IEEE EMC SOCIETY
ROCKY MOUNTAIN CHAPTER
OCTOBER MEETING
TUESDAY OCTOBER 20, 1998
Mr. Robert W. Dockey
Hewlett-Packard Company
"New Techniques for Reducing Printed Circuit Board
Common-Mode Radiation"
WHERE: NIST - Boulder
325 Broadway
Boulder, Colorado 80303
TIME: 7:00 p.m.
DIRECTIONS: From Denver take I-25 to Highway 36. Go NW on Highway
36 to
Boulder. Continue on Highway 36 to Baseline Road. Take Baseline Road
West
one block to 27th Way. Turn South (left) on 27th Way and continue past
Broadway into the NIST parking lot. Enter through the Main doors
(facing
Broadway) and check in with the guard at the front desk, the guard will
direct you to our meeting room.
DINNER: 5:30 p.m. - 6:45 p.m. Rafferty's Restaurant
2805 Pearl, Boulder, Colorado 80301, Phone; (303) 447-2665
Rafferty's is on the corner of 28th (Highway 36) and Pearl.
From
Baseline road, continue North on Highway 36 (28th Street) 5 stop lights.
Take next Right into the parking lot of Rafferty's. See you there!
RSVP to Kevin Hight at 303-417-5534 by noon on the 19th.
For more information please contact one of the IEEE Rocky Mountain
Chapter
EMC Society Officers listed below.
Chairman: Jeff Doolittle 303-449-4165
Vice Chairman: Joe Kramer 303-449-4165
Secretary: Kevin Hight 303-417-5534
Treasurer: Wes Smith 970-962-7472
EVERYONE IS WELCOME !
AUTHOR BIOGRAPHY
Robert Dockey
Bob 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, Colo.
INTRODUCTION
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.
"New Techniques for Reducing Printed Circuit Board Common-Mode
Radiation"
ABSTRACT
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.
Lyle Luttrell
Regulatory Agent/Component Engineer
Breece Hill Technologies
[email protected]
ph 303-664-8286 fax 303-449-1027
http://www.breecehill.com
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