Can a Single Probe Accurately Measure RF Power on Multilayered PCBs?

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Discussion Overview

The discussion revolves around the ability of a single probe to accurately measure RF power on multilayered PCBs. Participants explore the mechanisms by which RF energy is detected despite being primarily contained within the layers of the PCB, addressing both practical and theoretical aspects of RF measurement techniques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes the effectiveness of single-conductor probes in measuring RF power on the surface of multilayered PCBs, questioning how this is possible given that RF energy is primarily contained within the PCB layers.
  • Another participant emphasizes that no transmission line is completely effective at shielding, raising concerns about the signal level being sampled and its potential insignificance if it is significantly attenuated.
  • A different participant queries how the probe can measure power from waves that are mostly contained within the dielectric layers, suggesting that the percentage of energy that leaks into the probe may not be consistent, which could affect measurement accuracy.
  • A later reply mentions that the analyzer uses Earth ground as a reference and suggests that while a differential probe might be preferable, the single-ended probe appears to function adequately for the user's needs.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness and accuracy of single probes for RF measurement on multilayered PCBs. There is no consensus on the reliability of the measurements or the mechanisms involved.

Contextual Notes

Participants highlight potential limitations related to the effectiveness of shielding in transmission lines and the variability of energy leakage into probes, which may impact measurement accuracy.

Who May Find This Useful

This discussion may be of interest to engineers and technicians involved in RF design and testing, particularly those working with multilayered PCBs and spectrum analysis.

RFbeginner
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I often use Spectrum Analyzers to probe RF signal powers at different points on multilayered PCBs. These probes have a single conductor (or at least only require a single conductor). Placing the probe on a signal trace on the top exposed surface of the PCBs instantly shows us the RF power at that point. BUTTTTT How can this be when the RF energy is flowing within the PCB (between two conductors and amongst the DIELECTRIC)? It would make sense if we connected one probe on the top conductor and the other probe on the other conductor (usually a ground plane), that way we can multiply a I and V value, but this works on 1 probe. They just touch these magic probes on the top layer and BINGO.. they get the RF POWER reading. How can these devices somehow guide the underlying RF Energy to dive into them
 
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I have done a LOT of troubleshooting with a spectrum analyzer. First and foremost, no transmission line whether it be conventional coax or strip-line on a PC board is going to be 100% effective at shielding. Second, how far down on the spectrum is the signal? In other words, what is the signal level sampled? If it is 40 dB down, then the sampled signal is 1/10,000th of the strength of the actual signal. Pretty insignificant.
 
How

How does the probe measure the power of the wave that is mostly sandwiched between the layers of dielectric in the PCB? I do realize that a certain % of the energy leaks into the probe, but if that % was not fixed, how do we get proper measurements from the Spectrum Analyzer?
 
The analyzer is using Earth ground as a reference, and so is your PCB. Of course, you'd be better off using a differential probe, but it sounds like your single-ended probe gets the job done for you.

- Warren