Teledyne LeCroy HFP1000 - Manual (Page 25)

Operator’s Manual
922251-00 Rev A 19
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
oscilloscope probe.
Figure 2
Probe Input Equivalent Circuit
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 dramatically
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 measurement.
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,
capacitance or both.