Signal Integrity and Radiated Emission of High-Speed Digital Systems

9

PCB Radiated Emission

The essential equations for calculating the spectrum in the frequency domain of typical signal and noise waveforms in digital systems are given in this chapter. These equations will be useful for calculating the emission profile of radiating structures such as a PCB and attached cables.

The basic concepts for predicting radiated emission from a PCB to be compared with the limits of the standards are described. The distinction between differential-mode (DM) and common-mode (CM) emission is outlined. Analytical procedures are given and validated by experimental results. Closed-form expressions for calculating radiated fields from wire structures, as often found in PCBs with attached cables, are provided in Appendix D.

Radiated emission from typical trace structures in PCBs are investigated. Models based on Transmission-Line (TL) and long-dipole theory for predicting emission profiles are outlined. Experimental results obtained by some test boards are compared with simulation results. The importance of properly connecting the ground planes between them and with the circuits in order to avoid common-mode emissions is underlined.

It will be shown that the radiated emission from ICs can be predicted by a simple formula based on small dipole antenna formulation. Experimental results will be provided to demonstrate that radiated emission is independent of the decoupling capacitors in the absence of cables attached to the board.

Radiated emission from a PCB, including ...