12 Methods of Increasing Immunity to Interfering Signals

“Rest, rest, perturbed spirit”

(Shakespeare: Cymbeline)

Despite careful design and protections to limit the emission of interfering signals by disturbing circuits, some residual noise will still contaminate sensitive circuits by conduction or field coupling. Therefore, the next task is to improve the immunity of perturbed circuits, in order to make them more noise tol- erant without sacrificing their electrical performance. Traditional methods include balancing, filtering, and grounding. Although these were discussed in Chap. 10 (in the context of reducing emission from sources of interferences), they will be revisited here with focus on how to use them in order to increase electromagnetic immunity.

12.1 Balancing

Deﬁnition. A balanced circuit is a two-conductor circuit that is symmetric with respect to a longitudinal axis (ultimately passing through the grounds of its terminal ports). The basic condition required is symmetry, which also applies to the transmission path and to the source and load.

Balanced Line. In a balanced line, the ﬁelds around the conductors are symmetric and there is no special ground conductor. For instance, two-wire lines are balanced, but a coaxial line is unbalanced.

Explanation. The property of symmetry is essential because the ultimate goal of balancing is to make the noise pickup equal in the two longitudinal circuit halves. When this condition is achieved, any external noise is trans- formed into a common-mode signal, which cancels in a load which has a symmetric conﬁguration with respect to ground. 390 12 Methods of Increasing Immunity to Interfering Signals

Fig. 12.1. Generic conﬁguration of a balanced circuit

For instance, in Fig. 12.1 suppose that Vp1,Vp2 are the inductive noise voltages picked up by the conductors, which generate common-mode currents Ip1,Ip2, respectively. The useful signal is delivered by the symmetric source (E1,E2) and causes a current (Is) through the loop passing through the terminals A–B–C–D; hence, it is a diﬀerential-mode current (which is not inﬂuenced by load symmetry). It follows that the total output voltage across the load is

VAB =RL1 Ip1 − RL2 Ip2 +IS(RL1 +RL2) (12.1)

The ﬁrst two terms represent the external noise contributions; they cancel provided that RL1 =RL2 and Ip1 =Ip2. The former condition implies that the load must have a symmetric conﬁguration with respect to ground; the latter requires identical conductors equally exposed to interfering signals (which is a critical condition). To conclude, perfect balancing requires:

– A signal source that is symmetric with respect to ground (E1 =E2, Rs1 =Rs2). – A load that is symmetric with respect to ground (RL1 =RL2). – Identical conductors, equally exposed to noise.

Example. To illustrate the diﬃculties of achieving perfect balancing, consider a system (Fig. 12.2) of two metal traces on a board, denoted (A), (B), coupled to a third one (P), which is the source of interference (perturbing system). The cross-coupling has two components. Consider the capacitive coupling: the signal (Vp) carried by conductor P is transmitted through CPA ,CPB to 12.1 Balancing 391

Fig. 12.2. Capacitive coupling in a quasi-balanced system

its neighbors and becomes a perturbation. Let us denote by VPA the fraction of VP reaching conductor A; thus

RA VPA =VP (12.2a) RA +1/jωCPA

At low frequencies, we may suppose that RA |1/jωCPA |, and (12.2a) becomes ∼ VPA = VP (jωCPA RA) (12.2b) Similarly ∼ VPB = VP (jωCPBRB) (12.2c)

When the system is symmetric, RA =RB, but due to the location of the disturbing conductor (closer to A than to B), CPA > CPB, and unequal contamination results. If inductive coupling is considered, the effect of the magnetic flux gener- ated by the current flowing through conductor P is stronger on conductor A than on conductor B (due to different separation distances). Note that this effect, rather than cancelling the unbalance of capacitive coupling, adds to it instead. This shows that even when the circuit has perfectly symmetric structure with respect to ground, it is not perfectly balanced until the coupling to the source of interference is the same throughout. Possible solutions are: – modify location of traces to obtain, as much as possible, equal exposure to interfering signals; – when wire conductors are used instead of traces, twisting represents an effective way to reduce unbalance.

Balance Ratio. This concept is employed in communication systems, which are designed for a minimum of stray pickup from the outside. The degree to which a transmission device deviates from an ideal balanced condition is expressed by the balance ratio or simply balance [193]. Note that this 392 12 Methods of Increasing Immunity to Interfering Signals

Fig. 12.3. a Measuring the unbalance of a transmission system; b equivalent load for well-balanced systems corresponds to the common-mode rejection ratio (CMRR) in the theory of operational amplifiers. In practice, this parameter can be measured by using the circuit in Fig. 12.3, where we apply a common-mode voltage Vc and measure the resulting differential-mode voltage Vd on the load. For a perfectly balanced configuration, Vd should be zero. The greater the value of Vd,themore unbalanced the circuit. Often expressed in decibels, the balance ratio (BR) is

V BR = 20 log c [dB] (12.3) Vd

Whenever the source internal resistance is low, Vc is roughly equal to the voltage (V) measured between each conductor and ground; deﬁnition (12.3) can be expressed as V BR = 20 log [dB] (12.4) Vd Equation (12.4) is better suited when the distance separating the source and the load is signiﬁcant, since both measurements are made at the same end. For circuits with high BR, it is advisable to replace the load with the equivalent circuit proposed in Fig. 12.3b, where X is a highly balanced reactor (for instance, a transformer).

Concluding Remarks – A well-balanced circuit exhibits a BR factor of the order of 60 to 80 dB. – In operational ampliﬁers, a high BR guarantees a high rejection of common- mode interfering signals and therefore increased immunity. – As a general rule, balancing is employed when shielding has been implemented but fails to suﬃciently increase circuit immunity. 12.2 Filtering 393 12.2 Filtering

As shown in Chap. 9, ﬁlters can be employed either to suppress the conducted emission of perturbations from noisy circuits or to improve immunity of critical circuits. The former aspect has been addressed in Chap. 10; now, we shall focus on the latter.

Overview. Depending on the particular type of action, filters are used in two situations: 1) When the DC power supply is distributed by lines (planes) having a non- negligible internal resistance and inductance, abrupt current demand due to a large number of simultaneous switching events can give rise to consid- erable inductive noise. As a result, a transient voltage drop in the power supply results, which may introduce logic failures and/or degrade the drive capability of transistors. This noise, propagating along the power and ground planes, may contaminate critical circuits sharing the same power/ground system. In this case, decoupling filters must be provided at all sensitive circuit power pins. 2) In electronic equipment, shields are employed to protect critical circuits; however, the cables and wires passing through the shield can transport not only useful signals, but also picked-up interfering signals. To avoid contamination of the protected circuits, careful filtering of wires and grounding of interconnect screens must be provided.

12.2.1 Decoupling Filters

Decoupling the DC Supply. This is mandatory whenever the user has no control over the design of the DC supply and its associated distribution system. In practice, two kinds of filters are used to decouple circuits: RC filters and LC filters. Both are presented in Fig. 12.4, where filter ground should be connected to the ground of the protected circuit. Essentially, they are low-pass pi-section configurations intended to suppress voltage fluctuations due to abrupt transient currents in the DC supply system. To be effective, the HF series impedance of the filter must be as high as possible, but the DC series impedance must be negligible (to avoid losses in the DC supply). This compromise is rather difficult to obtain with RC filters, since the value of R is not frequency-dependent. From this point of view, LC filters are better suited, but they have the following drawbacks: – their inductance is sensitive to spurious magnetic fields; – they present a self-resonant frequency. Provisions have to be made to ensure that the normal operation of the filter will not be affected by it. 394 12 Methods of Increasing Immunity to Interfering Signals

Fig. 12.4. Decoupling the DC power supply input of every protected circuit with: a RC-ﬁlters; b LC-ﬁlters

All in all, RC ﬁlters are preferred, mainly because they are less suscepti- ble to electromagnetic interference. Contrary to common wisdom, the value adopted for capacitor C should not be taken much higher than the calculated value. As a general rule, adopt the smallest decoupling capacitor that will do the job (since the parasitic series inductance of any capacitor increases substantially with its value, and this will impair ﬁlter performance).

Amplifier Decoupling. To illustrate the necessity of decoupling, consider the single stage amplifier of Fig. 12.5a, where the parasitic inductance of the +VCC trace (between nodes 4 and 5) is denoted by Lp. It is common practice to draw the AC equivalent circuit of the amplifier by considering node 4 to be at ground potential (since any ideal voltage source has zero internal AC resistance, and the trace inductance is neglected).

Fig. 12.5. a Common-emitter stage; b AC equivalent circuit 12.2 Filtering 395

However, if the trace parasitic inductance (Lp) is taken into account, the equivalent circuit becomes like that shown in Fig. 12.5b. In this context, Lp is responsible for undesired coupling between the output loop (0–4–2–0) and the input loop (0–4–3–0). This kind of coupling is called “common-impedance coupling,” because current from at least two circuits ﬂows through the same impedance. This common impedance causes feedback, which, as a general rule, is not anticipated in the design. A practical solution to this problem is to insert a capacitor between node 4 and ground, to bypass the trace parasitic inductance. This is called a decoupling capacitor.

Decoupling Capacitors. The type of decoupling capacitor should be carefully selected according to the frequency range of the particular application. For instance, in analog circuits mica and ceramic capacitors with small values are practical in the frequency range of up to 200 MHz, while metallized paper capacitors can be used only at low frequencies. If broadband operation is required, often two capacitors are connected in parallel (for instance, a higher-value wound aluminum foil capacitor for low-frequency bypass and a ceramic capacitor of lower value, for high-frequency bypass). In digital applications, remember that the decoupling capacitor must be eﬀective at frequencies up to 100 times the primary clock frequency of the logic system. Hence, the parasitic inductance of the capacitor itself, as well as the inductance of its connecting leads, are of prime concern. With the mod- ern trend toward continuously increasing clock frequency (which is already in the GHz range for commercial personal computers), traditional discrete capacitors are obviously no longer suitable for decoupling. Several solutions are proposed instead: • Buried capacitors incorporated in a multilayer printed circuit board have been designed using standard PCB processes [201] for portable and hand- held communication products. They consist of two parallel conductor plates (typically situated in the internal power and ground planes) with dielectric material between them and connected by plated via holes. Three diﬀerent sizes of buried capacitors were reported as being embedded in 530 mm×575 mm panels, corresponding to 9 nF, 21 nF, and 95 nF (the latter when the entire panel area is used as a single capacitor). The small number of required decoupling capacitors for the entire board allows one to clear space on the board and reduce the overall cost of the PCB. Fur- thermore, a reduction by 80% of the parasitic inductance associated with buried capacitors has been reported, relative to their discrete counterparts • On-chip decoupling capacitors, employed in the design of high-performance CMOS microprocessors. The “white space” available on the chip is used to create MOS decoupling capacitors in the power/ground planes. In this way, decoupling capacitors are somehow placed blindly, with no guaran- 396 12 Methods of Increasing Immunity to Interfering Signals

tee that they are located in the right places, where the inductive noise of the power supply is maximum and must be suppressed. Hence, op- timization of decoupling capacitors deployment is required, and can be treated either as a post-ﬂoorplan step or as an integral part of the ﬂoor- planning [202]. This ensures that the decoupling capacitors are eventually optimally placed, very close to the clusters of high-switching activity.

Multistage Amplifiers. In this case, the parasitic impedance of traces distributing the same supply (+VCC) to various stages can cause problems. The multiple common-mode impedance couplings provide paths to return a fraction of the output signal of a certain stage to the input of a previous stage; if the feedback is positive, instability can ensue. To decrease the risk of oscillations, we must decouple the overall amplifier, as well as every stage (Fig. 12.6). Note that filters A, B, C should be placed as close as possible to the supply leads (pins) of stages 1, 2, 3, respectively. Note also that good-quality RF grounds should be used for ground connection of all stages. In practice, whenever the gain of the first stage is much greater than the gain of the following stages, enough protection is obtained by decoupling only the first stage.

Fig. 12.6. Multistage ampliﬁer feedback decoupling

12.2.2 Filtering of Wires and Cables

Filtering of Wires. It is important to filter each conductor entering a shielded enclosure, because it can collect spurious signals from the outside and transport them inside. Coaxial feed-through capacitors (CT) should be used in input filters (Fig. 12.7), and an additional internal shield (IS) is sometimes necessary to confine the radiation of this filter. 12.2 Filtering 397

Fig. 12.7. Filtering of conductors entering a shielded enclosure

Fig. 12.8. Filtering a DC supply line entering a shield

The ﬁltering of a balanced line (or a DC supply line) entering the shielded enclosure is illustrated in Fig. 12.8, where CT denotes feedthrough capacitors. As for general advice when ﬁlters are not purchased, but “home-made”: it is essential to keep component leads as short as possible, especially capacitor leads and ground connections.

Shielded Cables. Generally, shielded cables are decoupled by properly grounding the cable shield. Two categories of shielded cables exist: – Coaxial cables; these are unbalanced lines where the outer conductor is employed as a shield, but also as a return path for the inner conductor. It is advisable to ground the shield at the generator end for low-frequency applications. Multipoint grounding of the shield is preferred for high-frequency applications. – Shielded twisted pairs; in this case the screen protects both conductors and is not used as a return path. These cables are employed to connect two distant systems (or circuits), as illustrated in Fig. 12.9. 398 12 Methods of Increasing Immunity to Interfering Signals

Fig. 12.9. Connecting two systems with a shielded twisted pair

Table 12.1. Grounding a shielded twisted pair

Objective l/λ ≤ 0.1 l/λ > 0.1 Reduce emission Ground at T Grounds at T and R Increase immunity Ground at R Grounds at T and R

Depending on the electrical length of the cable and which equipment has priority in protection, optimal grounding of the cable shield is indicated in Table 12.1 [196].

12.3 Grounding

Explanation. When properly used, grounding is a powerful technique to increase the electromagnetic immunity of a system. During design, one of the traditional goals is to minimize the size of ground loops in order to reduce the electromagnetic susceptibility to radiated interfering signals. Whenever large ground loops still exist, they are merely the result of particular constraints imposing to interconnect several ground points, which are not in close proximity. Problems can appear when the interconnected grounds have slightly different potentials, since a current will flow in the ground system as a result of the voltage offset between ground points. If the ground also provides the return path for the useful signal, this current contaminates the useful signal. In practice this happens either when interfacing two different systems (Fig. 12.10), or when the system drives a distant load. Note that any mon- itoring device inserted between two systems can be considered an interface; the useful signal is transmitted from device 1 to device 2 through conductor 1–2 and the return is provided by 2–1, which connects together the two grounds. In many practical situations, although the two grounds are not expected to have the same potential, no current is allowed to flow through the interface. 12.3 Grounding 399

Fig. 12.10. Connecting two devices through an interface

This problem is particularly critical in medical equipment, where it may happen that an electrode applied to the human body must be connected to a particular ground whose potential might diﬀer from the earth potential by as much as several hundreds volts!

Breaking the Ground Loop. To avoid these potentially harmful situations, the ground loop can be broken by inserting a transformer, an opto-

Fig. 12.11. Breaking the ground loop by inserting: a a transformer; b an optocoupler; c an isolation ampliﬁer. Note that in all cases circuit A is galvanically isolated from circuit B 400 12 Methods of Increasing Immunity to Interfering Signals coupler, or an isolation ampliﬁer (Fig. 12.11). The choice depends upon the frequency of the transmitted signals, but other considerations will be detailed.

Inserting an Isolation Transformer. For high-frequency analog or pulse signals, a transformer is mostly recommended (Fig. 12.11a). They have ferrite-type cores with high permeability, requiring a few turns of copper windings (hence, low mutual capacitive coupling between coils). When intended for low-frequency applications, this option is not economical, since it requires bulky transformers. In this case, beyond additional weight and size, the external shielding of the transformer (to avoid pickup of spurious magnetic ﬁelds), as well as the internal shielding between the coils, dramatically increases cost, and contribute to degradation of performance. Figure 12.12 shows two examples of connecting isolation transformers: for an unbalanced line (Fig. 12.12a) and for long-distance communication of digital data over wire lines (Fig. 12.12b), where a transformer is needed at each end [197].

Fig. 12.12. Examples of connecting isolation transformers

Inserting an Optocoupler. The main drawback of isolation transformers is the residual parasitic capacitance that still exists between the circuits to be isolated. When this may be harmful, an optocoupler must be adopted. An optical coupler (Fig. 12.11b) combines a light-emitting device with a light-sensitive device in one package. A light- emitting diode (LED) trans- forms the electrical signal applied to its input into light, which is sensed by a detector (photodiode) and ampliﬁed by a transistor or a Darlington pair (Fig. 12.13). This double conversion of energy is the key to near-perfect isolation between circuits, and as a result electromagnetic susceptibility is greatly improved. Typical performance parameters for an optocoupler are: maximum isolation voltage 2.5 kV, isolation resistance 1 TΩ and residual coupling capacitance of order 1 pF. Globally, it represents an especially good choice for digital applications (linking computers and control devices); for analog applications, its linearity is not satisfactory.

Inserting an Isolation Amplifier. For all analog applications where linearity is essential, isolation amplifiers represent a good solution, provided that the transmitted signal does not saturate the amplifier. Usually, isolation 12.3 Grounding 401

Fig. 12.13. Optocoupler amplifiers are purchased. A block diagram of a typical isolation amplifier is presented in Fig. 12.14, where the potentials of the grounds on each side of the separation line can be completely different. Based on the transmission modality of the signal inside the amplifier (across the separation line), these devices can be grouped into three categories: 1) Amplifiers with internal coupling transformer, which suppress any DC component in the transmitted signal (this is the case of isolation amplifiers fabricated by Analog Devices). A high-frequency carrier is frequency-modulated (or pulse- width-modulated) by the transmitted signal, whose bandwidth does not exceed 10 kHz. Isolation up to 3.5 kV between input and output is achieved; note that

Fig. 12.14. Breaking the ground loop with an isolation ampliﬁer 402 12 Methods of Increasing Immunity to Interfering Signals

only one DC supply is needed, since the second one is a converter whose coil shares the same core as the first. 2) Amplifiers with internal input/output optical coupling (such as the IS0100 Burr–Brown isolation amplifier). According to the data sheets, the maximum isolation voltage is about 750 V for IS0100; to improve linearity, a second photodiode is excited by the light emitted by the input LED, and the resulting signal is employed to cancel the nonlinearity of the main conversion process. 3) Amplifiers with internal capacitive coupling, where the transmitted signal is frequency-modulated (like the Burr–Brown IS0106 amplifier, for which a maximum isolation voltage of 3.5 kV and a bandwidth of 70 kHz are specified).

Fig. 12.15. a Partial isolation technique; b common-mode choke

Remark. Whenever galvanic isolation between input and output circuits is not required (or is impossible to achieve), a cheaper technique consists in inserting high series impedances in both conductors. In high-frequency circuits this is usually implemented by winding the cable (or the pair of conductors) several times through a toroidal ferrite core (Fig. 12.15a). As a result, two common-mode coupled inductors (Fig. 12.15b) appear in series with the transmission line, providing some isolation between its terminal ports.

12.4 Practical Advice on Reducing Noise and Interference at the Circuit Level

Comment. In this section we discuss several design guidelines used to minimize interference and noise. It should be emphasized that focusing the effort solely on interference reduction does not guarantee a noiseless system, since intrinsic noise still contaminates the signals. Therefore, fighting interference must be correlated with fighting intrinsic noise (by adopting minimum noise design techniques), and these two activities cannot be dissociated in 12.4 Practical Advice on Reducing Noise and Interference 403 practice. Bear in mind that all users desire high-quality, noise-free equipment, not explanations in case of failure that one of the two aspects of noise was underestimated during design! Considering the important difference between the external and intrinsic noise amplitudes, it is obvious that intrinsic noise becomes a problem only when interfering signals are no longer a problem! For this reason, in the following, a collection of principles and techniques aimed at the global objective of noise reduction is presented. Although they represent good practice, these rules should be carefully applied, depending on the particular conditions inherent in every design. Very often, more than a single technique is required to satisfy requirements, hence many of them must be simultaneously implemented.

12.4.1 Interference Control

Overview. The reduction of electromagnetic interference is best achieved when trying to solve this problem during the design process and not latter. The basic rules involve circuit partitioning, selection of components (with criteria for both intrinsic noise and interfering signals in mind), PCB layout, selection of cables, grounding, shielding, and filtering (if the system is fabricated as an integrated circuit, substitute “layout” for “PCB layout” and “routing the interconnects” for “selection of cables”). Any interference problem involves an aggressor circuit and a victim. Often the aggressor is a well-designed circuit that satisfies all the electrical requirements, but the electromagnetic compatibility problem has been neglected. Furthermore, the situation is aggravated because testing is conducted on each module separately (or on the module in question connected to the modules with which it must normally operate), but very seldom on the system as a whole. We thus dispose of very limited means of detecting the aggressiveness of a particular circuit, early in the process. The victim is also a well-designed electronic circuit, dedicated to low-level signal processing. Hence, it must be sensitive to weak signals, which is of course its main vulnerability: susceptibility to interfering signals. Therefore, protection is needed to avoid (or, at least, to limit) penetration of interfering signals. Taking into account that the same circuit (or interconnection) can act as aggressor during some time intervals and victim during others, it follows that an exact classification is impossible (and perhaps pointless in any event). From another stand point, in most practical situations the circuit designer has no control over equipment design, since this task is usually performed by a different team. Ideally, the electromagnetic compatibility problem should be solved at the circuit design level, where the solutions adopted to control interference are easier to implement, less expensive, and above all, more efficient. In the following section, a non-exhaustive list of rules is proposed, to be considered during the circuit design and/or prototyping step. 404 12 Methods of Increasing Immunity to Interfering Signals

12.4.2 Guidelines for Circuit Design

Although this section appears at the end of Chap. 12, it should also be considered as a ﬁnal conclusion to part II of this book, dedicated to interfering signals. Since controlling conducted and radiated interference is only one side of ﬁghting noise, minimum-noise design principles (referring to intrinsic noise) should be simultaneously considered. Consequently, rules and principles aimed at these both major objectives [194,197–200] are proposed in the following sequence:

Partition the circuit to be designed into critical and non-critical sections: – identify sections prone to generate interfering signals (aggressors). They must be isolated from the rest of the circuit both electrically and physically. Electrical isolation is achieved by proper design, but physical isolation is achieved by layout and shielding; – identify noise-sensitive sections (victims); – locate victims away from aggressors (for instance, on diﬀerent layers of a multilayer PCB); – choose proper I/O points.

Select the Circuit Components of Digital Sections with the goal of reducing emission of interfering signals: – use slow logic families in regions where there is no impact on operation; – select high-immunity logic families in noise-sensitive sections; – group logic families according to functionality and apply segregation by speed.

Select a Circuit Topology that minimizes both interference and intrinsic noise by paying attention to the following points: – restrict bandwidth to the minimum required to transmit only useful signals; – minimize the signal level of aggressor circuits (while remaining consistent with the required S/N ratio); – maximize the signal level of noise-sensitive circuits; – adopt impedance values consistent with both performance and minimum crosstalk requirements; – in RF, microwave, or digital applications, consider matching line terminations. For analog circuits: – select the input-stage conﬁguration, active devices, and bias points for minimum noise; – use resistors with low noise index in all sensitive areas. 12.4 Practical Advice on Reducing Noise and Interference 405

For logic circuits: – reduce fan-out on clock circuits by using buﬀers; – insert a watchdog circuit on every microprocessor. For both categories: – select power supplies of good quality (noiseless).

PCB Layout. In general, the differential-mode emission can be controlled by circuit layout. The key to achieving reduction of differential-mode radiation is to minimize loop areas. Therefore: – minimize distance between circuit components; – minimize trace and component lead lengths; – ensure proper signal returns by using one or more ground planes; – minimize loop areas subject to large di/dt (for instance, all clock paths must have adjacent ground returns); – minimize node areas subject to large dv/dt; – provide a separate power plane. Another objective of layout is to avoid crosstalk; this can be achieved by observing the following rules: – increase separation between traces; – avoid parallel runs of traces or minimize parallel trace lengths; – when parallel traces are unavoidable, increase separation between traces, or better, insert a ground trace between them; – in multilayer PCB, route adjacent layers orthogonally; – minimize ground inductance by using a ground plane or ground grid; – provide matched terminations on traces carrying fast-switching signals or microwave signals; – reduce signal drive level; – place clock interconnects away from I/O regions; – locate all I/O circuitry in one area of the PCB and provide a separate ground to this area; – place I/O drivers next to the connector. Miscellaneous: – minimize loops associated with noise-sensitive circuits; – avoid sharp angles or 90◦ angles in traces; – don’t leave unused inputs of logic gates floating: connect them either to ground or to the power supply; – where signal and power traces must cross, make the crossing so that the traces are orthogonal; – provide a separate ground for sensitive circuits, and place them away from ground plane edges; 406 12 Methods of Increasing Immunity to Interfering Signals

– place decoupling capacitors next to each IC; – don’t leave any metallic area on the PCB floating. Of the various techniques employed to control interference, note that in practice they are used in the following order: first proper grounding is achieved, then shielding is eventually taken into account, and finally filtering is added. The same order is adopted in the following:

Grounding. The ground system of any electronic equipment should be created during the design step, then updated and checked during layout, prototyping, and fabrication. The main objectives are to ensure normal operation of the system, guarantee safety conditions, and provide protection against hazards (like ESD, lightning, etc.). Here is a non-exhaustive list of suggestions: – for circuit dimensions less than 0.03 λ, use single-point grounding; – for circuit dimensions larger than 0.15 λ, use multipoint grounding; – for circuit dimensions between 0.03 λ and 0.15 λ, provide hybrid grounding (especially for broadband circuits); – use floating ground only in low-frequency applications; – create separate ground systems for signal returns, signal shield returns, power supply returns, and hardware (chassis, racks, and cabinets) ground. These returns can eventually be tied together at a single ground point. – isolate the ground of logic circuits from the ground of analog circuits; – provide a “clean” ground for decoupling all interfaces; – provide quality bonding of screens, connectors, cabinets, and filters, and ensure that it is not damaged by operation under adverse conditions; – minimize the length of ground interconnects and give them adequate ge- ometries; – avoid common-ground impedances; – use a ground grid or plane on logic boards to minimize common-mode ground interference; – break ground loops if problems appear; – use balanced differential configurations to minimize common-mode ground interference.

Shielding. This technique is used to control near- and far-ﬁeld coupling. Here are several basic rules: – Since shielding is an expensive technique, determine whether a shield is necessary or not. If it is, select the type of shielding required for the frequency range of the particular application. – Use copper or aluminum for electric ﬁeld shields. A material thick enough to support itself usually provides good protection. – Use iron or high-permeability alloys for magnetic shields. 12.4 Practical Advice on Reducing Noise and Interference 407

– Enclose the most sensitive circuits or noisy systems in additional internal shielding. – For plane-wave shielding, use electromagnetic gaskets to seal apertures in the metal construction. – Seams should be welded or overlapped. – Avoid resonant metallic enclosures (dimensions of λ/2 are prohibited). – Avoid large apertures in the shield. Whenever possible, replace a large opening with several small apertures.

Filtering. Since filters control the spectral content of signal paths, they represent an efficient technique for eliminating conducted interference. The following points should be considered: – filtering is most effective when applied to the interference source (switches, motors, etc.). All transient interfering signals should be treated at the source. – select the proper mains filter for each particular power supply; – filter all I/O lines with common-mode chokes or capacitors; – decouple the DC supply of each board (module) with a separate pi-section filter; – adopt the smallest-value decoupling capacitor that will do the job; – ensure a good ground to each filter; – minimize lead lengths and associated wiring of filters.

Cables. The cabling system requires particular attention since the key to minimizing common-mode radiation is to reduce the common-mode current on all cables of a given set of equipment. Thus, all cables entering or leaving the equipment require treatment to control common-mode emission, and the following rules should be considered: – select the proper type of cable for every application; – minimize cable lengths; – for differential-mode input configurations, select equal cable lengths and run them adjacently; – provide enough separation between cables carrying weak signals and cables transmitting high-level signals. If they must cross, make the crossing orthogonal; – avoid running signal and power cables in parallel; – avoid resonant cable lengths; – run cables close to metallic grounded walls; – pass cables away from windows or apertures in shielding; – use signal cables and connectors with adequate shielding; – use chassis ground or the I/O ground for cable bypassing (never bypass cables to signal ground); – for balanced configurations, adopt twisted pairs; 408 12 Methods of Increasing Immunity to Interfering Signals

– for RF signals, match all cable terminations; – avoid pigtails when connecting cable shields to ground; – provide cable damping with ferrite suppressors; – provide ground termination of cable shields via a 360◦ solid contact with the shield enclosure.

Connectors and Mechanical Switches. Metal-to-metal junctions in coaxial connectors, waveguide joints, and mechanical switches can cause poor electrical contact due to corrosion, oxidation, dust, and impurities on surfaces. They are mainly responsible for contact noise and intermodulation interference (rusty bolt eﬀect). To prevent them: – avoid deposit of dust on the surfaces involved in mechanical contact; – clean them periodically with special chemical products; – don’t hesitate to replace them when wear makes operation unsafe.

12.5 Increasing System Immunity to Interference: Bluetooth Approach

Until now we have been concerned with techniques to reduce interference at the circuit level. A quite diﬀerent approach is employed to reduce interference at the system level in Bluetooth wireless networks.

Deﬁnition. Bluetooth is a short-range, low-power radio link (10–50 m) between several devices operating in the unlicensed 2.4 GHz industrial, sci- entiﬁc, and medical (ISM) frequency band [203].

Explanation. This wireless communication network is intended for home and oﬃce applications [204] such as cordless headphones; cordless keyboards; telemetry of physiological signals to health support systems; sharing voice, data, and video among computers, digital cameras and camcorders, TV sets, printers; and so on. Since the 2.4 GHz band is considerably crowded by other consumer appli- ances (such as microwave owens, baby monitors, etc.), interference is expected to be large enough to adversely aﬀect the security of transmission.

Approach. Data transmission is performed using GFSK (Gaussian frequency-shift keying) modulation where a positive frequency deviation corresponds to 1 and a negative frequency deviation means 0. To increase immunity to interference, Bluetooth uses a pseudo-random hopping sequence over a large number of 1-MHz frequency channels. To support this frequency hopping, the ISM band is splitted into 79 1-MHz channels in the United States and most european countries, and 23 1-MHz channels in France, Japan and Spain (to accomodate smaller ISM frequency bands). 12.5 Increasing System Immunity to Interference: Bluetooth Approach 409

In a point-to-point link (or point-to-multipoing link) the master unit selects the hopping sequence and imposes it to the slave unit. Each channel involved in the hopping sequence is active only during a time slot of 625 µs, then the next on the list channel is selected for another time slot of 625 µs, and so on.

Conclusion. Even if data can be corrupted by interference when transmitted over a particular channel, global damage is limited due to the continuous frequency hopping and the timing involved in the hopping. This concept, combined with the general Bluetooth architecture, packet structure, data en- cryption, and access coding techniques largely improves the security of data transfer between two devices. In this way the transmission quality is main- tained despite strong interference potential.

Summary

• Differential-mode emission can be controlled by circuit layout. The key to reducing differential-mode radiation is minimizing loop areas. • Common-mode interference is controlled by balancing the topology of analog circuits. The key to minimizing common-mode radiation is reducing the common-mode current on all equipment cables. • In logic circuits, use a ground grid or plane to minimize common-mode ground interference. • Balancing represents an efficient way to pick up equal interfering signals on both conductors of the line. Then, with terminal differential configurations, common-mode interference is eliminated. • Filters control the spectral content of signal paths and provide an efficient way to eliminate conducted interference. • All transient interfering signals should be filtered at the source. • Apply decoupling filters at the DC power supply input on every board, or even on every circuit of a board, to avoid coupling through the source and/or interconnect impedance. • Adopt the smallest-value decoupling capacitor that will do the job. • The main objectives of grounding are to ensure normal operation of the system, guarantee safety conditions, and provide protection against hazards. • Provide a “clean” ground for decoupling all interfaces. • Bypass all interconnects (cables, wires, etc.) not to the signal ground, but to chassis ground or the I/O ground. • Whenever it is required to interconnect several ground points which are not necessarily at the same electrical potential, break the ground loop for DC but ensure unimpeded transmission of AC signals. • Galvanic isolation is achieved by inserting transformers, optocouplers, and isolation amplifiers, or by using capacitive coupling. Part III

Case Studies