How the Instrument Works
System Architecture
System bandwidth
The bandwidth of the combined analyzer and probe system must be
sucient to accurately reproduce the input signal. Otherwise, time-interval
measurements are inaccurate.For example, if the analyzer and probe have
a combined rise time of 1 ns, and the signal also has a 1 ns rise time,the
measured rise time is:
p
(1
ns
)
2
+(1
ns
)
2
=1
:
41
ns
The answer is in error by 41 percent.
If the
analyzer and
probe have
a
combined
rise
time
of
330
ps
,
and
the
signal
has a
1ns
rise time
,
the
measured
rise
time
is:
p
(1
ns
)
2
+
(330
ps
)
2
=
1
:
05
ns
Now
the
error
is
only
5
percent.
There
are
three
rules
worth
memorizing.
1.
The
combined
system
rise
time
(analyzer
and
probe)
should
be
less
than
1/3
the
rise
time
of the
measured
signal
for
an
error
of
less
than
5
percent,
or
less
than
1/7
of the
rise
time
of
the
measure
signal
for
an
error
of
less
than
1
percent.
2.
Rise
time
and
bandwidth are
inversely related
as
shown
in
the
preceding
equations
.
3.
Rise
times
add
approximately
as
the
square
root
of
the
sum
of the
squares.
The wide bandwidth of the analyzer compared to the bandwidth of a typical
probe means that, for most cases, the combined bandwidth can be assumed to
be the same as that of the probe alone.
13-17
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