SPECTRUM MANAGEMENT AND EMC DESIGN TECHNIQUES OF ELECTRONICS SYSTEM

Introduction

Recent past has witnessed a proliferation of electronic system in the military domain as well as commercial sector. This warrants co existence of electronic system with other friendly users. The electromagnetic spectrum is a finite natural resource and hence requires planning , co-ordination and proper management so that the system deployed could function in the intended environment . Achieving electromagnetic compatibility is an essential and integral part of research , development , testing and operational management of spectrum dependent systems. The primary concern of electromagnetic compatibility design is, electromagnetic shielding (Schelkunoff, 1943), circuit filtering (White,1973) and good ground design (Denny,1983). The engine that drives modern electronic system is located on a printed circuit board (PCB) and is comprised of potential interference sources (Keenan, 1985). The interconnect cables are the best emitter and receptor if care is not taken while routing.

Noise and interference becomes hazardous for a missions success where it is essential to understand the transmitted messages from friendly emitters i.e. better signal to noise ratio. The source of noise and interference can be from both external as well as internal sources. The interference may be inadvertent or it may be deliberate attempt on the part of an adversary to disrupt an operators ability to communicate.

There are various techniques to combat noise and interference such as.

* Boosting the effective radiated power

* Providing means for optimizing operating frequency

* Choosing a suitable modulation scheme

* Designing a receiver that reject interfering signals

* Time, space and frequency management

Hence, parameters of immunity and emission needs to be addressed from the beginning of basic design. The system designers must under stand the various noise that occur. Average values of atmospheric noise is a function of time of day and season and has greatest potential in the 1 MHz to 5 MHz range. The environment is full of radio spectrum emitted by man made multi sources, even a friendly source can degrade the desired performance. The circuit should be designed to eliminate as many of these problems. Further the circuit board should allow the easy addition of noise suppression components at the later stage. In subsequent section the paper elaborates upon the EMC design techniques i.e. shielding, filtering and grounding to suppress EMI.

EM Shielding

An electromagnetic wave consists of two oscillating field, E-field and H-field. The E-field can easily interact with high impedance voltage driven circuitry, such as straight wire. Where as , H-fields are mostly interacting with low impedance current driven circuitry, such as wire loops. Any barrier placed between an emitter and a susceptor that attenuates the strength of the interfering signal is known as EMI shield. The energy loss in a shield is a function of permeability, conductivity and the thickness of the barrier material, and frequency as well as distance of interference source.

Thus total shielding effectiveness

(SE_dB) = Reflective loss (R_dB) + Absorption loss (A_dB) + re Reflection loss (B_dB) --------------- (i)

A shielded enclosure, to house the sub system, is fabricated from materials that possess the desired physical and electrical characteristics. To achieve desired shielding effectiveness care is taken on seams and joints. The efficiency of coupling is the function of hole/opening size in relation to impinging wavelength. Any openings in an enclosure can become efficient coupling path at some particular frequency or a band of frequency. A good rule of thumb is therefore followed to avoid openings larger than / 50 for microwave. Since most EMI coupling problems are broad band in nature, the frequency of concern would be highest threat frequency within the band width envelop.

Electromagnetic energy leakage through an aperture is maximum or shielding is zero when wavelength (λ ) is equal to twice the opening ( d). The frequency at which it occurs is known as cut off frequency. If, C, is the propagation velocity of em waves then

f_c(critical frequency ) = C/2d.

For λ ≥ 2 d, the attenuation is expressed as ;

R _dB = 20 logλ /2 d,

where 2 ≥ d ≥ t (the material thickness)

As a rule of thumb net shielding is assumed 0 dB at the cut off frequency and a linear increase of 20 dB per decade in shielding as the frequency decreases. The highest shielding effectiveness is calculated for a box without any aperture.

When a noise source or unwanted signal source is close to the aperture of the box reduction in shielding effectiveness value is inevitable. The effective cut off frequency is reduced proportionally to the ratio of the distance from the aperture.

F = (C /2d) (r/d) ---------------- (2) And

R dB = (20 log λ /2d) (r/d) ---------(3)

Where λ /2 ≥ d

If there are more than one aperture of the same size in a solid metal barrier, it reduces the total effective shielding. The spacing between two adjacent aperture if kept λ /2, the loss is minimum or zero for all practical purposes.

The reduction in shielding due to multiple apertures is approximately proportional to the square root of the total no. (n) of equal sized aperture.

R_dB = 20 log λ/2d – 20 log n^1/2

Enclosure Penetration

Electromagnetic energy can penetrate the wall of shielding box through various coupling mechanism such as

* Direct penetration

* Aperture penetration

* Coupling through cable

* Coupling through antenna surface area

To maintain the shielding integrity of the enclosure at the entry point of cable, electronic filters or shielded cables are used.

To maintain the shielding integrity of an enclosure with feed through or non conductive shaft or air vent, wave-guide technique is used. The attenuation characteristics of a wave guide below the cut off frequency is a function of the depth to width ratio. When depth to width ratio increases, the shielding increases.

Grounding

Grounding is probably the most important aspect of EMI control. Often ground conductor impedance at the frequencies of interest are ignored, as a result, the shielding effectiveness gets degraded. The goal is to maintain a very low impedance path. Special attention is given to the use of dissimilar metals to preclude the effects of galvanic action. To avoid ground loop signal returns are required to be attached at one point. However, multipoint ground also yield better result in specific cases.

Viewing Window

Viewing windows are laminated with conductive screen between optically clear plastic sheet or glass sheets and also by applying an optically clear conductive layer to a transparent substrate like stannous chloride.

EMI Filters

Filters are used to control conducted EMI. Generally, higher filter insertion loss are required for military equipment operating in severe electromagnetic environments or mission critical scenarios. This requires generally physically larger filters. A basic knowledge of filter design and its varying characteristics help in selecting the type of filter. When common mode filtering is required, line-to-ground capacitors and common core inductors are used. When differential mode filtering is required, line-to-line capacitors and discrete series inductors are used. The installation of a filter is extremely critical. Filter case to frame ground connection need low impedance path over the frequency range of the filter. Input-to-output leads must have maximum physical isolation. In the case of power line and input/output line filters, the filtered line must be as close as possible to the enclosure entry point.

Filter attenuation is highly dependent upon source and load impedances. Generally 50Ω source and load impedances are considered while actual impedance are reactive and vary considerably over the frequency range of interest. The mathematical computation for filter selection is always misleading. When a filter mismatches its source and load impedances, minimum transfer of signal power occurs. When the source impedance is high, the filter input impedance should be low or shunt capacitive. If the source impedance is low, the filter input impedance should be high or series reactive. The same mismatch is allowed between the load impedance and the filters output impedance. Virtually all off – the-self power line filters are designed to handle common mode noise and many provide both common and differential mode filtering. Thus only conduction test could determine the interference potential and the types of filter required.

Printed Circuit Board Design

The PCB design has special significance to control EMI. The location of active component, the routing of traces, impedance matching, grounding and shielding of certain active components are the key features. A printed circuit board is designed considering control to the following:

* Emissions from the PCB circuitry * Susceptibility of the PCB circuits to external interference

* Coupling between PCB circuits and other near by circuits in the device

* Coupling between circuits on the PCB

This is accomplished primarily by paying special attention to the board lay out and design, minimizing impedance discontinuities and where ever possible, by using low amplitude signals.

For clock frequency above 10 MHz it is necessary to use multiplayer design with an embedded ground layer. Components should be located such that noisy and sensitive circuits can be isolated. Clocks should be centrally located to minimize onboard traces. I/O chips should be located near the associated connectors. Output circuit is protected through damping with a register, inductor, or ferrite bead mounted close to the driver. Circuit types like digital, analog, and power; require separation as well as careful ground. When a PCB lay out starts behaving like a transmission environment minimizing impedance discontinuities is the remedial measure. Good decoupling practices are to be followed through out the PCB. For by passing, suitable choice is 0.1 to 1.0f (microfarad) ceramic by pass capacitor. They are mounted close to the IC. The power bus loop becomes receptor of magnetic field, thus, it is required that routing the power bus as close as possible to its return. Power lines are required to be filtered at the PCB interface.

Internal Cable

For a properly designed PCB, the requirement for shielding of internal cable is minimum, however, if at all cable shielding is needed, care must be taken to ground the cable shield. Under no condition cable shields are used as signal return path. For certain unbalanced circuit co axial cables are used and the shields are used as return signal path. But at both end of the co axial cable the circuits are so designed that the cable does not radiate. If, in such a case, a co axial cable radiates, another shield must be added and the outer ground is bonded to the chassis ground.

Class of EMC problems:

EMC problems can be classified as,

* Radiated interference

* Conducted interference

* Radiated susceptibility

* Conducted susceptibility

* Electrostatic discharge.

The primary purpose of EMC design analysis is the problem solving approach considered by system designer at the beginning of the concept paper. EMC design ensures the reliability of the system in the intended environment. For military application, the most commonly used military standards is MIL-STD 461 D/E. The military emission limits are much lower and the susceptibility criteria more severe than those found in commercial standards. Pieces of equipment/sub system/components acquired from vendors should comply the industrial standards in particular EN 61000-6-2 and EN 61000-6-4.

Growth in commercial wireless applications such as; cellular, personal communication system, mobile telephony, broadcast and others has placed increasing em hazard on defence systems. Military information superiority requires technical developments in areas like; passive and active remote sensing, high speed data links, high resolution radars and wide band mobile links. Thus to ensure EM spectrum supportability in defence it is bounden duty of system designer to pursue EMC regulations and procedures.

Conclusions

Military electronics is the back bone of operation commanders. The powerful transmitters as well as sensitive receivers, most of the time, work on the same platform. The EMC requirement is pretty tough, shielding and gasketing is always needed, irrespective of how well the circuit board has been designed. Good PCB design technique with shielding and grounding scheme is the principal requirement for controlling emission and enhanced immunity. Defence has to rely on force multipliers such as automation, smart weapons and situational awareness which require rapid and sustained information transfer. Therefore it is imperative to opt for EMC design of Electronic system.

References

[1] Schelkunoff, S A Electromagnetic waves, Princeton, NJ, 1943 2. White Donald, R J; A Hand Book Series on electromagnetic Interference and compatibility, Vol.3, Gainesville, VA: Don White Consultants, 1973

[2] Denny, Hugh W; Grounding for the control of EMI, Gainesville, VA: Don White Consultants. Inc; 1983

[3] Keenan R Kenneth; Decoupling and lay out of Digital Printed circuits, Pinellas Park, FL; TKC 1985.