Probe Types
There are three common types of oscilloscope probes. Each type has
different loading effects. First, there is the low-impedance resistive divider
probe, like the HP 54006A. Second, is the compensated, high-resistance
passive divider probe, like the HP 10430A. Third, there is the active probe,
like the HP 54701A.
Resistive Divider Probes
Resistive divider probes are designed for
oscilloscopes with a 50-
Ω
input impedance. The tip of the probe has a
450-
Ω
or 950-
Ω
series resistor. The cable is designed for a 50-
Ω
transmission line. Because the cable is terminated in 50
Ω
at the
oscilloscope input, it looks like a purely resistive 50-
Ω
load when viewed
from the tip. Therefore the resistive divider is flat over a wide range of
frequencies, limited primarily by the parasitic capacitance and
inductance of the 450-
Ω
or 950-
Ω
resistor and the fixture that holds it.
The input resistance of the probe to the circuit under test is either 500
Ω
or 1 k
Ω
.
Resistive divider probe
Figure 1-11
How the Oscilloscope Works
System Bandwidth
1–26
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