Estimation of fields radiated by a PCB from close magnetic field measurements

A method is developed to predict the maximum electric and magnetic fields radiated by a printed circuit board (PCB) by measuring the amplitude of magnetic fields very close to the PCB on a planar surface. The vertical component of the magnetic fields in each cell of an imaginary planar grid a few centimetres above the PCB is modelled as due to the magnetic dipole moments situated directly below the measuring point on each cell of another imaginary planar grid on the plane of the PCB. The dipole loops are assumed to be coplanar with the plane of the PCB. Magnetic dipole moments in each cell of the grid are calculated from the magnetic field value measured directly above it. From the dipole moments the amplitude of the electric and magnetic fields can be calculated at any distance from the PCB. To verify the method, two devices were tested: a simple PCB containing a loop with a crystal oscillator and resistor, and a fully populated microprocessor device in its casing. The vertical component of the close magnetic fields are measured using a commercially available close-field magnetic field probe and spectrum analyser. The frequency range from 30 MHz to 110 MHz are covered. The calculated maximum electric field radiation at 3 m from the devices are compared with the directly measured electric field radiation in a semi-anechoic chamber. The calculated and measured values are within 5 dBμV/m of each other for the oscillator circuit and within 9 dBμV/m of each other for the microprocessor device, for each frequency tested