High Frequency Measurements; Input Loading; Inductive Loading (Lead Length) - LeCroy ZS1000 Instruction Manual

High impedance active probe

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5 High Frequency Measurements

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

A significant element in this circuit is the inductance shown in the input

ground leads of the oscilloscope probe.

Figure 5-1. 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 wave-

forms. 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:

ZS1000-OM-E Rev C

High Frequency Measurements

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Resonance

2π LC

5–1

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Zs1500