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2RFSYSTEMBOARDLEVELINTEGRATIONFORMOBILEPHONESGordonJ.AspinTTPCommunicationsLtd.Melboum ScienceParkRoyston, SG8 6EE, U.K.Abstract:This paperhighlights the importanceof system design choices in thedevelopment ofRF chip sets for cellular mobile phones. By way ofexample, the designoftheBRIGHTfamilyofRFchipsetsforGSMisdescribedPerformanceresultsarepresented based on these chip sets.2.1INTRODUCTIONTraditionally, the RF system represents one ofthe most difficult challenges for compa-nies developing digital cellular phones, particularly for companies with limited devel-opmentexperience.Itis nowpossibletobuymoreorlesscompletebaseband-chip-setsolutions together with associated software from a number ofvendors.However,al-though off-the-shelf radio-chip-set solutions are available, they nevertheless require asignificant level of design expertise on the part of the handset designer in order to re-alise a manufacturable product whichmeets the necessaryperformance requirementsThispaperdescribestheresults of some of our worktomeetthechallenge ofrealisinganRFchip set which makesdesigninga GSMmobilephonea straightforwardtask9
2 RF SYSTEM BOARD LEVEL INTEGRATION FOR MOBILE PHONES Gordon J. Aspin TTP Communications Ltd. Melbourn Science Park Royston, SG8 6EE, U.K. Abstract: This paper highlights the importance of system design choices in the development of RF chip sets for cellular mobile phones. By way of example, the design of the BRIGHT family of RF chip sets for GSM is described. Performance results are presented based on these chip sets. 2.1 INTRODUCTION Traditionally, the RF system represents one of the most difficult challenges for companies developing digital cellular phones, particularly for companies with limited development experience. It is now possible to buy more or less complete baseband-chip-set solutions together with associated software from a number of vendors. However, although off-the-shelf radio-chip-set solutions are available, they nevertheless require a significant level of design expertise on the part of the handset designer in order to realise a manufacturable product which meets the necessary performance requirements. This paper describes the results of some of our work to meet the challenge of realising an RF chip set which makes designing a GSM mobile phone a straightforward task. 9
10CIRCUITSANDSYSTEMSFORWIRELESSCOMMUNICATIONSThis work, the development ofthe BRIGHT' RF chip set family, has been carried outincollaborationwithHitachiSemiconductorofJapanThe initial devices were targeted at single-band products-—the main GSM marketat that time.Even more important commercially,however, has been the emergence ofdual-band900/1800capabilityasakeymarketrequirement.Fortunately,theBRIGHTarchitectureisparticularlywell suitedtomulti-band operation,andthelatest-genera-tion devices,BRIGHT2,are designed to support this modeThe objective of the development has been to achieve the maximum level of inte-gration in the radio system, compatible with the objectives oflowest total system cost,lowestpartcount,andeaseofdesignandmanufacture,withintheavailableICprocesstechnology.Theapproach is asystems approachwiththefocusupon achieving ade-vice design in which the radio requirements ofthe whole product are most simply andelegantlymet.2.2DESIGNAPPROACHWhat makes RF IC design particularly interesting is that the silicon represents onlya small part ofthe total RF system but can makea verylarge differencetohow easyor difficult it is todesign the rest of the system.It is therefore vital to understand thetotal system requirements when designing the chip.It is also important to be able tovalidate the design at both device and system level. This is achieved by means of aSystemEvaluationBoard--essentiallyacompletehandset-onwhichallthesystemperformanceparameters can be measured and confirmed (Fig.2.1).Validation of thesystem is often one ofthe most expensive parts ofthe development.Thisradio SystemEvaluationBoard is designedtobecompatiblewithBasebandEvaluationBoardsusedforbasebandchipsetandprotocolsoftwaredevelopmenwork.The two boards can beplugged togetherto emulate complete handset opera-tion.As well as testing in our own laboratories we have even been able to take sucha system through“Type Approval" at a GSM test house to verify radio system perfor-mance.2.3KEYGSMSYSTEMSPECSAs with all modern radio standards, there aremany specification points for GSM thatneedtobemetinorderfortheradiotoconformtothestandard.Howeverasmallnumberofthese specification pointsturn outtobecritical totheradioarchitectureanddesign.2.3.1TransmitterphaseerrorAsadigital phasemodulation system,GSMcontrolshowcloselythetransmittedphasefollows the ideal modulated phase trajectory. It does this by specifying the phase errorin terms of an RMS value(5°)and a peak value (20°),across theuseful part of theburst.Thephaseerrorisaverycritical parameterforthetransmitter,because somanythings can contribute to it, including digital modulator phase error, synthesiser settling'BiCMOS Radio IC for GSM by Hitachi and TTPCom
10 CIRCUITS AND SYSTEMS FOR WIRELESS COMMUNICATIONS This work, the development of the RF chip set family, has been carried out in collaboration with Hitachi Semiconductor of Japan. The initial devices were targeted at single-band products — the main GSM market at that time. Even more important commercially, however, has been the emergence of dual-band 900/1800 capability as a key market requirement. Fortunately, the BRIGHT architecture is particularly well suited to multi-band operation, and the latest-generation devices, BRIGHT2, are designed to support this mode. The objective of the development has been to achieve the maximum level of integration in the radio system, compatible with the objectives of lowest total system cost, lowest part count, and ease of design and manufacture, within the available IC process technology. The approach is a systems approach with the focus upon achieving a device design in which the radio requirements of the whole product are most simply and elegantly met. 2.2 DESIGN APPROACH What makes RF IC design particularly interesting is that the silicon represents only a small part of the total RF system but can make a very large difference to how easy or difficult it is to design the rest of the system. It is therefore vital to understand the total system requirements when designing the chip. It is also important to be able to validate the design at both device and system level. This is achieved by means of a System Evaluation Board — essentially a complete handset—on which all the system performance parameters can be measured and confirmed (Fig. 2.1). Validation of the system is often one of the most expensive parts of the development. This radio System Evaluation Board is designed to be compatible with Baseband Evaluation Boards used for baseband chip set and protocol software development work. The two boards can be plugged together to emulate complete handset operation. As well as testing in our own laboratories we have even been able to take such a system through “Type Approval” at a GSM test house to verify radio system performance. 2.3 KEY GSM SYSTEM SPECS As with all modern radio standards, there are many specification points for GSM that need to be met in order for the radio to conform to the standard. However, a small number of these specification points turn out to be critical to the radio architecture and design. 2.3.1 Transmitter phase error As a digital phase modulation system, GSM controls how closely the transmitted phase follows the ideal modulated phase trajectory. It does this by specifying the phase error in terms of an RMS value (5°) and a peak value (20°), across the useful part of the burst. The phase error is a very critical parameter for the transmitter, because so many things can contribute to it, including digital modulator phase error, synthesiser settling 1 BiCMOS Radio IC for GSM by Hitachi and TTPCom
11RFSYSTEMBOARDLEVELINTEGRATIONFORMOBILEPHONESOFigure2.1BRIGHT2systemevaluationboard.time,switchingtransients,I/Qgain andphaseimbalance,up-converterphasenoiseand spurious modulation, and power amplifier AM-to-PM conversion, etc.Inputtingtogetheraradiosystemdesign,budgetsareallocatedtoeachoftheseparameters. Some, however, can be difficult to quantify theoretically (e.g. switchingtransients)andmayonlybecomeapparent oncethedesign is realised inhardware.Thetask ofthe system designer is to identify a system architecture in which uncontrollableand unquantifiable effects are minimisedA classic example ofa problematic architectureis thedirect-up-convertertransmit-ter, in which a baseband I/Q modulator is mixed up to the final frequency in a singlestage.As a result, the up-converter's local oscillator runs also at the final frequencyMaintaining sufficientisolationbetweenthemodulatedpoweramplifieroutputandtheunmodulated low-power local oscillator is very difficult, particularly within a minia-turehandset.2.3.2Transmittermodulationspectrum (Fig.2.2)Like the phase error, the modulation spectrum is affected by many factors, includingdigital modulator spectrum,I/Qgain and phase imbalance,up-converterphase noiseand spurious modulation,power amplifier AM-to-PM conversion, transmitter noisefloor, etc. A good system design will minimise these effects inherently in the design.2.3.3Transmitternoiseinthereceiverband(Fig.2.3)Toavoid interference between handsets in close proximity,GSM limits theamountofspuriousradiationemittedfromtheantennaintothereceiveband.Inthenon-extendegband, this is limited to -79 dBm in a 100kHz bandwidth, or -129 dBm/Hz.Reflected
RF SYSTEM BOARD LEVEL INTEGRATION FOR MOBILE PHONES 11 time, switching transients, I/Q gain and phase imbalance, up-converter phase noise and spurious modulation, and power amplifier AM–to–PM conversion, etc. In putting together a radio system design, budgets are allocated to each of these parameters. Some, however, can be difficult to quantify theoretically (e.g. switching transients) and may only become apparent once the design is realised in hardware. The task of the system designer is to identify a system architecture in which uncontrollable and unquantifiable effects are minimised. A classic example of a problematic architecture is the direct-up-converter transmitter, in which a baseband I/Q modulator is mixed up to the final frequency in a single stage. As a result, the up-converter’s local oscillator runs also at the final frequency. Maintaining sufficient isolation between the modulated power amplifier output and the unmodulated low-power local oscillator is very difficult, particularly within a miniature handset. 2.3.2 Transmitter modulation spectrum (Fig. 2.2) Like the phase error, the modulation spectrum is affected by many factors, including digital modulator spectrum, I/Q gain and phase imbalance, up-converter phase noise and spurious modulation, power amplifier AM–to–PM conversion, transmitter noise floor, etc. A good system design will minimise these effects inherently in the design. 2.3.3 Transmitter noise in the receiver band (Fig. 2.3) To avoid interference between handsets in close proximity, GSM limits the amount of spurious radiation emitted from the antenna into the receive band. In the non-extended band, this is limited to –79 dBm in a 100kHz bandwidth, or –129 dBm/Hz. Reflected
12CIRCUITS AND SYSTEMS FOR WIRELESS COMMUNICATIONS100-10-20dB-30-40-50-60-70-200-600-4000200400600FrequencyfromCarrier (kHz)Figure2.2Transmitspectrummask+33dBm40200-20-67dBm-40GSM900-79dBm-60-80-100Tx (880-Rx(925-Rx(935-915MHz)935MHz)960MHz)+30dBm40200-20-71dBm口GSM1800-40-60-80-100Tx(1710-Rx(1805-1785MHz)1880MHz)Figure2.3Transmitnoiseinreceivebandsback to the input of thepower amplifier thistypically corresponds to a noiselevel of- 162 dBm/Hz or 12 dB above the ideal thermal noise floor.In practice, modulatorscannot achieve this level of performance,and typicallyaround20dB or more offilter-ing is required in a duplexer-which is a relativelybulkyand expensive component
12 CIRCUITS AND SYSTEMS FOR WIRELESS COMMUNICATIONS back to the input of the power amplifier this typically corresponds to a noise level of – 162 dBm/Hz or 12 dB above the ideal thermal noise floor. In practice, modulators cannot achieve this level of performance, and typically around 20 dB or more of filtering is required in a duplexer—which is a relatively bulky and expensive component