Design Requirements for Integrated Mw Avionics Receivers - Maloratsky

36 High Frequency Electronics

High Frequency Design

AVIONICS RECEIVERS

Design Requirements for Integrated MicrowaveAvionics Receivers

By Leo G. MaloratskyAerospace Electronics, Co.

The acronym avion-ics is a combina-tion of AVIation

and electrONICS. Thegeneral direction of amodern electronics isintegration or size reduc-tion. With recent reduc-tions in the size of physi-

cal circuits, we may also require an abbrevia-tion of electronics acronyms. Therefore, themainstream of aviation systems withmicrowave integrated circuits can be namedby the acronym avmic, formed from letters ofwords AViation Microwave IntegratedCircuits. The author used the more commonacronym avionics in the title of the article tobe better understood by the readers. Thechanging of terminology (if necessary) is adecision for the experts in the future.

Avionics has come to include communica-tions, navigation, flight control, collisionavoidance, military applications, landing aids,guidance and lighting, as well as electronicssystems involved with the display and man-agement of multiple systems [1-6]. Avionicsrefers to electronic systems on aircraft, artifi-cial satellites and spacecraft that providecommunications, navigation and guidance,display systems, flight management systems,sensors and indicators, weather radars, elec-trical systems, and various computersonboard modern aircraft and spacecraft. Itincludes hundreds of systems fitted to air-craft to meet individual roles.

The avionics industry is now a majormultibillion dollar industry and accounts foraround 30% of the total cost of the aircraft.Integration of avionics systems is a significant

issue because the reduction in weight can betranslated into longer range on less fueland/or more passengers. Also, other veryimportant issues are cost reduction, increasedsafety, improved aircraft performance, andsize reduction.

Avionics Receiver RequirementsThese are the following specific require-

ments for the avionics RF and microwavereceiver (RX):

• Narrow frequency band (from 0.2% forthe Transponder up to 6% for the TCAS)or moderate frequency band (23% for theDME);

• Large dynamic range (up to 90 dB for theWeather Radar);

• Small or moderate manufacture quantity(from several hundred samples to tens ofthousands samples per year);

• Strong resistance to environmental con-ditions (temperature, vibration, humidi-ty, etc.);

• Very low size and weight;• Low cost

For avionics microwave RXs, high integra-tion, low cost as well as power and mass effi-ciency are particularly important. The devel-opment of new avionics RX is driven by theneeds of all areas of space applications. Thekey drivers can be summarized as follows:

• Higher dynamic range, more resolution,more channels.

• Low power consumption: spacecraft oper-ate in an environment where the avail-ability of electrical power is very limited,

This article discusses vari-ous approaches to solvingdesign problems in RF and

microwave avionicsreceivers, with recommen-dations for design strategy

of the optimal receiver

From August 2011 High Frequency ElectronicsCopyright © 2011, Summit Technical Media, LLC



AVIONICS RECEIVERS

and the cost of power in terms ofspacecraft mass is very high.

• Low mass: miniaturization ofRX is often an enabling factorfor demanding avionics sys-tems, where miniaturizationoften goes hand in hand with areduction of power consump-tion, which allows achieving fur-ther mass savings.

• Low RX cost: an important fac-tor for reducing avionics sys-tems cost; the reduction of massand power consumption allowssignificant overall cost savings.

Microwave typically refers toradio frequencies from 1 to 300 GHz.Microwave avionics systems includeMicrowave Landing System (MLS),Radio Altimeter (RALT), GlobalPositioning System (GPS), TCAS,XPDR, DME, Automatic DependentSurveillance-Broadcast (ADS-B),Universal Access Transceiver (UAT),Traffic Information Services (TIS-B),Flight Information Services—Broadcast (FIS-B), Joint TacticalInformation Distribution System/Multi-functional Information Distri-bution System (JTIDS/MIDS), WXR.

The principal functions withinmicrowave RXs are: filtering, amplifi-cation, frequency down conversion,isolation (ISO) between transmitter(XMTR) and RX (if necessary), pro-tection from parasitic signals, chan-nel switching (if necessary), and self-test. The RXs of avionics systems areconnected to antenna (ANT) throughcables. These networks have a differ-ent architecture, which is represent-ed in Figure 1. All microwave RXscan be implemented in the two fol-lowing architectures: without XMTR(or separated from XMTR) and com-bined RX/XMTR or transceiver(XCVR) with common antenna sys-tem. The XCVR (Fig. 1b) requires thespecial duplexer for separation of aRX from a XMTR. The multichannelRX (Fig. 1c) is used in systems withantenna array, multi-antenna sys-tems, etc. In the modern high sensi-

tivity systems, the active ANT (Fig.1d) can be used.

The main characteristics of themicrowave RX depend on aircraftsystem requirements. The systemspecification should be transforminto microwave receiver require-ments. Table 1 shows the specifica-tions of the microwave avionics RXs.

Receiver ComponentsMicrowave RXs are based on the

use of:

• Planar transmission lines;• Distributed and lumped passive

components;• Passive devices;• Control devices;• Active solid-state devices;• Passive and active antennas.

More than 30 different types ofplanar transmission lines have beendeveloped over the past four decades,often in guest of additional BW, lowerinsertion loss (IL), smaller size, orother performance advantages.There are several reasons for thewide use of planar transmission linesin microwave RX [7-12]. First of allthey are broadband while providingcompact dimensions and lightweight. Foremost, they are generallyeconomical to produce as they arereadily adaptable to hybrid andmonolithic integrated circuit fabrica-tion technologies at RF andmicrowave frequencies. Each planartransmission line offers certainadvantageous features with respectto other types (see Table 2).

All planar transmission lines

Figure 1 · Basic receiver architectures used in avionics RXs.

Table 1 · Microwave receiver performance of different avionics systems.



AVIONICS RECEIVERS

have strip conductor(s) implementedon a relatively thin substrate. Typicalsubstrate materials are slabs ofdielectric, ferrite, or high-resistivitysemiconductor. In most cases, thereare metal ground planes that caneither be printed on the same sub-strate or be a part of the metal hous-ing. Planar transmission lines can beclassified as being uniform or nonuni-form; homogeneous or inhomoge-neous in their surrounding area; loss-less or lossy; shielded or non-shield-ed; planar, multilayer, or three-dimensional. They are also based ondifferent substrate types, includingdielectric, ferrite, or semi-insulatingmaterials. In a uniform line, the char-acteristic impedance does not varywith position along the line. Anonuniform printed transmissionline exhibits characteristic impe-dance that varies as a function of theposition along the line. Usually, thischange in impedance is achieved bychanging the conductor strip width.Tapered transmission lines can befabricated with smooth changes inconductor width and characteristicimpedance as functions of distancealong the line.

Passive RF and microwave inte-grated circuits comprise distributedelements, lumped elements, or combi-nations of both these types of ele-ments. Distributed elements consistof segments of transmission lines ofthe different types we have discussedin the preceding chapters. Thesetransmission-line segments can be ofvarious lengths ranging from smallfractions of a wavelength (λ) to sever-al wavelengths. A planar lumped ele-ment refers to single-layer circuitthat consists of different configura-tions of conductors printed on sub-strates.

Lumped elements are small insize with linear dimensions that areusually less than λ/10 or λ/16 (λ isguide wavelength). At this size, trans-mission line effects do not play a sig-nificant role in their function. Thereare no appreciable variations in

phase over the lumped element, andthe current distributions and associ-ated potential drops are essentiallyuniform. Lumped elements have theadvantages of small size, low cost,wide-band performance, largeimpedance transformations, mini-mum interactions between elements,but have low Q-factor and power han-dling capability than distributed ele-ments. Lumped elements are espe-cially suitable for monolithic integrat-ed circuits, where size requirementsare of most important. Lumped ele-ments are usually easily realized inRF or low-frequency microwave appli-cations. At higher microwave frequen-cies, and particularly at millimeter-wave frequencies, lumped elementsare very difficult, or even impossible,to achieve because of dimensionallimitations in fabrication technolo-gies and parasitic [5]. A lumped ele-ment has parasitics because of thenot insignificant physical dimen-sions. The effects of the parasiticcapacitance and inductance increaseas the operating frequency increas-ing. Lumped elements are not purelyinductive, resistive, or capacitive, buthave aspects of all three due to unde-sirable parasitic. Because of theseparasitic effects, lumped elements

will resonate at some frequency. Theupper limit for the use of lumped ele-ments is approximately 30 GHz.

Lumped elements have been usedto realize many circuit functions,such as matching networks, filters,transformers, dividers/combiners,directional couplers, baluns, phaseshifters, switches, bias chokes, high-power oscillators, LNA, power ampli-fiers, etc. Planar lumped elementsare used in hybrid microwave inte-grated circuits (HMICs) and mono-lithic microwave integrated circuits(MMICs). In applications where it ispossible to utilize lumped elements,the advantages are usually smalldimensions, wideband characteris-tics, and low production cost.Lumped-element circuits exhibit alower quality factor Q than distribut-ed circuits. The lumped elements canbe reactive or lossless, resistive orlossy or combination of both. “AnAchilles’ heel” of lumped-element cir-cuits, low Q-factor, limits applicationof these circuits, but this situationcan be improved by combinations oflumped elements with distributedelements, discrete elements, groundplane modification, etc. Planarlumped element Q-factor depends onthe following factors: parasitics,

Table 2 · Comparison of various planar transmission lines.



AVIONICS RECEIVERS

physical dimensions, transmissionline, dielectric performance, conduc-tor performance (loss, structure, con-figuration, resistance, and dimen-sions), and ground plane structure.Table 3 illustrates performances ofplanar lumped elements and dis-tributed elements.

Basic passive components thatare used in modern avionicsmicrowave receivers are directionalcouplers, power dividers/combiners,filters, baluns, ferrite isolators andcirculators. Passive components areprevalent in microwave front-ends.For example, it is estimated that inan avionics front-end (see Table 4),passive components account for 63%of the component account, 71% of thesize, and 70% of the cost. In a single-mode wireless phone, passive compo-nents account for 90% of the compo-nent count, 80% of the size, and 70%of the cost.

System Functional Blocks1. Control Devices

Control devices for avionicsmicrowave receivers are switches,limiter circuits, phase shifters. Themicrowave switches are widely usedin avionics receivers (see Table 5).Limiters are primarily used in RXs toprotect power sensitive componentsand devices (e.g., LNA, mixer) fromnearby high power sources. In avion-ics RXs, these signals may arise fromXMTR and outside parasitic strongsignals. The author has experiencewith failures of the weather radar,which was damaged by the weatherradar of another aircraft on the run-way. This defect was eliminated bythe optimization of the limiter circuitin the weather radar RX.

RF and microwave phase shiftershave many applications in variousequipment such as phase discrimina-tors, BFNs, power dividers, balancedamplifiers, and phase array anten-nas. A phased array antenna has alarge number of radiating elementsthat emit phased signals to form aradio beam. The radio signal can be

electronically steered by the activemanipulation of relative phasing ofindividual antenna elements. Phaseshifters are fundamental part of anyphased array antennas. They allowan array to scan a beam or reconfig-ure a shaped beam. Phased antennaarrays, such as the avionics TCASANT, consist of a number of individu-al elements, each one requiring aphase shifter that applies the neces-sary phase shift to steer the antennabeam. In some avionics amplitudemonopulse systems, a phase shifterprovides different directional and

omnidirectional antenna modes [13,14].

2. DuplexersThe transceiver duplexer is a key

element in avionics systems, radars,antenna arrays and wireless commu-nication systems. The duplexer pro-vides protection of the front-end andtransmitter from unwanted parasiticsignals [15]. A microwave transceiverduplexer connecting a single ANT tothe XCVR is often the key componentin phase array, radar, communica-tions and navigation systems. A

Table 3 · Performance comparisons of planar lumped elements and dis-tributed elements.

Table 4 · Passive components number in avionics front-ends.

Table 5 · Applications of switches in avionics RXs.


AVIONICS RECEIVERS

duplexer directs transmitter energyto a single antenna during the trans-mit mode and the energy received bythe antenna to the receiver duringthe receive mode. The duplexersshould meet the following objectives:

• Reduce the number of antennas(a single antenna is used fortransmitting and receiving)

• Reduce the number of cablesconnecting the transceiver withthe single antenna

• Reduce the potential transmit-ter pulling, intermodulation andnoise due to antenna mismatch

• Protect the RX from unwanted(parasitic) synchronous andnon-synchronous signals

• Reduce cost, maintenance andmechanical complexity

3. DiplexersA diplexer is a three-port passive

device that implements frequencydomain multiplexing. Do not confuse

the word diplexer with duplexer,which is the three-port network thatpermits a transmitter and receiver touse the same antenna, at essentiallythe same frequency band. A diplexermultiplexes two ports onto one port,but more than two ports may be mul-tiplexed. A typical diplexer consists oftwo fixed tuned BPFs that share acommon port. The common port andtwo outputs of the two filters formthe three terminals of the diplexer.Signals applied to the common portare separated in accordance with thepass band frequencies of the BPFs. Todesign a multiplexer, component fil-ters should be connected in such away that each filter appears as anopen circuit to each other filter. Forthe design of diplexers only one opencircuit condition should be satisfiedand this can be done using optimiza-tion of the T- or Y-junction. It hasbeen found that using the Y-junctionintroduces fewer losses than the T-junction. In integrated avionics RX

multiplexers are used to split wideBW signals of several avionics sys-tems from common ANT into differ-ent paths for each system. In an air-plane many different systems may beused simultaneously. Compact equip-ments that support multi-mode/multi-band front-end are very attrac-tive and will be widely used in thefuture. The front-end multiplexerconsists of several BPFs in n chan-nels sharing a common path.Received signals from common ANTare separated in accordance with thepassbands of the BPFs. Thus, signalsin different front-end channels areisolated from the each other.

4. Low Noise Amplifiers (LNA)A LNA is a key component in the

microwave RX. The LNA at themicrowave RX sets the system noisefigure (NF) or sensitivity. The LNAshould provide enough gain to over-come any noise added by componentsfollowing it, such as the mixer, BPF.

The NF of first amplifying stage ofLNA has a predominant effect, whilethe NF of the subsequent stages is oflesser importance. The desirable elec-trical characteristics of an LNA aregain (G), NF, isolation (ISO), returnloss (RL), dynamic range (DR), bandwidth (BW) and power consumption.Setting high G to the LNA will helpreduce the NF by minimizing thenoise contribution of the mixer andreduce the emission of spurious CWRF energy, but at the expense of high-er power consumption in the blocksand the risk of early mixer overload-ing. For lower LNA gain, a very lownoise mixer would be required tomaintain an acceptable NF. The G ofthe LNA should be the result of acompromise between the followingcontradictory characteristics: maxi-mum RX sensitivity, maximum possi-ble mixer input RF power, and mini-mum LO leakage at the antennaport. The amplifier tuned for the bestNF will not have the optimal inputRL. In the RX with the circulatorduplexer, the LNA reflection signal

does not reach the antenna inputbecause the circulator isolates theLNA from the antenna input duringthe receive mode. However, the RLshould be minimized in order to pro-vide a low mismatch loss, whichinfluences the RX sensitivity. Highreverse ISO of a LNA is important toattenuate local oscillator energy froma mixer to ANT. The LNAs G and NFcompromise is a classical trade-off.The G of the LNA should be a resultof the compromise between the fol-lowing contradictory characteristics:RX sensitivity, maximum possiblemixer input RF power, and maximumLO-RF leakage at the antenna port.Setting high G to the LNA theexpense of higher power consumptionin the blocks after the LNA and riskof early mixer overloading. Too muchpower into the mixer will cause com-pression, while not enough power willshrink the potential DR.Unfortunately, the LNA createsbroadband noise (hopefully verysmall) which will appear at the imagefrequency have its noise powerdirectly converted into the IF band.This has the effect of reducing theoverall receive sensitivity.

5. MixersA very important device in avion-

ics RX is the microwave mixer. Amixer is a nonlinear device that con-verts an RF signal to an intermediatefrequency (IF) by combining the RFsignal with a higher power signal oflocal oscillator (LO). This device mustsatisfy all required LO and RF inputfrequencies as well as all IF outputfrequencies. In the event of a largeLO signal pickup, the total NF will bedegraded. The level of LO signalpicked up at the antenna input mustbe kept below –70 dBm. The followingguidelines should minimize LO pick-up at the RX:

1. Use RF mixer with high LO-RFISO.

2. Use LNA with maximum G.3. Position the LO circuitry away

from the LNA circuitry.4. Use a grounded screen to cover

all of the LO components.

The choice of the mixer IF is veryimportant and will result in manytrade-offs. A low IF allows great sup-pression of adjacent channel interfer-ences, while a high IF gives substan-

tial image rejection and high DR. Inthe double-converting superhetero-dyne RX, high first IF gives goodimage rejection and low second IFgives good selectivity. The mixer com-pression level is the maximum powerlevel that we can put into RX outputwithout compromising the accuracyof the output signal. As the signal



AVIONICS RECEIVERS

level increases, the mixer transferfunction becomes nonlinear becausesignificant energy is lost to the dis-tortion product.

System FactorsThe basic super-heterodyne archi-

tectures of avionics RXs consist ofseveral key components:

• Matching circuits to allow allthe received energy from theANT to get to the next stage

• BPF and/or LPF to knock downout-of-band jammers

• LNA—its primary responsibili-ty is to set the receive sensitivi-ty of the RX, by offering highgain and very low NF

• RF mixer for down convertingfrom RF to IF

• Duplexer for connection of a sin-gle antenna to a RX and XMTR(if necessary)

• Diplexer for signal separation inthe combined multichannel RXs

The conventional antenna mod-ules are passive device, where the RXconnected after the lossy cable haslow sensitivity and therefore an air-craft range measurement is limited.The position of the first LNA in theRX limits the minimum NF (maxi-mum sensitivity) due to IL of the cir-cuit before the LNA. For example,according to [1], the existing avionicscable has a maximum loss of 3 dB.Also, the cable matching might be upto VSWR = 1.7 that lowers the elec-trical performance of the antennamodule. The components of themicrowave module are typicallydesigned using analog circuits. As itis extremely difficult to designmicrowave components over a broadrange of avionics system frequencybands, each microwave RX is typical-ly designed for a certain band of fre-quencies and the band width of thechannels. Table 6 shows characteris-tics of microwave front-ends of differ-ent avionics systems.

The last decade has been marked

by rapid developments in integratedavionics systems. The autonomic sys-tems are heavy, occupy a substantialamount of space and are very costly.The advantages of the integrated sys-tems include fewer units, smallersize, less weight, lower power use,reduced cable count, extensive built-in test equipment (BITE) and inte-grated features. Integratedmicrowave RX of the avionics sys-tems has common antenna module,common XMTR and single multi-channel RX. For microwave integrat-ed avionics systems, cost is the domi-nant factor. The combined microwavesystems require a multi-channel RX,which makes allowance for, one hand,in a large number of receiving chan-nels each with a large DR, and, on theother hand, low cost, small volume,

low weight, and low dissipation ofthese RX channels.

The passive and active antennamodules are very important compo-nents of avionics microwave RX. Theavionics systems with the TCAS, theRX includes an antenna module witha beam forming network. For theactive antennas, the RX includes anantenna module with the first LNAand T/R network. A conventionalTCAS antenna array is passive sys-tem. A TCAS XMTR with pulse powerof about 200 W provides a transmitrange of 45 nautical miles given thetypical directional antenna G. Duringreceive operation, signals received bythe antenna elements are conveyedat very low power levels, becausethere is no power boosting from thepassive antenna array. Receiver sys-

Table 7 · Antenna module specifications.

Table 6 · Microwave RX performance of different avionics systems.


AVIONICS RECEIVERS

tems incorporating passive antenna arrays have aninherent receive range that is dependent, among otherthings, upon the characteristics of the passive compo-nents provided along the receiver path, as well as noise,loss, sensitivity and other factors. The limited receive sen-sitivity is due in part to cable losses and noise within thereceive signal path. In certain applications, it is desirableto provide a TCAS receive range of 100 nautical miles,which may not be possible with conventional passiveantenna systems. This disadvantage can be eliminated byusing the active antenna.

Table 7 illustrates antenna module specifications fordifferent integrated avionics L-band systems. The ANTsof the integrated systems, including TCAS, can be operat-ed in two configurations. The top antenna module mustprovide the directional and the omnidirectional modes.Separate omnidirectional antennas are not required forthe added functionality. The bottom antenna module maybe either likewise directional/omnidirectional or some-times omnidirectional only to reduce cost. In integratedsystems without TCAS (XPDR/DME; XPDR/DME/UAT/ADS-B), the typical antenna is a vertically polarizedmetal blade with one BNC or TNC connector. Most of theL-band blade antennas are thin blades about a quarterwavelength in height due to aerodynamic considerations.

References1. R. P. G. Collinson, Introduction to Avionics Systems,

Second Edition, Kluwer Academic Publishers, 2003.2. M. Golio, Editor, Microwave and RF Product

Applications, CRC Press, 2003.3. L. G. Maloratsky, “RF Design of Avionics L-band

Integrated Systems,” Microwave Journal, Vol.52, No.10,Oct. 2009, pp.64-82.

4. A. Helfrick, Principles of Avionics, AvionicsCommunication, 2007.

5. M. Golio, J. Golio, RF And Microwave Applicationsand Systems, CRC Press, 2007: Chapter 13, J. L. Bartlett,“Microwave and RF Avionics Applications.”

6. (same as Ref. 1)

7. H. Howe, Jr. Stripline Circuit Design, Artech House,Norwood, MA, 1974.

8. S. K. Koul, B. Bhat, “Broadside-Edge CoupledSymmetric Strip Transmission Lines,” IEEE Trans .Microwave Theory Tech., Vol. MTT-30, 1982, pp.1874-1880.

9. T. C. Edwards, Foundations for Microstrip CircuitDesign, John Wiley & Sons, 1981.

10. C. B.Wadell, Transmission Line Design, Handbook,Artech House, Norwood, MA, 1991.

11. L. G. Maloratsly, “Setting Strategy for Transmis-sion Lines,” Microwaves & RF, Part 1, Sep. 2008, pp.100-112, Part 2, Oct. 2008, pp.84-86.

12. L. G. Maloratsky, Passive RF and MicrowaveIntegrated Circuits, Elsevier, 2004.

13. B. E. Dinsmore, D. M. Smith, “Apparatus andMethod for an Amplitude Monopulse DirectionalAntenna,” U.S. Patent No. 5,191,349, Mar. 2, 1993.

14. L. G. Maloratsky, “Switched Directional/Omni-directional Antenna Module for Amplitude MonopulseSystems,” IEEE Antenna and Propagation Magazine,October 2009.

15. L. G. Maloratsky, “Transceiver Duplexer DesignConsideration,” Microwave Journal, Oct. 2008, pp.68-86.

Author InformationLeo G. Maloratsky received his MSEE from the

Moscow Aviation Institute and his PhD from the MoscowInstitute of Communications in 1962 and 1967, respec-tively. Beginning in 1962, he was involved in the research,development and production of RF and microwave inte-grated circuits at the Electrotechnical Institute, and hewas assistant professor at the Moscow Institute ofRadioelectronics. Later, he worked as a staff engineer atAllied Signal and a principal engineer at RockwellCollins. In 2008 he joined Aerospace Electronics Co. He isauthor of four monographs, one textbook, over 50 articlesand 20 patents. His most recent book is Passive RF &Microwave Integrated Circuits, Elsevier, 2004. Dr.Maloratsky can be reached at: [email protected]

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