Loading & Coupling - how to measure high-speed circuit accurately

Near-field coupling and Loading of a high-speed circuit leads to measurement errors, as shown in this video. Even using a magnetic field loop, which is non-contact measurement could still load the circuit. Understanding the phenomenon is the first step towards better measurement.

[@idk2412]

Great video! Could you clarify what the lessons learned were? If I understood you correctly, one of them was that given you have a setup like you do, you can move your H/E field probe around and see which source causes a greater increase to the CMC noise trace we see from the RF clamp?

[@MachOneDesignEMC]

Hi, not really, I think perhaps you slightly missed the point. The point I wanted to make was, when making a measurement on a high speed board, coupling can affect your results. In other words, what you see on the scope may not be how the system behaves. Sometimes, in extreme cases, the person who performed the measurement can "fix" a problem by simply measuring the circuit, this is because the noise was bypassed by the engineer him/herself. This subject is quite complex, perhaps you can start from some basics, I am sure when you have seen enough issues in the filed, you will understand this a lot better.

[@yeaveny3629]

Thank you for the next very informative video. Please consider changing the microphone or using some amplification because the sound is extremely quiet

[@MachOneDesignEMC]

Yes, I wonder why! need to fix it for sure. Thanks for the feedback.

[@ramiroduran2636]

Hi there, may you post here the cable and connector type/PN or name used to connect the H-Field probe to the Oscilloscope/Spectrum Analyzer Channel? Thanks a lot for the video.

[@MachOneDesignEMC]

Hi, the cable is part of the Tekbox near-field probe set, www.tekbox.com/product/tekbox-tbps01-emc-near-field-probes/, the cable is co-aixal SMA SMB one.

[@MachOneDesignEMC]

telonic.co.uk/product/tekbox-smbf-smam-75-rg316u-smb-female-to-sma-male-75cm-50ω-rf-cable/

[@TheDutchGuyOnYT]

Could the probe be seen as an unintended rc snubber? And would it be useful to use the 1M instead of 50 Ω input?

[@MachOneDesignEMC]

Hi, definitely not the 1M ohm input, here, we are using a 50 ohm system in a sense that the transimisision line (the co-axial) is 50 ohm, and the RF current probe only works with 50 ohm. by selecting 1 M ohm, the characteristics of the RF monitoring probe will change completely and even place a negative impact on the signal you want to measure.

[@TheDutchGuyOnYT]

@@MachOneDesignEMCclear! Measurements always influence the system because the total sum of energy stays the same, except if it’s added like with an active probe.