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.