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5 High Frequency Measurements
INPUT LOADING
When you touch a probe to the circuit under test, the probe will affect your measurement because
of the probe's input impedance introduced into the circuit. All probes present resistive, capacitive
and inductive loading.
INDUCTIVE LOADING (LEAD LENGTH)
A significant element in this circuit is the inductance shown in the input ground leads of the oscillo-
scope probe.
The ground lead is the primary return path for the current resulting from the input voltage acting on
the probe's input impedance. The ground lead and input lead inductances act with the probe's
input capacitance to form series L-C network. The impedance of a series LC network will drop dra-
matically at its resonant frequency. This is the cause of the "ring" we often see after the leading
edge of pulses in measured waveforms. This effect is referred to as ground lead corruption.
Because it is impossible to eliminate either the L or C from this circuit, the method to improve
waveform fidelity is to raise the resonant frequency beyond the bandwidth of interest in the mea-
surement.
The resonant frequency of a simple LC circuit can be represented by:
The resonant frequency of a series LC circuit can be raised by decreasing the inductance, capaci-
tance or both.
Since the input capacitance is already very low and cannot be reduced, you can only try to reduce
the inductance. This can be accomplished by using the shortest possible input lead as well as the
shortest possible ground lead.
For example, to obtain the shortest possible ground lead when measuring IC related signals,
attach a small piece of copper clad material to the top of the IC package and connect this to the
HFP1500-OM-E Rev E
High Frequency Measurements
Figure 5-1. Probe Input Equivalent Circuit
F
=
Resonance
1
------------------
2π LC
5–1
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