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Douglas C. Smith

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Technical Tidbit - January 2004
Determining the Effects of Probing on Signals - Tuned Probe Simulators

  tuned probe tester - passive 10x probe
Figure 1. Passive 10X Hi-Z Probe with Tuned Probe Simulator

Abstract:  Probing can have significant effects on the signals being measured. One of these effects, resonance between probe input capacitance and lead inductance, can be simulated with a trimmer capacitor and its connecting leads. Such a "tuned probe simulator" can both provide insight on how probing affects signals and help troubleshoot circuit problems as well. Tuned probe simulators for passive and active probes are discussed.

Discussion: Probes whose input impedance is primarily capacitive, such as the old standby 10X Hi-Z probe and most active probes, can significantly affect circuits to be measured. This happens when the probe input capacitance resonates with the inductance of the probe connections to form a series tuned circuit. Such a circuit can have very low input impedance at the resonant frequency. The resonance can affect the measured waveform, induce error in the measured result, and both cause and "fix" circuit problems.

Figure 1 shows a standard "Hi-Z" 10X passive probe on the left and a tuned probe simulator on the right that can mimic the probe's effect on a circuit to be measured. To construct a tuned probe simulator, start with a test lead that is about the same length as the probe signal path dimensions. The total length of the tuned probe simulator should be approximately equal to the length of the probe tip and ground lead length. Cut the test clip in the middle and insert a variable trimmer capacitor that has a range from about 1/2 to about 2 times the input capacitance of the probe. Figure 2 shows such a construction for an active probe. The important factors are that the length of the simulator be about the dimensions of the probe's connections and the trimmer capacitor have a range of at least a factor of two above and below the probe input capacitance.

  Active Probe with Tuned Probe Tester
Figure 2. Active Probe with Tuned Probe Simulator

If the probe has an input damping resistor, as many recently designed active probes do, be sure to add the same amount of resistance in series with the trimmer capacitor. Typical values range from 100 to 200 Ohms. Likewise, for the probe in Figure 2, it would be a good idea to add a small ferrite core to the simulator that has characteristics similar to the one used on the probe. A 100 ohm resistor can be used as an approximate substitute if a suitable ferrite core is not available. A tuned probe simulator for an active probe, such as shown in Figure 2, may need to soldered to the circuit so the trimmer can be adjusted. Be careful to use a non-conductive tuning tool and not to touch the trimmer or its connections with fingers during the adjusting process.

Sometimes, connection of a probe to a circuit may cause the circuit to start working! One way this happens can be described with reference to Figure 3. The figure shows the input impedance of a typical active probe configuration that does not have a series damping resistor in series with its input. Notice the strong resonance just below 1 GHz where the input impedance dips as low as 17 Ohms, a typical value. The probe essentially has placed a notch filter on the circuit to be measured. If the source of the circuit malfunction has significant energy in the neighborhood of the resonance, the circuit may start to work when the probe is connected.


active probe input z graph
Figure 3. Active Probe Input Impedance Example
Input Z (2000-17 Ohms) vs. Frequency (100MHz-5GHz)

One way to determine if this is happening is to replace the probe with a suitably constructed tuned probe simulator. The trimmer capacitor is tuned through its range and if the circuit starts to work, then probe resonance is likely affecting the circuit. Likewise, a tuned probe simulator can be used to see if a probe will load a circuit, even if the probe is not available. If one is considering purchasing an active probe, a tuned probe simulator can allow you to estimate the effect on your circuits.

Summary and Conclusion: Probing a signal can affect it in several ways. One way this can happen is the resonance of probe input capacitance with the connection inductance resulting in a very low probe input impedance at the resonant frequency. Tuned probe simulators can be useful both as a troubleshooting tool to investigate this effect as well as to predict possible loading from a probe being considered for purchase.

History:  Originally, Henry Ott, described a "tuned probe tester" years ago in his EMC seminars. This article takes the concept further in applying the principle to active probes and its use in predicting circuit response to a probe before it it purchased.

Other articles on this website covering probing effects include:

Equipment used in this article includes:

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Copyright © 2004 Douglas C. Smith