A common laboratory tool for displaying and analyzing the waveform of electronic signals is an oscilloscope. The gadget essentially plots the immediate signal voltage against time on a graph.
Understanding Voltage Measurements with Oscilloscopes
When connecting a voltmeter or oscilloscope between two points, interpreting the displayed voltage requires a nuanced understanding. The displayed signal results from electromagnetic fields intersecting a loop formed by the meter leads, with a smaller loop area resulting in reduced voltage. While at DC this poses no issue, in the presence of changing fields, especially in intense electromagnetic fields, it becomes crucial to interpret measurements accurately.
In such fields, significant current flows in nearby conductors, including the shield of an oscilloscope probe. Even with a controlled loop area, a voltage drop in the probe shield can contribute to the signal, potentially misleading observers trying to comprehend an issue. Testing techniques involve shorting the probe tip to the probe shield or connecting the shorted probe to the hardware’s ground reference to identify excess probe shield current.
Proper connection of the probe shield near the signal of interest is crucial to avoid unintended loops. Meaningful measurements at high frequencies demand good judgment and awareness of field phenomena. Battery-operated oscilloscopes offer some relief by eliminating a loop through a power transformer to remote ground, yet capacitance in free space may still allow probe currents. Grounding issues arise when connecting an oscilloscope to grounded circuits, leading users to employ adapters or “cheater plugs” to avoid ground loops.
Engineers often take risks, such as grounding oscilloscopes or metal housing at elevated voltages, for the sake of functionality. Differential inputs in oscilloscopes are designed for accurate subtraction of signals (A–B or B–A), but it’s essential to recognize that common ground connections are needed for these operations. Oscilloscopes are not intended to handle signals originating from different environments, emphasizing the importance of understanding and mitigating common-mode signals in complex setups.