• • • TEST & MEASUREMENT • • • Getting and Understanding the Best
High Frequency Probe Measurement Whether engineers are looking to design, debug, or perform compliance tests, being able to measure with confidence is critical
By Hal Paver, Research and Development Engineer, Keysight Technologies O
scilloscopes offer the ability to capture the true performance of the device under test, but in order to achieve accurate
results it is essential to minimise any errors. This article will explore common probing mistakes and how to correctly position a probe to make better measurements.
What you’ll learn:
• The causes and impacts of probe loading. • The influence of probe position on measurements.
• How to minimise the probe effects on the circuit for more accurate measurements.
To understand circuit behaviour, it is crucial to
use a high input impedance active differential oscilloscope probe. These probes effectively minimise any disruptions to the circuit that is being measured, facilitate differential signal measurements, and often reduce measurement anomalies associated with single-ended probes. All of this results in better measurements and faster insights, enabling engineers to create, test, and validate designs. However, using an active differential probe requires careful consideration of the probe connection with the device under test
Figure 1: Typical impedance vs frequency plot for an active differential probe.
(DUT). Being able to understand how taking measurements may affect the DUT, as well as how to interpret those measurements, can transform the oscilloscope probe into an essential time-saving tool and not a hindrance.
Impacts of Probe Loading As soon as the probe is connected to the device it becomes part of the circuit. The probe introduces both galvanic and parasitic impedances, this is called probe loading. Oscilloscope software can account for probe loading effects on the voltage waveform by applying correction in real-time, or the measurement can be exported to simulation software where the loading effects can be subtracted. It is more difficult, however, to account for parasitic capacitances that can couple energy into unshielded parts of a probe tip and the capacitive loads on the circuit itself. These parasitic effects can create measurement errors that are not readily apparent. This is where engineers need to consider probe positioning to reduce these effects, which will be discussed later. Probe loading affects the operation of the DUT
Figure 2: Probe tip orientations tested. The probe tip was nominally positioned in these 5 positions. The input signal was connected on the left side of the fixture. The right side of the fixture was attached to a 50-ohm oscilloscope channel.
itself. When the voltage probe is connected, it adds a shunt impedance to the ground at the circuit node where it is attached and a differential impedance between the probe contacts. If the probe has a high impedance, the effect on the signal will be minimal. If the impedance is low, at
34 ELECTRICAL ENGINEERING • OCTOBER 2024
electricalengineeringmagazine.co.uk
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