秘密的射頻電路設計

Radio-frequency (RF) electronics differ from other electronics because the higher
frequencies make some circuit operation a little hard to understand. Stray
capacitance and stray inductance afflict these circuits. Stray capacitance is the
capacitance that exists between conductors of the circuit, between conductors or
components and ground, or between components. Stray inductance is the normal inductance
of the conductors that connect components, as well as internal component
inductances. These stray parameters are not usually important at dc and low ac
frequencies, but as the frequency increases, they become a much larger proportion
of the total. In some older very high frequency (VHF) TV tuners and VHF communications
receiver front ends, the stray capacitances were sufficiently large to tune the
circuits, so no actual discrete tuning capacitors were needed.
Also, skin effect exists at RF. The term skin effect refers to the fact that ac flows
only on the outside portion of the conductor, while dc flows through the entire conductor.
As frequency increases, skin effect produces a smaller zone of conduction
and 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 both
from the circuit and within the circuit. Thus, coupling effects between elements of
the circuit, between the circuit and its environment, and from the environment to
the circuit become a lot more critical at RF. Interference and other strange effects
are found at RF that are missing in dc circuits and are negligible in most lowfrequency
ac circuits.