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The Quantum Mechanics of Probe Loading

First off, welcome to the Tektronix Blog. For my first post I want to address something that is simple yet widely, and wildly, experienced by test and measurement industry designers and customers alike. Many people ask me various forms of the same question, and prior to working for Tektronix I, too, would often overlook this aspect of my design process. What is probe loading, and is it really that important?

Rather than answer right away, I would like to draw an analogized parallel to the quantum phenomenon known as the observer effect and the popular double-slit experiment, below is a video that explains this. Not only do they provide a good functional comparison, but quantum theory is a topic people can’t help but have an “Oh yeah, I’ve know that about that, but I could learn more” reaction that entangles their attention. Consider yourself ensnared.

In quantum mechanics, the act of observing something has the potential to change that something’s behavior, such as in the double-slit experiment where unobserved light behaves as a wave. The light is not bound by an observed state, so it is able to simultaneously pass through both slits at the same time in a sort of probabilistic wave. This creates an interference pattern consisting of several bands of light. In the observer effect phenomenon, when each photon is observed passing through the slits individually, the light propagates as a particle, forcing one physical state in which light can only travel through one slit. As a consequence, the pattern on the wall is a dual band, one for each slit as more particles are sent.

Why a test measurement system affects the operation of a device under test (DUT) can be explained by substituting industry terminology for parts of the quantum story. A probe would be an eye that observes the behavior of the electricity, which would take the place of light. An oscilloscope functions as the observer’s brain, processing and storing information as it is captured. Of course, the functionality of the DUT would be the light’s pattern substitute.

While a probe and scope connected to a device does not change the classification of the DUT’s physical existence, it is a good-outside-the-industry example of why a DUT may behave differently when a test solution is connected to make a measurement. How a signal changes on a device as a result of probe loading should be considered the consequences of electrical observation. Simply put, the DUT’s circuitry is always changed when a probe/scope combination is connected to it. The secret is to minimize the loading effect while maintaining the required speed and sensitivity to capture the desired signal.  

Probe loading is seemingly the most inconspicuous characteristic of a probe’s specifications. How a probe affects a circuit is a commonly underestimated design consideration—often forgone altogether. Consider how a probe is able to acquire signals: by being attached to the circuit and sharing its energy. This thought leads to the realization that the probe becomes part of the circuit, consequently changing the circuit, its transfer function, and possibly how it performs and operates. With preventative considerations, a design can incorporate, minimize or basically nullify the effects of probe loading.

As such, to avoid misleading measurement results, planning and careful preparation are required. While the specifics of what to look for when attempting this planning require a setting more formal than this, I will leave you with two thoughts. First, when considering a test measurement solution, make sure the solution has a high enough resistive load and a low enough capacitive load to try and eliminate DUT functionality changes. Second, think about how difficult it would be to measure the pressure in a tire without letting any air out. That should give you an idea of what it’s like to be me. 

Below is a video, I helped Tektronix Support create, which shows the importance of probe loading.