RF Circuit Design, 2nd Edition

Chapter 11: RFIC (Radio Frequency Integrated Circuit)

11.1 Interference and Isolation

11.1.1 Existence of Interference in Circuitry

An electrical circuit consists of basic parts such as resistors, capacitors, inductors, transistors, and so on. Each of these has its specific function in the circuitry. One should be aware of the fact that the parts' performance is different in DC and AC operational situations. When a circuit operates with a DC current flowing through it or DC voltage across it, every part appears as a resistor and is ruled by Ohm's law. When a circuit operates with AC or RF signals, every part appears as a small antenna since there is AC current flowing through it or AC voltage across it. All these alternating electromagnetic fields radiate from the part to its whole surroundings, which would disturb the desired signal at any node in the circuitry.

According to the theory of electromagnetic radiation, the interference thus produced in the low-frequency range is negligible and usually can be ignored. However, in the high-frequency range, such as the RF range, the interference due to the radiation is considerable and it does interfere with the circuit at every node in the circuitry.

It must be noted that these electromagnetic fields disturb the circuitry along two paths. One is that of the electromagnetic field radiated into the surrounding space and then returned to the circuitry. This can be categorized as an “air path.” Another path is that of the electromagnetic ...