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mounted at the "root" of the star, and smaller capacitors are mounted at each of the star branches. e value of these latter capacitors depends on the operating frequency range of the RF IC and their specific functionality (i.e., interstage vs. main supply decoupling). Selecting Decoupling or Bypass Capacitors Real capacitors have limited effective fre- quency ranges due to their self-resonant fre- quency (SRF). e SRF is available from the manufacturer, but sometimes must be char- acterized by direct measurement. Above the SRF, the capacitor is inductive and, therefore, will not perform the decoupling or bypass function. When broadband decoupling is required, standard practice is to use several capacitors of increasing size (capacitance), all connected in parallel. e smaller value capacitors normally have higher SRFs (for example, a 0.2 pF value in a 0402 SMT package with an SRF = 14 GHz), while the larger values have lower SRFs (for example, a 2 pF value in the same package with an SRF = 4 GHz). Guidance for selecting capacitor values is furnished in Table 1. Bypass Capacitor Layout Considerations Since the supply lines must be AC ground, it is important to minimize the parasitic induc- tance added to the AC ground return path. ese parasitic inductances can be caused by layout or component orientation choices, such as the orientation of a decoupling capacitor's ground. e vias connecting the VCC pad on the top layer to the inner power plane (layer) potentially impede the AC ground current return, forcing a longer return path with result- ing higher parasitic inductance. Any AC cur- rent flowing into the VCC pin passes through the bypass capacitor to its ground side before returning on the inner ground layer. is con- figuration presents the smallest total footprint for the bypass capacitor and related vias. Final Note Designers working with companies devel- oping products with RF-dominant products may already have proven experience in RF, but, for the greater majority of us, there is still a lot to learn. To prepare for developing the circuit board requiring controlled impedance, the designer can gain a great deal of guidance from the designated circuit board supplier, the actual company responsible for fabricating the circuit board. DESIGN007 Vern Solberg is an indepen- dent technical consultant, specializing in SMT and micro- electronics design and manu- facturing technology. To read past columns, click here. 18 DESIGN007 MAGAZINE I APRIL 2025 Table 1: Low end of useful frequency range is defined as less than 5Ω of capacitive resistance. (Source: Maxim Integrated)