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16 DESIGN007 MAGAZINE I APRIL 2025 RF Transmission Conductors • Care must be taken to prevent unintended coupling between signal conductors. Conductors should be kept as far apart as possible and should not be routed in close proximity for extended distances. • Coupling between parallel microstrip conductors will increase with decreasing separation and increasing parallel routing distance. • Conductors that cross on separate circuit layers should have a copper ground plane layer to provide separation. • Conductors that carry high power levels should be spaced away from all other sig- nal conductors whenever possible. High-speed Digital Signal Conductors ese lines should be routed separately on a different layer than the RF signal lines to prevent coupling. Digital noise (from clocks, PLLs, etc.) can couple onto RF signal lines, and can be modulated onto RF carriers. Alter- natively, in some cases digital noise can be up-/ down-converted. Power and Ground Decoupling Decoupling/bypass capacitors should be provided at the main VCC distribution node, as well as at VCC branches. e choice of the bypass capacitance values will be made based on the overall frequency response of the RF IC and the expected frequency distribution nature of any digital noise from clocks and PLLs. ese lines should also be separated from any RF lines that will transmit large amounts of RF power. Conductor Planning for RF Applications ere are two basic types of conduc- tors employed for transmitting RF signals: microstrip and stripline. e primary differ- ence between these two conductors is the position of the signal conductor(s) in relation to the reference planes. Impedance control is most relevant when high-frequency signals cross over transmission lines on adjacent lay- ers. Microstrip conductors will be routed on the outer surface of the substrate while strip- line conductors are sandwiched between two reference planes. e stripline illustrations furnished in Figure 1 are typical transmission lines implemented for controlled impedance applications. Impedance control is most rel- evant when high-frequency signals cross over transmission lines on adjacent layers. e stripline function is identical to microstrip, but the RF signal is surrounded top and bottom by ground. e ground planes provide isolation to minimize outside interference with the RF signal transmission on the stripline. e spacing between the two conductors of a differential pair should be no more than dou- ble the width of the conductors. As an exam- ple, a 0.10 mm (~0.004") wide differential pair Figure 1: Stripline profile variations.