Prediction of Radiated Emissions from High Speed Digital Circuits

This report examines the application of theoretical models to predict the levels of emissions caused by currents induced on the tracks of a printed circuit board (PCB) when driven by typical clock circuits.

A clock generator circuit and its interface with the PCB was modelled to determine the magnitude and form of the currents excited at the inputs to the PCB tracks, at both fundamental clock frequency an harmonic frequencies.

Using the calculated excitations, the current and field distributions on the PCB were determined and used to predict the radiated fields.

Two numerical analysis techniques are considered, one using the method of moments and one based on the finite difference time domain (FDTD) approach.

These techniques were combined with the P-SPICE circuit simulator software using a novel methodology which enabled both circuit imbalances and electromagnetic field distributions to be accurately modelled.

A range of PCBs are examined, including both single and multilayer types, with straight and angled signal tracks; simple layouts being employed to enable physical effects to be identified. The boards were drive by a 50MHz square-wave clock, using a realistic high-speed CMOS configuration.

Boards were fabricated and measured on an open field test site and the results compared with the predictions over a wide frequency band (100MHz-1GHz).