You are invited to IEEE Electromagnetic Compatibility Society Wednesday, December 2nd, 1998, 6:30 p.m. Chomerics Inc, Woburn, MA New Techniques for Reducing Printed Circuit Board Common-Mode Radiation Mr. Robert Dockey, EMC Engineering Group, Hewlett Packard Co.
Have you ever had the experience of believing that you had done everything humanly possible to reduce the radiated emissions from an unshielded printed circuit assembly and yet still measure unacceptable margins? If so, this presentation could help explain the reasons why. Even when all of the common emission suppression measures like power supply decoupling, cable filtering, loop area control and full ground planes have been utilized, there is still one dominant coupling mechanism remaining which can be manipulated to improve the margin. Unfortunately it may also set a lower limit on the possible emission level which can be obtained from a specific design. This mechanism is referred to as _Ground Plane Voltage Gradient_ contamination. This presentation will attempt to describe and analyze this radiated emission mechanism and present several design techniques which can be used to deal with it successfully. A multi-layer printed circuit board with a "good ground plane" can produce common-mode radiation similar to a dipole antenna. This ground plane is commonly thought of as a low impedance path for returning currents and one which is of constant potential across its area. In fact, these currents give rise to voltage gradients in the plane which act as sources of common-mode current. The majority of the RF current flowing along a signal trace on a multi-layer printed circuit board (PCB) returns on the ground plane directly beneath the signal trace. However, a small portion of the ground-plane current also can return via indirect paths causing the PCB and attached cables (if present) to produce common-mode radiation similar to a dipole antenna. Several new techniques to reduce these emissions by lowering the inductance of the ground return or by bypassing the common-mode current on the cables are presented. These cost-effective techniques can be employed on two-sided or multi-layer PCBs. Previous independent work by German, Ott and Paul experimentally investigated the radiated emissions from a printed circuit board (PCB) with a digital circuit that produced current on a signal trace that returned via an adjacent ground-return trace. They demonstrated that if this two-wire transmission-line is slightly unbalanced, it will radiate as an asymmetric dipole antenna producing common-mode radiation at much greater levels than the differential-mode radiation from the current loop. A direct prediction of this radiation was later performed by Hardin, Paul and Naishadham. In 1993, Dockey discovered that a relatively small PCB with a solid ground-plane could also produce common-mode radiation. On this truncated Microstrip transmission-line, the majority of the signal-trace current returns on the ground plane beneath the signal trace. However, this current encounters the finite inductance of the ground plane and produces a voltage gradient. The voltage gradient, commonly called the ground-noise voltage, then causes a small portion of the signal-trace current to flow through the distributed stray capacitance of the ground plane. This presentation will elaborate on these findings and propose several methods which can be used to effectively mitigate the radiation mechanisms. Robert Dockey is the EMC Engineering Group manager at the Hewlett Packard division in Vancouver Washington. He has a BSEE from the University of Missouri at Rolla and is certified as an EMC engineer by the National Association of Radio and Television Engineers. He is the author of three technical papers on various EMC subjects and is a member and Distinguished Lecturer of the IEEE EMC Society. Bob has been with Hewlett Packard for 13 years as both an EMC engineer and engineering manager. Previously, he spent 13 years as a TEMPEST engineering manager for TRW in Colorado Springs, Colorado. The EMC Chapter meeting will be held in the cafeteria at Chomerics, 77 Dragon Court, Woburn, Massachusetts on Wednesday December 2nd, 1998 at 7:PM. Complimentary pizza and beverages will be offered at 6:30 p.m. IEEE members and non-members are welcome. If you need further information contact John F. Luchini at (781) 401-1570 or Chomerics at (781) 935-4850.
