Radio frequency interference (RFI) is a well known problem. Generally it refers to quasi monochromatic (one frequency) signals that enter into our circuits and corrupt our signals.

In the lectures we covered the basic principle behind electromagnetic shielding. To recap: providing we lock our circuit in a metal box which is seamless (so external currents flowing on the outer surface do not cross to the inside) and whose shell thickness exceeds the skin depth, then no external influence can be detected on the inside.

In practice the following situations unavoidably arise.

• Radiation from outside the circuit can enter by direct radiative pickup. Usually this enters at the antenna, the enclosure through lack of proper electromagnetic shielding, through power supply feeds or by faults or misapplications of cables and connectors. Let's look at this in detail

• Within our own circuit, RFI can occur from one part to another through inductive or capacitive coupling. This is also referred to respectively as magnetic or electrostatic pick up. A related issue here is mains pickup. Large currents at 50 Hz (the power line frequency) circulate in earth loops leading to superposition of 50 Hz on wanted signals. Low frequency pick up like mains induced earth loops can be easily eliminated at radiofrequency by high pass filtering. High frequency pick up can only be avoided by careful circuit design or by using on board shielded enclosures.

• Finally, voltage measurement and amplification is plagued by poorly defined earth or a failure to recognise the true origin of the signals being measured. The best one can do here is to start with a good earth plane for your circuit and to make sure that tracks and wires carrying RF signals are as short as can be.