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FEBRUARY 2023 I DESIGN007 MAGAZINE 25 ondary winding rather than radiating. Just as waveguides and traces guide electromagnetic energy, so does the core. Transformers completely isolate the primary from the secondary. Transformers transfer the electric energy into magnetic energy (primary windings) and then back to electric energy (secondary winding). e transformer's core captures >90% of the magnetic energy and delivers it to the secondary windings. AC Coupling of High-speed Serial Links A capacitor is typically placed in series with both differential signals of high-speed SERDES serial links to remove common mode voltage differences between ICs or different technolo- gies (Figure 5). Any capacitor placed in series with the signal path tends to pass the high-fre- quency AC portions of the signal, while simul- taneously blocking the low-frequency DC por- tions. ese capacitors are essential to a variety of high-speed interfaces. And, as the next gen- eration of designs target data rates of 56Gbps and above, it becomes increasingly important to characterize channel transitions accurately to ensure a high confidence of success. However, capacitors block electron flow. A capacitor in a circuit causes equal and oppo- site charges to appear on the plates, charging the capacitor and increasing the electric field between the plates. No actual charge is trans- ported between its plates. Nonetheless, a mag- netic field exists between the plates as though a current were present. One explanation is that displacement current flows in the dielectric and this current produces the magnetic field in the region between the plates. is idea was conceived by James Clerk Maxwell in his 1861 paper "On Physical Lines of Force, Part III" in connection with the dis- placement of electric particles in a dielectric medium. Maxwell added displacement cur- rent to the electric current term in Ampère's Circuital Law. In his 1865 paper "A Dynamical eory of the Electromagnetic Field," Max- well used this amended version of Ampère's Circuital Law to derive the electromagnetic wave equation. is derivation integrates electricity, magnetism, and optics into one single unified theory. e displacement cur- rent term is now seen as a crucial addition that completed Maxwell's equations and is necessary to explain many phenomena, most particularly the existence of electromagnetic waves. Figure 4: Basic transformer with primary and secondary coils surrounding an iron core. Figure 5: Common mode current (I CM ) is the displacement current.