wrote:
Dave M wrote:
nobody wrote:
Gary wrote:
On May 30, 3:12?pm, Plasmah77
wrote:

The 60 MHZ specification on your
scope means its vertical amplifier and display will start to
degrade at 60 MHZ or higher in frequency.
No. It is 3dB Bandwidth. Degrade will start earlier. You will have
lost half power or in voltage around 30% of the signal.


Therefore using a probe with a 60
MHZ rating will allow you to realize all of your scope's
capabilities.
There is also a capacitance specification that should match. It is
however unlikely that it should not. Go ahead with the 100MHz
probes if you get them at a fair price.
Here are some guidelines to determine more exactly what the
interaction between the scope and probe is:
Bandwidth is BW
Risetime is Tr
BW = 0.35/Tr
Tr(overall) = Sqrt(Tr(scope)^2 + Tr(probe)^2)

Then if
Scope Tr at 60 MHz = 5.9nS
Probe Tr at 100 MHz = 3.5 nS
Overall Tr = 6.80 nS, making overall -3db bandwidth = 51.4 MHz
Though this sounds plausible, and it's thought through,
I think the result is mistaken.
A 60MHz scope is not a 60 MHz scope only if used with
(say) 3 GHz probes; its a 60 MHz scope if used with the probes as
provided or specified by the maker.

Brian W

It's not mistaken... in fact, it's well documented. Here are some
attributions that elaborate on the effects of a probe on the overall
bandwidth of a scope/probe combination.
http://books.google.com/books?id=xHA...0probe&f=false

http://www.adler-instrumentos.es/ima...%C3%B1al.pd f
pg 3

http://www.freelists.org/post/si-lis...nt-equipment,9

http://www.analog.com/library/analog...cd/vol41n1.pdf pg 13

As you can see from the documents, the scope and probe bandwidths do
interact as the RMS sum of the two. The vertical bandwidth or
risetime of scopes is specified at the scope's input connector. If
the bandwidth specification includes the probe, it will be specified
as such. In those cases, the scope's bandwidth will be specified
separately, and will be higher than the scope/probe combination.
Vertical bandwidth on many high quality scopes will be described in
their manuals or spec sheets when using a variety of probes, and
will reflect the equivalent bandwidth accordingly.


I'm interested in seeing info on the Tektronix way that proves that I am
mistaken. I provided several sources that defend my statements. Did you
read the sources that I provided links for? All quite credible sources.
Did you do the math, or is your conclusion just an opinion? How would you
calculate the combined risetime/bandwidth of a scope/probe combination?

Please don't interpret my questions as being confrontational, I'm genuinely
interested in learning if the technique that I used for years in calibration
labs was, in fact, correct or totally wrong. The technique that we used was
this:
Using a high bandwidth scope, measure its risetime without the probe being
connected (scope connected directly to a fast-rise pulse generator).
Connect the probe being calibrated to the scope input, and connect the probe
tip directly to the pulse generator output.
Measure the resulting pulse risetime.
Using the formula that I gave previously (rearranged to find the probe's
risetime), calculate the probe's risetime and bandwidth.
This method of measuring the performance of a probe worked quite well for
the lab and our customers for the years that I was a cal technician
(commercial and military).

--
David
dgminala at mediacombb dot net

"brian whatcott" wrote in message
...
The idea that a 55 MHz sine wave of 5 volt P-P amplitude would only register
about 3.6 volts on a 60 MHz scope is an alien idea to me.
That would render such a scope all but useless....

Have you used many older scopes? Effectively the older the scope, the more
like that a scope labeled "60MHz" really did mean that the scope itself was
3dB down at that point -- and of course the probe will just make things worse.
Things did get blurry over time, with HP and Tek starting to spec scopes based
on a combination of the probe and mainframe used ... and perhaps not use 3dB
as a reference anymore (all the bandwidth extension tricks they used tended to
make the frequency response no longer look like a simple single-pole roll-off,
and one could suggest that a 3dB point definition of bandwidth isn't that
useful if you're already dropping at, e.g., 12dB or 18dB/octave rather than
the expected 6 at that point...). And finally, once scopes started "going
digital" in the late '80s and beyond, the response of the scope itself often
became a lot closer to a brick-wall rather than the single-dominant-pole
roll-off the old analog scopes often saw, at which point -- if you had a good
enough probe -- the frequency response at the stated bandwidth might very well
be 1dB down (...which is still 10% down in voltage, though!).

I suggest that in American usage, I would be more likely to expect a
60 MHz sine wave to be not more than 5% down as indicated on a 60 MHz scope
with its maker's standard probes.

If you buy a new scope today, that's very likely the case... but if you get an
old boat anchor, it'd be VERY surprising if it were anywhere close to "only"
5% off.

---Joel

"Dave M" wrote in message
...
I'm genuinely interested in learning if the technique that I used for years
in calibration labs was, in fact, correct or totally wrong. The technique
that we used was this:
Using a high bandwidth scope, measure its risetime without the probe being
connected (scope connected directly to a fast-rise pulse generator).
Connect the probe being calibrated to the scope input, and connect the probe
tip directly to the pulse generator output.
Measure the resulting pulse risetime.
Using the formula that I gave previously (rearranged to find the probe's
risetime), calculate the probe's risetime and bandwidth.
This method of measuring the performance of a probe worked quite well for
the lab and our customers for the years that I was a cal technician
(commercial and military).

If all you have available is a pulse generator, this is a decent enough
method -- just know that the formula to convert between rise time and
bandwidth assumes a single-pole frequency rolloff, which is usually -- but not
always -- a reasonable assumption.

A more insightful means of measuring scope bandwidth is to take an RF
generator that can produce a repetitive frequency sweep (with a leveled
output), have the start of it trigger the scope, and then adjust the timebase
settings so that you get, e.g., 10MHz or 100MHz or some other convenient MHz
sweep per division. In other words, the scope's display effectively becomes a
Bode plot of the system (scope + probe combined) response.

---Joel

Joel Koltner wrote:
"Dave M" wrote in message
...
I'm genuinely interested in learning if the technique that I used
for years in calibration labs was, in fact, correct or totally
wrong. The technique that we used was this:
Using a high bandwidth scope, measure its risetime without the probe
being connected (scope connected directly to a fast-rise pulse
generator). Connect the probe being calibrated to the scope input,
and connect the probe tip directly to the pulse generator output.
Measure the resulting pulse risetime.
Using the formula that I gave previously (rearranged to find the
probe's risetime), calculate the probe's risetime and bandwidth.
This method of measuring the performance of a probe worked quite
well for the lab and our customers for the years that I was a cal
technician (commercial and military).

If all you have available is a pulse generator, this is a decent
enough method -- just know that the formula to convert between rise
time and bandwidth assumes a single-pole frequency rolloff, which is
usually -- but not always -- a reasonable assumption.

A more insightful means of measuring scope bandwidth is to take an RF
generator that can produce a repetitive frequency sweep (with a
leveled output), have the start of it trigger the scope, and then
adjust the timebase settings so that you get, e.g., 10MHz or 100MHz
or some other convenient MHz sweep per division. In other words, the
scope's display effectively becomes a Bode plot of the system (scope
+ probe combined) response.
---Joel

Thanks for the elaboration... makes sense. And yes, after wideband leveled
generstors became available, we did use them to measure frequency response
on scopes, but still relied on the risetime method to characterize probes.
The reason for that was that the leveled generators that we had didn't cover
the full spectrum of the scopes and probes that we encountered; we had a 30
picosecond risetime pulser that handled everything we saw.

It has been several years since I worked in a cal lab, so the technology has
progressed a good deal since. Are there any texts that describe the
technique(s) by which the response of any given scope/probe combination can
be calculated and verified? Not that I need to do it any more, just
interested in knowing.

--
David
dgminala at mediacombb dot net

Dave M wrote:
... Here are some
attributions that elaborate on the effects of a probe on the overall
bandwidth of a scope/probe combination.
http://books.google.com/books?id=xHA...0probe&f=false

http://www.adler-instrumentos.es/ima...%C3%B1al.pd f
pg 3

http://www.freelists.org/post/si-lis...nt-equipment,9

http://www.analog.com/library/analog...cd/vol41n1.pdf pg 13

As you can see from the documents, the scope and probe bandwidths do
interact as the RMS sum of the two. The vertical bandwidth or
risetime of scopes is specified at the scope's input connector. If
the bandwidth specification includes the probe, it will be specified
as such. In those cases, the scope's bandwidth will be specified
separately, and will be higher than the scope/probe combination.
Vertical bandwidth on many high quality scopes will be described in
their manuals or spec sheets when using a variety of probes, and
will reflect the equivalent bandwidth accordingly.


I'm interested in seeing info on the Tektronix way that proves that I am
mistaken. I provided several sources that defend my statements. Did you
read the sources that I provided links for? All quite credible sources.
Did you do the math, or is your conclusion just an opinion? How would you
calculate the combined risetime/bandwidth of a scope/probe combination?

Please don't interpret my questions as being confrontational, I'm genuinely
interested in learning if the technique that I used for years in calibration
labs was, in fact, correct or totally wrong. The technique that we used was
this:
Using a high bandwidth scope, measure its risetime without the probe being
connected (scope connected directly to a fast-rise pulse generator).
Connect the probe being calibrated to the scope input, and connect the probe
tip directly to the pulse generator output.
Measure the resulting pulse risetime.
Using the formula that I gave previously (rearranged to find the probe's
risetime), calculate the probe's risetime and bandwidth.
This method of measuring the performance of a probe worked quite well for
the lab and our customers for the years that I was a cal technician
(commercial and military).

I started looking for a reference that I could trust - and I found it in
a Tektronix note - a note that totally supports your position.

Ooops! I was wrong - with a misplaced sense of what one can measure with
Tektronix scopes. Sorry.... I appreciate your level tone though!

http://www.tek.com/Measurement/App_N...55_18024_0.pdf


Brian Whatcott Altus OK

On May 31, 3:13�pm, nobody wrote:
Gary wrote:
On May 30, 3:12 pm, Plasmah77
wrote:

The 60 MHZ specification on your
scope means its vertical amplifier and display will start to degrade
at 60 MHZ or higher in frequency. �

No. It is 3dB Bandwidth. Degrade will start earlier. You will have lost
half power or in voltage around 30% of the signal.

Therefore using a probe with a 60

MHZ rating will allow you to realize all of your scope's capabilities.

There is also a capacitance specification that should match. It is
however unlikely that it should not. Go ahead with the 100MHz probes if
you get them at a fair price.

The question was would a 100 MHZ probe work with a 60 MHZ scope. The
asker claimed to be a beginner. I made a general comment about why it
would. I saw no reason to muddy the waters by talking about half
power points and 3dB bandwidths. Trying to make it simple.

Gary N4AST

On Jun 1, 5:45*pm, brian whatcott

http://www.tek.com/Measurement/App_N...fs/bw_rt/55_18...

Brian Whatcott *Altus

Hey OM:
Great link there to the Tech article from tek tronics

As pointed out in that article, a scope is worthless for measuring
voltages. Towit a 60Mhz scope is only good to 10 Mhz, going by their
standards, published, in that article.

73 OM
de n8zu

"Bob Spooner" wrote in message
...

"Plasmah77" wrote in message
...

I need to buy a probe for my oscilloscope which is 60 MHZ.

My question is can I buy a probe that rated 100 MHZ or must I make sure
it is rated 60 MHZ the same as my system?

Thanks




--
Plasmah77

You can buy a probe with a higher frequency rating than your scope has and
it will work fine, it just will be more expensive.
Regards,
Bob
Actually, it will also work "better."

Pete