"Sal M. Onella" wrote in message
...
It may be that a 100 MHz scope has better than a 3.5 nsec risetime,
given that it is sped'ed for response flatness to that limit and its
response actually extends beyond !00 MHz. In retirement, I no longer
have access to test equipment that would support my point.

See the following article:
http://www.eetimes.com/design/microw...ight-Bandwidth

There are two paragraphs in the article of importance he

"All oscilloscopes exhibit a low-pass frequency response that
rolls-off at higher frequencies, as shown in Figure 1. Most scopes with
bandwidth specifications of 1GHz and below typically have what is called
a Gaussian response, which exhibits a slow roll-off characteristic
beginning at approximately one-third the -3dB frequency. Oscilloscopes
with bandwidth specifications greater than 1GHz typically have a
maximally-flat frequency response, as shown in Figure 2. This type of
response usually exhibits a flatter in-band response with a sharper
roll-off characteristic near the -3dB frequency.

"Closely related to an oscilloscope's bandwidth specification is its
rise time specification. Scopes with a Gaussian-type response will have
an approximate rise time of 0.35/f(sub)BW based on a 10- to 90-percent
criterion. Scopes with a maximally-flat response typically have rise time
specifications in the range of 0.4/f(sub)BW depending on the sharpness of
the frequency roll-off characteristic. But it is important to remember
that a scope's rise time is not the fastest edge speed that the
oscilloscope can accurately measure. It is the fastest edge speed the
scope can possibly produce if the input signal has a theoretical
infinitely fast rise time (0 ps). Although this theoretical specification
is impossible to test (since pulse generators don't have infinitely fast
edges) from a practical perspective, you can test your oscilloscope's
rise time by inputting a pulse that has edge speeds that are 3 to 5 times
faster than the scope's rise time specification."

73, Dr. Barry L. Ornitz WA4VZQ