LeCroy ZS1000, Инструкция по эксплуатации

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High Frequency Measurements
ZS1000-OM-E Rev C
5–3
At higher frequencies the capacitive loading can affect the amplitude as
well as the waveshape of the measured signal by introducing an expo-
nential response to the waveform.
For a simple RC network the time constant of this exponential response
is:
where C
total
is the combined probe and circuit capacitance and R
total
is
combined circuit and probe resistance.
For a setup where C
t
= 0.9 pF and a source resistance is 250
Ω,
the
measured rise time will be 0.495 ns, which will correspond to a band-
width of 909 MHz, assuming no inductive loads.
(
t
rise
=2.2 x 0.9 X 10
-12
x 250
Ω
= 0.495 ns
)
(
parallel combination of 250
Ω
and 1 M
Ω
is still 250
Ω
)
Figure 5-2. Probe input equivalent circuit
To illustrate the effect of capacitive loading at higher frequencies:
At a frequency of 750 MHz the reactance of the 0.9 pF capacitance is
236
Ω
, and at 1.0 GHz the reactance has been lowered to 177
Ω
If, at a given frequency, the source impedance is large with respect to
the input impedance, a measurable reduction in the output signal ampli-
tude may occur.
t
rise
2.2 C
total
×
R
total
×
=
Input inductance
V
Source
250
Ω
0.9 pF 1 M
Ω
V
out
Z
probe
Z
probe
Z
source
+
--------------------------------------
V
in
×
=