18CIRCUITSANDSYSTEMSFORWIRELESSCOMMUNICATIONS3259国B8BiocZ7MHGSM13SMHEDC:35MH1CHD156121F380-915MPA ModuleFigure2.8BRIGHT2architecture.PhaseTrajectoryError12010.0PeakPhaseErrorPL58.0aWRMSPhaseErrorPL56.0PeakPhaseErrorPL154.0RMSPhaseErorPL152.00.00255075100125Channel No.GSM90ophaseerror.Figure2.92.5.2TransmittermodulationspectrumFig.2.11illustrates thetypical modulation spectra achieved with theBRIGHT2designat 900 MHz.The spectrum falls comfortably within the specification mask.Similarresultsareobtainedat1800MHz
18 CIRCUITS AND SYSTEMS FOR WIRELESS COMMUNICATIONS 2.5.2 Transmitter modulation spectrum Fig. 2.11 illustrates the typical modulation spectra achieved with the BRIGHT2 design at 900 MHz. The spectrum falls comfortably within the specification mask. Similar results are obtained at 1800MHz
19RESYSTEMBOARDLEVELINTEGRATIONFORMOBILEPHONESPhaseTrajectoryError108a品真6PeakPhaseErorBYYIYPHASERMSPower Level 02wanAeehhwihe(+30dBm)0500550600650700750800850900Channel No.Phase TrajectoryError86sarJTPeak Phase ErrorPHASERMS2PowerLevel15eyyrwia(OdBm)0500550600800850900650700750Channel No.Figure2.10GSM1800phaseerror.2.5.3ReceivesensitivityFig.2.12showsthemeasured receivesensitivityinbothbands plotted as afunctionof channel number. For GSM900, the specification is -102 dBm, for GSM1800, thespecification is-100dBm.A minimum margin of 4dB is achieved on these figures,with 5-7 dB typical margin. This provides sufficient margin for production and tem-perature tolerances.Theunderlying sensitivity variation displayed in thegraphs is dueto ripple inthefront-endSAWfilter.Therearetwowell-known"deafchannelsinGSM9O0receivers, channels 5 and 70, at the 72nd and 73rd harmonic of the GSM referencefrequencyof 13MHz.Theseareclearlyvisibleinthefigure.Theotherdeaf channelis thought tobedueto a local GSMbase station transmitting on this channel in thevicinity ofthe laboratory
RF SYSTEM BOARD LEVEL INTEGRATION FOR MOBILE PHONES 19 2.5.3 Receive sensitivity Fig. 2.12 shows the measured receive sensitivity in both bands plotted as a function of channel number. For GSM900, the specification is –102 dBm, for GSM1800, the specification is –100dBm. A minimum margin of 4dB is achieved on these figures, with 5–7 dB typical margin. This provides sufficient margin for production and temperature tolerances. The underlying sensitivity variation displayed in the graphs is due to ripple in the front-end SAW filter. There are two well-known “deaf” channels in GSM900 receivers, channels 5 and 70, at the and harmonic of the GSM reference frequency of 13 MHz. These are clearly visible in the figure. The other deaf channel is thought to be due to a local GSM base station transmitting on this channel in the vicinity of the laboratory
20CIRCUITSANDSYSTEMSFORWIRELESSCOMMUNICATIONSnTTEH2OdBUnUG180HKR10.50dmRL0.2dBm10dB/902.000MHzDRCENTER 982.000HHzSPAN1.800MHzARDN30KHTUBN30kHzS4P 50.emsFigure2.11Transmittermodulationspectrum.GSM900Sensitivityazaepaea-105-106-107-108-1090255075100125Channel No.GSM1800Sensitivity%-102-104106d-108110512612712812912Channel NoFigure2.12Receivesensitivity2.5.4BlockingperformanceFig.2.13showstheblockingperformanceforBRIGHT2inthe900and1800MHzbands.The specification points are 2%BER and23dBm for GSM900,-26dBm
20 CIRCUITS AND SYSTEMS FOR WIRELESS COMMUNICATIONS 2.5.4 Blocking performance Fig. 2.13 shows the blocking performance for BRIGHT2 in the 900 and 1800MHz bands. The specification points are 2% BER and –23 dBm for GSM900, –26 dBm
21RFSYSTEMBOARDLEVELINTEGRATIONFORMOBILEPHONESfor GSM1800.Around 2-3dB margin is thus achieved in each band.The blockingperformanceof BRIGHT2intheGSM900band isfurther illustrated inFig.2.14.Awanted signal is applied,3dBabovethereference sensitivitylevel,andthen thelevelofblocking signal which causes the receiver BER todegradeto the nominal 2%level,ismeasured.Thewanted signal is on channel61,i.e.at947.2MHz.BERvs3MHzBlockingLevel7650--3MHz (GSM)+3MHz (GSM)-3MHz (DCS)%3+3MHz(DCS)21-19-18-17-27-26-25-24-23-22-21-20BlockerLevel (dBm)Figure2.13Receiverblockingperformance.The GSM requirements areas follows-43dBmat±0.6MHz,i.e.<946.6MHz&>947.8MHz■-33dBmat±1.6MHz,i.e.<945.6MHz&>948.8MHz-23dBmat±3MHz.i.e.<944.2MHz&>950.2MHzThe most difficult requirement is the 3 MHz blocker, and for this, a minimum of15dBmargin isdemonstrated.Sincethese measurements weremade, it was iden-tified that the first mixer was not particularly well-matched (hence the ripple), andsubsequentlybetterperformancewithmorethan3dBmarginhasbeendemonstrated.2.6FUTUREOPTIONSLooking to the future, there are a numberofpossible options.Firstly,the levels ofinte-gration can be increased,for example by including on chip the VCOs or the basebandinterface.The challengewithputtingVCOs on chip is torealise circuits of sufficientlyhighQto meet the phasenoise requirements; also low-loss varactors are not normallyrealisable in a BiCMOS IC process.Nevertheless, there is much interest in this areaIncluding thebaseband interface within theRF chip is more straightforward inprinciple. Whether this makes commercial sense is more questionable-integration
RF SYSTEM BOARD LEVEL INTEGRATION FOR MOBILE PHONES 21 for GSM1800. Around 2–3 dB margin is thus achieved in each band. The blocking performance of BRIGHT2 in the GSM900 band is further illustrated in Fig. 2.14. A wanted signal is applied, 3 dB above the reference sensitivity level, and then the level of blocking signal which causes the receiver BER to degrade to the nominal 2% level, is measured. The wanted signal is on channel 61, i.e. at 947.2MHz. The GSM requirements are as follows: The most difficult requirement is the 3 MHz blocker, and for this, a minimum of 1.5 dB margin is demonstrated. Since these measurements were made, it was identified that the first mixer was not particularly well-matched (hence the ripple), and subsequently better performance with more than 3 dB margin has been demonstrated. 2.6 FUTURE OPTIONS Looking to the future, there are a number of possible options. Firstly, the levels of integration can be increased, for example by including on chip the VCOs or the baseband interface. The challenge with putting VCOs on chip is to realise circuits of sufficiently high Q to meet the phase noise requirements; also low-loss varactors are not normally realisable in a BiCMOS IC process. Nevertheless, there is much interest in this area. Including the baseband interface within the RF chip is more straightforward in principle. Whether this makes commercial sense is more questionable — integration
22CIRCUITSANDSYSTEMSFORWIRELESSCOMMUNICATIONSBlockerlevelfor2%RBERSweptBlockingSignal (400kHzsteps)-15Wanted atCh61,-99dBm1-30930936940945955950960965blocker freq(MHZ)Figure2.14Receiversweptblockingmeasurementsdoes not help reduce the total number of pins in the package very much, and the areafor a 64-pin QFP is actuallygreater than for a 48-pin QFP plus a 20-pin SSOP.Also,becausethebaseband interfacemay be realised in smaller geometries than theRFdevice, thetotal power consumption may be increased by integration.A more fruitful line is perhaps to consider greater functionality.In this regardtriple-band phones may be a market requirement soon, a requirement which may besupported as afurther extensionof theBRIGHT architectureIn conclusion, it is clear that radiodesign remains one of the most challenging andexciting areas in this industry.Whatever happens in wireless communications over thenextdecade, elegant design at both circuit and system level will continueto be a majorfactorinthesuccessofproducts.asithasbeentodatewithBRIGHT
22 CIRCUITS AND SYSTEMS FOR WIRELESS COMMUNICATIONS does not help reduce the total number of pins in the package very much, and the area for a 64-pin QFP is actually greater than for a 48-pin QFP plus a 20-pin SSOP. Also, because the baseband interface may be realised in smaller geometries than the RF device, the total power consumption may be increased by integration. A more fruitful line is perhaps to consider greater functionality. In this regard, triple-band phones may be a market requirement soon, a requirement which may be supported as a further extension of the BRIGHT architecture. In conclusion, it is clear that radio design remains one of the most challenging and exciting areas in this industry. Whatever happens in wireless communications over the next decade, elegant design at both circuit and system level will continue to be a major factor in the success of products, as it has been to date with BRIGHT