LeCroy HFP1500, Instruction Manual

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High Frequency Measurements
HFP1500-OM-E Rev E
5–3
(
parallel combination of 250
Ω
and 100 k
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 851 MHz the reactance of the 0.7 pF capacitance is 267
Ω
, and at 1.5 GHz the
reactance has been lowered to 152
Ω
If, at a given frequency, the source impedance is large with respect to the input impedance, a mea-
surable reduction in the output signal amplitude may occur.
where:
Z
probe
is the probe’s input impedance and
Z
source
is the source impedance
As an example:
At 851 MHz, where the probe input impedance has reduced to 267
Ω,
and a source resistance of
250
Ω
the probe output amplitude is reduced to:
###
Input inductance
V
Source
250
Ω
0.7 pF 100 k
Ω
V
out
Z
probe
Z
probe
Z
source
+
--------------------------------------
V
in
×
=
V
out
267
267 250 +
------------------------
0.52 V
in
×
= =