Secrets of RF Circuit Design

Radio-frequency (RF) electronics differ from other electronics because the higherfrequencies make some circuit operaTIon a little hard to understand. Straycapacitance and stray inductance afflict these circuits. Stray capacitance is thecapacitance that exists between conductors of the circuit, between conductors orcomponents and ground, or between components. Stray inductance is the normal inductanceof the conductors that connect components, as well as internal componentinductances. These stray parameters are not usually important at dc and low acfrequencies, but as the frequency increases, they become a much larger proporTIonof the total. In some older very high frequency (VHF) TV tuners and VHF communicaTIonsreceiver front ends, the stray capacitances were sufficiently large to tune thecircuits, so no actual discrete tuning capacitors were needed.Also, skin effect exists at RF. The term skin effect refers to the fact that ac flowsonly on the outside porTIon of the conductor, while dc flows through the entire conductor.As frequency increases, skin effect produces a smaller zone of conductionand a correspondingly higher value of ac resistance compared with dc resistance.Another problem with RF circuits is that the signals find it easier to radiate bothfrom the circuit and within the circuit. Thus, coupling effects between elements ofthe circuit, between the circuit and its environment, and from the environment tothe circuit become a lot more critical at RF. Interference and other strange effectsare found at RF that are missing in dc circuits and are negligible in most lowfrequencyac circuits.