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86 PCB007 MAGAZINE I OCTOBER 2022 However, inductors can play a very active role in circuit design. So, strap in and take your Dramamine because the ride now gets a little bumpy. Inductors or coils usually play a role as trans- formers or sometimes electromagnetic motors. Using a primary and secondary winding, these devices can step-up or step-down voltages or create an electromagnetic field that can spin an armature in a motor assembly. However, in PCB design they can do some very inter- esting things. e primary and secondary of a coil are created by making "turns" of a wire around a form or core. e ratio of the num- ber of turns in the primary to the number of turns in the secondary determines the charac- teristics of performance. We don't see the use of transformers on a PCB per se, but they per- form other functions quite well. Although coils can be used to boost or aug- ment voltages, they are primarily used to manipulate signals. is is particularly impor- tant in RF designs. In high-speed digital or RF circuits, stray RF or parasitic compo- nents are extremely detrimental to prod- uct performance. Here is where the inductors play a very important role. When you play in the RF arena you deal with VSWR, reac- tance and attenuation. Remem- ber that I said a coil is a device that has a core and a primary and secondary winding? Well, an active coil could be created using just the PCB. PCB manufacturing uses dielectrics between lay- ers (hey, a core material) and depending on the design, an inductor can be made. We can run a trace between layers, say 1 to 3, back to layer 1, back to layer 3, and so on until we have the number of "turns" we need. We do the same for the other winding. We have created a primary and secondary winding around a core. An induc- tor! Todd, you say, that's all fine and good but what are we doing? As it sits, we can boost a signal or imped- ance match a circuit with that. But we don't stop there. If we add resistance or capacitance to this circuit, we start getting into reactance. Coils act differently when coupled with resis- tors or capacitors. We start playing in the world of inductive reactance and capacitive reac- tance. is is where it gets important with RF and microwave applications. Reactivity plays a significant role. Whether the circuit has induc- tive reactivity or capacitive reactivity depends on the matching. In an RF application, if the inductive reactivity equals the capacitive reac- tivity the circuit resonates, and full power is presented to the feed point of the transmitting port or antenna (if applicable). Figure 2 illustrates a Smith Chart where reactivity is plotted. When either inductance or capacitance in the circuit is higher than the Figure 2: Smith Chart.

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