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Motorola Semiconductor Application Note

AN1050

Designing for Electromagnetic Compatibility (EMC)

with HCMOS Microcontrollers

By Mike Catherwood

Motorola, Inc.

Austin, Texas

Introduction

The operating speed of present high-density complementary metal

oxide semiconductor (HCMOS) devices is approaching that of the

fastest bipolar logic families of only a few years ago. Associated with this

increase in performance ar some new design challenges for the

microcontroller unit (MCU)-based system designer. This application

note addresses one of these issues, the electromagnetic compatibility

(EMC) of the finished product.

EMC may be considered from either an emission or a susceptibility point

of view. Although the following discussion relates primarily to emission

control (in particular, radiated emission), most techniques to limit

emission also reduce susceptibility. Furthermore, minimizing

electromagnetic interference (EMI) will reduce overall system noise, the

benefits of which are higher digital noise immunity and accurate

operation of local analog subsystems (for example, better design margin

and a more reliable end product).

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Application Note

EMC can exist only when the system functions correctly within the

intended electromagnetic environment and does not exceed the EMI

levels specified in the appropriate standards documents. EMI, which

encompasses interference in a bandwidth of dc to daylight, is a

generalization of a much older term, radio frequency interference (RFI),

which is now defined to encompass 10 kHz to 3 GHz. Failure to consider

EMC during early phases of the design process may result in expensive

modifications (possibly with many additional components), printed circuit

board (PCB) re-layout, product introduction delays, and EMC consultant

fees to conform to the required standards.

Legal Requirements

The Federal Communications Commission (FCC) has a set of standards

to regulate EMI in electronic equipment and systems for use in the

United States. Compliance with the appropriate sections of these

regulations is mandatory to market or sell a product except for certain

subclasses of digital devices that are temporarily exempt. Engineering

models (including field-trial prototypes which are not sold) are also

exempt; however, the display of a product at an electronics show is

considered a marketing function subject to regulations.

FCC rules and regulations (part 15, subpart J of title 47 of The Federal

Regulations ) apply to almost all digital devices (see reference 1),

defining standards and operational requirements for all devices capable

of emitting radio frequency (RF) energy within the range 450 kHz to

1 GHz.

Equipment for use within West Germany must comply with a different set

of standards defined and administered by the Verland Deutcher Electro-

Techniker (VDE). Digital equipment is generally required to meet both

VDE0871 standards. In other countries, compliance to a standard is not

always mandatory; however, the European Economic Community (EEC)

member states intend to introduce a mandatory RFI performance

standard after January 1, 1992. The current proposal is based on

Internal Special Committee on Radio Interference specification

CISPR22 and is referred to as European norm EN55022. As the FCC is

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Application Note

RFI Problem Overview

a member of the CISPR, and has voted in favor of the CISPR22

standard, it is likely that the FCC will ultimately adopt the same

standards. CISPR22 is somewhat more stringent than FCC part 15,

subpart J, in the 88-MHz to 230-MHz frequency range, though it is less

stringent than some aspects of the VDE0871.

RFI Problem Overview

The frequency spectrum of a periodic waveform has been shown,

through Fourier analysis, to be composed of discrete frequency

components that include the fundamental (

o) and multiple harmonics

o). For a typical trapezoidal waveform, the relative amplitude of

each frequency component is related to the fundamental frequency, the

rise time, and mark-to-space ratio (duty cycle) of the waveform (see

reference 2). Doubling the frequency, halving the rise time, or halving the

mark-to-space ratio will double (+6 dB) the amplitude of a specific

harmonic frequency.

It is possible to graphically predict the harmonic spectrum of a specific

trapezoidal waveform by plotting the amplitude of two corner frequencies

and a reference (0 dB) point. This plot is referred to as a Fourier

envelope, a Bode plot, or a nomogram. An example is shown in

Figure 1 where:

0 dB reference = 20 log(2A

)dB

f1 = 1/

f2 = 1/

t r

where:

V = amplitude V

P = pulse width s

t r = rise time

T = period

= (P + t r )/T

P), the amplitude of the harmonics falls off

at –20 dB/decade. Above f2 (1/

t r ), the amplitude of the harmonics falls

off at –40 dB/decade. For many applications, these latter harmonics are

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(n x

At frequencies beyond f1 (1/

Application Note

t r . For example, an HCMOS device which

can produce an external periodic signal with edge times on the order of

2 ns can generate significant harmonics (for example, have a bandwidth)

of up to 160 MHz. Any printed circuit board (PCB) tracks, component

leads, cables, or connectors attached directly or capacitively to signal

sources, such as those previously described, can act as antennas and

radiate the harmonics with varying degrees of efficiency. Radiated

emission from a system may be either differential-mode or common-

mode radiation; common-mode radiation is typically more difficult to

reduce.

= (P + t r )/T

20 LOG (2A

)

f1 = 1/ π

–20 dB/OCTAVE

20 LOG (2At r /P)

f2 = 1/

t r

–40 dB/OCTAVE

LOG FREQUENCY (Hz)

(a) Nomogram

90%

10%

t r

(b) Trapezoidal Waveform

Figure 1. Nomogram of a Trapezoidal Waveform

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considered small enough to be ignored; thus, the bandwidth of a system

is generally defined to be 1/

Application Note

Differential-Mode Radiation

Differential-mode radiation is caused by the flow of RF current loops

around the system conductors. For a small loop area, the far-field

electric term, when operating in a field above a ground plane (free space

is not a typical environment), can be shown to be approximately (see

reference 3):

E = 2.6 (A I L ƒ

2 )/R

V/m

(1)

where:

A = loop area cm 2

I L = loop current A

= frequency

MHz

R = distance

For a constant current and loop area, the electric field at a prescribed

distance is proportional to the square of the frequency (for example, it

increases at 40 dB/decade). Adding this term to the Fourier envelope

indicates that the differential-mode radiated emission increases at

20 dB/decade up to f2, after which it remains flat. R is fixed by both the

FCC and VDE rules and regulations, and

Common-Mode Radiation

Common-mode (CM) radiation is caused by unintentional voltage drops

in a circuit, which cause some grounded parts of the circuit to rise above

the real ground potential (see Figure 2 ) . Cables connected to the

affected ground act like antennas and radiate the components of the CM

potential. The far-field electric term can be shown as follows (see

reference 3):

(

I CM L)/R

V/m

(2)

where:

L = antenna length m

I CM = common-mode current A

= frequency

R = distance

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is usually not a system

variable; however, A and I L can be reduced through thoughtful board

layout and careful circuit design.

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