A two-conductor cable

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electronics, Instrumentation & Communication · Communication systems and RF engineering

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Problem

A 1 m1\,\mathrm{m} two-conductor cable, conductors 1.3 mm1.3\,\mathrm{mm} apart, carries a 30 MHz30\,\mathrm{MHz} clock with a differential-mode current iDM=5.0 mAi_{\rm DM}=5.0\,\mathrm{mA}. A current probe clamped around both conductors together reads 20 μA20\,\mu\mathrm{A} at 30 MHz30\,\mathrm{MHz}. The cable fails a radiated-emission check at 30 MHz30\,\mathrm{MHz}. ((a)) Say which current the clamp reading is, iDMi_{\rm DM} or iCMi_{\rm CM}, and why. ((b)) Using the chapter's field estimates ∣EDM∣≈1.32×10−14 f2A iDM/r|E_{\rm DM}|\approx1.32\times10^{-14}\,f^2A\,i_{\rm DM}/r and ∣ECM∣≈1.26×10−6 fℓ iCM/r|E_{\rm CM}|\approx1.26\times10^{-6}\,f\ell\,i_{\rm CM}/r, find the ratio ∣ECM∣/∣EDM∣|E_{\rm CM}|/|E_{\rm DM}|. ((c)) Decide which mode carries the interference, and whether a snap-on ferrite clamp around the cable is the right remedy.

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