HP 54620A, User'S And Service Manual

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Probing the Circuit Under Test
You may encounter problems during your use of the HP 54620A/C that are
related to probing. These problems fall into two categories: probe loading and
probe grounding. Probe loading problems generally affect the circuit under
test, while probe grounding problems affect the accuracy of the data to the
measurement instrument. The design of the HP 54620A/C probes minimizes
the first problem, while the second is easily addressed by good probing
practices.
Input Impedance
The probes used in the HP 54620A/C are passive probes, which offer high
input impedance and high bandwidths. They usually provide some
attenuation of the signal to the analyzer. This is typically 20 dB.
Passive probe input impedance is generally specified in terms of a parallel
capacitance and resistance. The resistance is the sum of the tip resistor value
and the input resistance of the test instrument (see figure 42). The
capacitance is the series combination of the tip compensating capacitor and
the cable plus instrument capacitance in parallel with the stray tip
capacitance in parallel with the stray tip capacitance to ground. While this
results in an input impedance specification that is an accurate model for DC
and low frequencies, the high-frequency model of the probe input is more
useful (see figure 43). This high-frequency model takes into account pure tip
capacitance to ground as well as series tip resistance, and the cable’s
characteristic impedance (Z
O
).
DC and Low-Frequency Probe Equivalent Circuit
Ω
Figure 42
Ensuring Accurate Measurements
Probing the Circuit Under Test
139