"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