High-Frequency Probe Equivalent Circuit
The impedance plots for the two models are shown in figure 44. By
comparing the two plots, you can see that both the series tip resistor and the
cable’s characteristic impedance extend the input impedance significantly.
The stray tip capacitance, which is generally small (1 pF), sets the final break
point on the impedance chart.
Impedance versus Frequency for Both Probe Circuit Models
The HP 54620A/C probes are represented by the high-frequency circuit
model shown in figure 43. They are designed to provide as much series tip
resistance as possible. Stray tip capacitance to ground is minimized by the
proper mechanical design of the probe tip assembly. This provides the
maximum input impedance at high frequencies.
Ω
Ω
Ω
Figure 43
Ω
Ω
Ω
Ω
Ω
Ω
Figure 44
Ensuring Accurate Measurements
Probing the Circuit Under Test
140
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