Components, layout, and construction of RF circuits

        The components and circuits of RF circuits are different from those of other frequencies, which essentially stem from the countless parasitic inductances and capacitors in the circuit, which account for a large part of the inductance and capacitance that the circuit work can do. Consider a tuning circuit consisting of a capacitance of 100pF and an inductance of 1uH. According to the formula you will learn in the following sections. The theoretical resonance frequency of this combination is 15.92 MHZ. However, if the circuit is poorly laid out, it will result in a parasitic capacitance of 25pF: this capacitance can be either from conductance and coupling between the capacitor and the base or other parts, or from the input capacitance of a transistor or integrated circuit (IC) amplifier (the input capacitance of a common RF IC is typically 7pF). So, what impact will this additional 25pF capacitance do to our circuits? This capacitor is connected in parallel to a discrete capacitor of 100pF, resulting in a total capacitance of 125pF. If 125pF is used instead of 100pF to recalculate, the resonance frequency obtained from the resonance formula becomes 14.24MHZ.

        A similar situation occurs with distributed inductances. Any conductor with current flowing through it will exhibit some small inductive effects. In low-frequency circuits (and even in some low HF circuits), this inductance is so small that it is not considered at all. However, the random frequency rises from the 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 way to replace the actual wire interconnection with a printed circuit board. An example is a circuit that uses #28 iron-nickel-cobalt alloy enameled wire with poor performance, but the performance improves when it is reimplemented with a printed power eyeboard.

     Shows a printed circuit of a simple RF amplifier. The only feature of this circuit is that the printed lines are very wide and the spacing is very narrow. This method reduces the distributed inductance and makes the circuit more designable. The top of the printed circuit board is all copper except for the reserved supply device and the bottom line. This layer is called the ground plane of the board.