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Douglas C. Smith

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Technical Tidbit - November 2003
Heisenberg and Signal Measurements - Part 2, The Frequency Domain

  Measured and actual voltage from source
Figure 1. Node Voltage (red, with dip) and Probe Response (blue, peaked) Without Damping Resistor
(vertical scale = 5dB/div probe gain)
(horizontal scale = 100 MHz to 4.8GHz)

Abstract:  Last month's Technical Tidbit presented simulation results in the time domain showing how probing a signal can significantly effect it. This month, simulation data is presented in the frequency domain to again show that under realistic conditions, active probes without damping resistors can significantly affect the signal to be measured. As before, the result can be more serious than just signal loading.

Discussion: Figure 1 shows simulation results of probe response (peaked signal in blue) and the actual voltage at the measured node (red with dip) for a 1 Volt source having an impedance of 25 Ohms in series with 5 nH without a probe damping resistor (a series resistor ~100-200 Ohms at the probe tip). The frequency range of 100 MHz to just under 5 GHz is displayed. The source resistance was chosen to represent a gate output resistance and the inductance to represent nominal package inductance. Many modern chip packages have signal path inductance ranging from 2 to 8 nH*.

The probe model used in the spice simulation is a manufacturer's multi-element model of a 4 GHz active probe fitted with a 5 cm extension adapter that included a damping resistor at the tip of about 200 Ohms. The simulation modeled the probe's connections and input circuitry, but not the probe amplifier response for simplicity. For the plots in Figure 1, the damping resistor was removed. Without the damping resistor, the probe's input impedance and response is similar to active probe designs that do not include a damping resistor. The probe response has a peaked response that is common with even short probe connections on many active probes. In this case the probe response has a 14 dB gain peak at about 800 MHz! The probe response in Figure 1 agrees well with measured results on active probes without damping resistors, thus helping to verify the simulation results.

However, the important detail in Figure 1 is the actual signal on the node during the measurement, the red trace with a dip at 900 MHz. With the 25 Ohm + 5nH signal source used in the simulation,  the node voltage fell about 9 dB at 900 MHz, low enough to possibly cause signal integrity problems. And, this happened while the probe was displaying the signal with about double the original amplitude with no probe connected. Both the probe gain peak and the dip in the measured node voltage are sensitive to the amount of source inductance, but even a few nH of inductance can be a problem. Of course, the node voltage cannot be observed on a real circuit because it is changed by probing it, but the simulation suggests a real possibility of problems caused by an active probe without a damping resistor at the tip.

Figure 2 shows the probe response (blue trace falling off at high frequencies) and node voltage (red trace gradually increasing at high frequencies) when the ~200 Ohm damping resistor was present in the circuit. The probe response is smooth with a well behaved rolloff. Note the inductance of the 5 cm extension causes the 3dB rolloff point to be only about 1.2 GHz rather than the 4 GHz bandwidth the probe actually is capable of. Although the damping resistor can smooth the probe response as well as reduce effects in the measured circuit, it cannot restore bandwidth lost due to inductance in the probe connections. Probe connections should be kept as short as possible for this reason and other reasons. That being said, the frequency responses in Figure 2 are very good compared to those in Figure 1. Only passive probes can achieve better performance in terms of tolerance to probe connection inductance.


  Node and Measured Voltage With Damping Resistor
Figure 2. Node Voltage (red, upper) and Probe Response (blue, lower) With Damping Resistor
(vertical scale = 5dB/div probe gain)
(horizontal scale = 100 MHz to 4.8GHz)

Summary and Conclusion: Active probes, or any type of scope probe for that matter, can have significant effects on the measured signal that are quite distinct from the response of the probe itself as evidenced by the frequency domain plots above. Active probes should always have damping resistance either built in or added and probe connections should be kept as short as possible.

Other articles on this website covering probing effects include:

Equipment used in this article includes:

* Inductance numbers from a private conversation with Michael King.

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Copyright © 2003 Douglas C. Smith