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