Components, layout, and construction of RF circuits

The components and circuits of RF circuits are different from those of other frequencies, essentially because the countless parasitic inductances and capacitors in the circuit account for a large part of the inductance and capacitance that the circuit works can do. Consider a tuning circuit consisting of a 100pF capacitor and a 1uH inductor. Follow the formula you will learn in the following sections. THE THEORETICAL RESONANCE FREQUENCY OF THIS COMBINATION IS 15.92MHZ. However, if the circuit is not well laid out, it will result in a parasitic capacitance of 25pF: this capacitance can be either derived from the coupling between the conductance and capacitance and the base or other part, or it can be the parasitic capacitance contributed by the input capacitance of a transistor or integrated circuit (IC) amplifier (a typical RF IC has an input capacitance of 7pF). So, what effect does this extra 25pF capacitance do to our circuit? This capacitor is connected in parallel with the 100pF discrete capacitor, so the total capacitance is 125pF. Recalculate with 125pF instead of 100pF, then the resonance frequency obtained by the resonance formula becomes 14.24MHZ.

A similar situation occurs with distributed inductance. Any conductor with current flowing through exhibits some small inductive effects. In low-frequency circuits (and even in some low-HF circuits), this inductance is so small that it is completely unconsidered. However, the random frequency rises from HF to the VHF region, and the distributed inductance accounts for a large proportion of the total inductance.

Layout in RF circuits is important because it reduces the effects of distributed inductance and capacitance. A good approach is to replace the actual wire interconnect with a printed circuit board. An example is a circuit that performs poorly with %S28 iron-nickel-cobalt enameled wire, but improves performance when re-implemented with printed power eyeboards.

Shows the printed circuit of a simple RF amplifier. The only feature of this circuit is that the printed wiring is very wide and the spacing is very narrow. This approach reduces the distributed inductance and makes the circuit more designable. The top of the printed circuit board is all copper, except for the reserved place where the supply device is connected to the bottom line. This layer is called the ground connection of the board.