Improving Signal to NoiseRatios in Defence

Electronics

Chris ReynoldsAVX Application Manager

As a vertical market, “Defense Electronics” encompasses a huge range ofsystems, from portable communications to transportation, avionics and ship borneradar. While the power architectures are as diverse as they could be, all havecommon operational demands – they must be robust (shock, vibration, temperatureextremes), highly reliable, able to power-up after periods of dormancy andbased on components with minimal obsolescence.

AVX can improve power distribution quality, over a wide spectrum of signal typesfrom digital to RF power, through implementation of various low inductance, lowESR power filter capacitors. These innovative input and output capacitors employnovel technology designed to positively impact power supply noise in battery, fuelcell, and avionic systems. In addition to capacitor solutions, there are otherpassive designs for circuit protection that are necessary to maintain signalintegrity while containing ESD events and controlling EMI emission andsusceptibility.

First, we will take a look at digital signal processing. Traditionally, avionic andsatellite power applications are associated with the 28v bus, (or 14v for vehicular),which in turn are converted for low voltage power distribution where required. It isthis area that is seeing rapid growth due to the increase in digital content of bothcontrol systems and payload (including programmable arrays and analog digital(ADC or DAC) conversion for sensors). New designs continue to adopt ASICSwith higher processing speeds, requiring Multilayer Ceramic Chip Capacitors(MLCC) used for decoupling to have lower parasitic elements i.e. Low EquivalentSeries Resistance (ESR) and Low Equivalent Series Inductance (ESL). Thecloser to the core ASIC or programmable Array, the more critical becomes thecontrol of the ESL. As capacitors are 2-terminal devices, the base ESLcharacteristic arises from the geometry of the part – the two terminals effectivelydefine a current loop for the signal, and the larger the part, the larger the loopand hence the ESL. A basic way to counter this is to use a “reverse geometry”Low Inductance chip Capacitor (LICC) which has the terminations on the sides,rather than the ends of the parts. In a 2:1 aspect ratio part such as a 1206 size,using a reverse geometry version, 0612, will reduce inductance by a factor of 2(typically from 1nH to 500pH) for the same capacitance / voltage design and thesame amount of real estate. Lower inductances still can be achieved by usingsmaller outline part with smaller loops (0508 instead of 0805, 0306 instead of0603 etc), but this comes at the cost of decreasing capacitance value – andcapacitance retention at the ASIC operating frequency is still a requirement. So,for faster speeds, new component designs are needed where the inductivecomponent can be separated from the capacitative. There are three ways to dothis – by inductance cancellation, by having very small signal loops and byminimizing inductive coupling to the pcb ground plane. A good example ofinductance cancellation is the Inter-Digital Capacitor (IDC). This is a reverse

geometry design, but instead of having solid terminations, the part is terminatedalong the side with stripes, or fingers, each alternate pair of stripes beingconnected to alternating pairs of plates in the multilayer stack. This means thatinductive cancellation can take place, and by having a Via In Pad (VIP) designthe loop to the ASIC i/o can be minimized. This type of design can result in<200pH ESL for an 0612 4.7μF / 6v device.

Figure 1

In order to minimize current loop, a new series of Land Grid Array (LGA)components has been developed having extremely tightly controlled terminationdimensions. Using a vertical internal electrode system means that the averagecurrent loop between ports is extremely small and has the added advantage ofminimizing ground [plane coupling. These parts can achieve <30pH for a 0.1μF / 4vfor a small 0306 4.7μF / 6v device.

Figure 2

Image 3, LGA Series

For many avionics or government programs, reliability levelsbeyond commercial grade are required. While many of thesetechnologies do not yet have MIL spec equivalents,COTSplus versions that have voltage aging and conformancetesting to MIL standards are available. Another requirementis that all component terminations must contain 3% Pbminimum. While the commercial versions of all of the abovetechnologies are RoHS compliant (with 100% matte Sntermination), the COTSplus versions standardize on SnPbterminations.

As we come out from the core or ASIC, the DC output that holds up the systemrequires bulk capacitance with low ESR to minimize noise and ripple, with theideal SMT passive technology for maximum volumetric efficiency being tantalumchip.

Tantalum technology enables high CV (capacitance x voltage) to be achievedwith high volumetric efficiency (e.g. CV ratings from 330μF / 6v to 4.7μF / 50v ina 77mm3 package). Apart from filtering and decoupling applications, their highCV range also suits them to pulse applications.

Image 4, MH Series

For low ESR applications a new series recentlyincorporated into MIL-PRF-55365 is the CWR29 family.These include a very wide range of CV options up to 50vrated, with Weibull reliability grading and optional surgecurrent test. The typical ESR performance of these partsis well within specification; for example the 330μF / 6vrating noted above will have a maximum limit of 180mOhms, but typical performance well under 100mOhms.Beyond the MIL ratings, tantalum chip are also available anew DSCC drawing (DSCC 07016). This drawing is a newstandard defining high reliability versions of the latest LowESR “EIA” size tantalum chip.

All the above tantalum chips are characterized by a fully molded body withcompliant (wrap around) termination, decoupling them mechanically from the pcb,so that they are ideally suited to withstand high shock & vibration over a widerange of substrate materials with differing coefficients of thermal expansion(CTE). To match their mechanical performance with electrical robustness, for“mission critical” applications, all CWR series capacitors require full lotassessment which includes voltage conditioning (defined as Weibull reliabilitygrading – which is also a requirement for DSCC 07016 ratings). As tantalumdielectric cannot be subjected to a high level of overvoltage to accelerate lifeconditions, the Wiebull system uses a combination of voltage and temperatureacceleration factors while the lot is statistically monitored throughout the 42.5hr

min. test. This grading allows parts to be released to failure rates below (0.1 to0.001 )% / 1000hrs at 90% confidence limit.

Having optimized the characteristics for discrete capacitors, for medium powerapplications it's often necessary to use a bank of capacitors in parallel, or forhigher voltage bus lines (e.g. 28v) in applications where the derating rules areconservative, a parallel / series configuration of capacitors to achieve the neededtotal application capacitance, ESR target and required voltage derating.

A new series of parts that allows this is the TCP Series stacked tantalum module.These can be configured in standard two, four, or six unit stacks with othercustom configurations available. The advantage of using such stacks is that theycan be assembled with parts having inset ESR limits and, by matching the ESRin a stack, ensure that current sharing is better equalized. From the designperspective, this advantage gives a large reduction in real estate with bettersystem volumetric efficiency on the PCB.

Image 5, TCP Series

For additional safety, there is also a series available(TBW Series, available to DSCC drawing 04053)that includes a Weibull-grade reliability capacitormatched to an internal fast-acting (thin film) fuse.This gives the best of both options – establishedreliability performance and failsafe operation, withthe minimum of added ESR.. It is designed asboth a discrete capacitor and in module format

(and all with standard dimensions). In the module version, each element isindependently fused so in the event of one element failure, there is stillredundancy in the system.

For broadband filtering on power lines, MLCCs with resonances in the 1MHz to100MHz range are often used in parallel with the tantalum chips. The CDR series,available to MIL-PRF-55681, are ideal in this respect, and can also beaugmented by DSCC drawing for smaller size, COTSplus 0805 and 0603packages.

Circuit Protection

While current capacitor technologies are able to maximize the performance ofhigh-speed digital applications, the devices themselves are more at risk due toincreased EMI and ESD susceptibility, so on-board circuit protection is in turn anarea of major focus.

While the newer generations of low voltage / high speed ASICS are increasinglysusceptible, they are also being used in more ESD / EMI hostile environments.From an ESD point of view, the increasing number of secure handheld

communication devices, with multiple USB I/Os and increasing data streamsensitivity, will require protection against multiple ESD events throughout theiroperational lifetime. As EMI detection devices become more sensitive, they mustalso have emissions minimized for “stealth” operations. At the same time, theyrequire protection from any local EMI generator they may encounter in the field.

For ESD protection, the Transguard Series is a multilayer varistor (MLV) that hasbecome the mainstay of the electronics industry, due to its small size, standard“chip” outline (0402 – 1210 size) and robust characteristics (-65oC to +150oCoperation). The benefits of this technology are that, due to its unique materialcharacteristics, it will clamp ESD spikes faster than a silicon Transient VoltageSuppressor type (TVS) as well as survive thousands of events with zerodegradation These events are based on 8kV contact, or 14kV air human bodymodel discharges.

Image 6, MLV

Because the parts are supplied in standard SMTchip format, it means that arrays can also beprovided – typical solutions can have 2 to 4channels in 0508 or 0612 size for protecting multipledata lines with a single placement. Another benefitof the construction is that capacitance can becontrolled; in some application, the capacitancecan be maximized (10pF – 100pF range) so the

parts act as ESD protection with a small built in EMI capacitor. In otherapplications (data port or antenna protection), capacitance can be minimized.Typically, USB ports any antenna will benefit from low capacitance (3pF – 10pFrange), but new applications for HDMI data protection require sub pF capacitance– even lower than the typical parasitic capacitance of a silicon TVS device. Thelatest series of Sub pF (SPV) varistors are now available for harsh-environmentapplications.

For EMI control, AVX manufactures feedthru capacitors, also in standard SMTchip sizes, that maximize the intrinsic inductance from the geometry of theinternal feedthru channel and capacitor plates (standard class I or class IImaterials) to provide an LC filter with precise inductance value and essentially noinsertion loss in the circuit. Again, these are readily available as feedthru arraysto provide multi-line filtering. On application that benefits is filtering RGB signallines for LCD screen or CCD camera applications (especially if connected withflex cables – which can have high EMI cross-sections). With S21 of -20dBover the 500MHz to 2GHz range, the 0612 4-channel feedthru array is an idealsolution for on board filtering at the individual device level.

Image 7, Mini EMI Filter

For combined ESR / EMI there is anotheralternative – a standard chip with feed-thrugeometry, but manufactured fromTransguard (MLV) materials. This is theTransfeed series, has the fastest turn-ontime of any ESD protection device subpicosecond) and can achieve rejection to -60dB. This is an idea device for filtering &

protecting LNA circuits, where signal differentiation in the V range needs to bemaintained.

Capacitors for Radar applications

While the above has been aimed at the growing low voltage sector for defenseelectronics, the same principals — low ESR, low ESL and transient protection —also govern high power applications.

Radar is one example of an application that requires pulse power - energyderived instantaneously to a system so that minimal voltage droop occurs due tothe power demand.

For avionic applications operating from a 28v bus, high-voltage solutions andneeded such as tantalum chip banks, Wet Tantalum, or stacked ceramicswitchmode capacitors. When looking for the maximum capacitance/ lowest ESRoptions for radar, the switchmode ceramic solution will have the lowest ESR, butalso a lower capacitance value.

To improve filtering, low Inductance versions are available – the new TurboCapSeries combines stacked ceramic with vertical electrode for higher currentcapability. The wet tantalum (TWA series, available to DSCC 93026) solution willhave high capacitance; but the ESR of these devices is characterized in ohms inthe 10 kHz range. Tantalum chip has the optimum combination of capacitanceand ESR in the milliohm range at 100 KHz, but operation on a 28 V bus requiresa 50 V rated solution (or a new 63v rating) as a minimum and so a robust andreliable version is required.

Image 8, Wet Tantalum Image 9, TurboCap

For a high current pulse system, a combination of either TCP tantalum chipmodules, or TWA wet tantalum for the bulk capacitance, and TurboCap for thepulse delivery is ideal.

At this scale, the EMI filtering to needs to have high current handling capability,so discoidal capacitors assembled in hermetic feedthru configurations are used.These are available in discrete bulkhead or brackett assemblies, or multi-channelarrays.

Image 10, EMI Buckhead Series – Customized Array Filters and Filter Connectors

For these devices in avionics applications, transients remain anissue, but on a different scale – while digital circuits areconcerned with ESD, avionic control applications have to factorin lightning strikes. For commercial aviation, the trend is tocomposite material airframes, each region having a differentsusceptibility to lightning. Switchmode ceramic capacitors(SM0 to Turbocap) can now be tested to D0-160standards, depending on the strike level certification requiredby the customer application.