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MARCH 2022 I DESIGN007 MAGAZINE 19 tribution network above this frequency is to use ECM or, alternatively, die capacitance. ese ultra-thin laminates replace the con- ventional power and ground planes and have excellent stability of dielectric constant and loss up to 15 GHz. Planar capacitance requires a very thin dielectric with a high dielectric constant (Dk). is is contrary to the typical high-speed design that requires a low dielectric constant to improve dissipation losses. However, in this case, we need high capacitance, hence the high Dk. With 20 nF per square inch in capacitance density, 3M ECM is the highest capacitance density embedded-capacitance material on the market (Table 1). 4. Differential Pair Signal Coupling Electromagnetic radiation from digital cir- cuits can occur as either differential mode or common mode. Differential mode is typically equal and opposite, and therefore any radiat- ing fields will cancel. Conversely, common mode radiation from two coupled conductors is identical. It does not cancel but rather rein- forces. Unfortunately, differential mode propa- gation can be converted to common mode by parasitic capacitance, or any imbalance caused by signal skew, rise/fall time mismatch, or asymmetry in the channel. Also, return path discontinuities can create large common-mode loop areas that increase series inductance and electromagnetic radiation. In the case of differential pairs, the trans- formation from differential mode to common mode typically takes place on bends and non- symmetrical routing, near via and pin obstruc- tions, but can also be caused by small changes in impedance due to RPDs. If a differential pair is well balanced, then tight coupling will achieve an effective degree of field cancellation. However, if they are not perfectly balanced (Figure 2), then the degree of cancellation is not determined by the spac- ing, but rather by the common-mode balance of the differential pair. Most digital drivers have poor common-mode balance and therefore differential pairs oen radiate far more power in the common mode than in the differential mode. In such a case, one gains no radiation benefit from coupling the differential traces more closely together. Differential signals that are closely coupled will operate mainly in the differential mode with some common-mode radiation from imbalances in the signals. If the two traces are separated enough to prevent coupling, then both act as single-ended signals. So, a 100-ohm differential pair becomes two individual 50 ohm single-ended signals. is is fine, provid- ing the loop area is small and the impedance does not change along the length of the signals. 5. Induced Crosstalk As signal traces come into proximity of an aggressor signal, part of that signal is uninten- tionally electromagnetically coupled into the victim trace as noise due to the overlapping of EM fields. Crosstalk is three-dimensional and is dependent on the signal trace separation, the Figure 2: Differential mode signals can be converted to common mode.