DWA TECHNIQUE TO IMPROVE DAC OF SIGMA-DELTA FRACTIONAL-N FREQUENCY SYNTHESIZER FOR WIMAX

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1 Journal of Al-Nahran Unversty Vol.12 (2), June, 2009, pp Scence DWA TECHNIQUE TO IMPROVE DAC OF SIGMA-DELTA FRACTIONAL-N FREQUENCY SYNTHESIZER FOR WIMAX H. T. Zboon* and H.M. Azzaw *Department of Electrcal and Electronc Engneerng, UOT., Baghdad-Iraq. Abstract A frst order of DWA (Data Weghted Averagng) algorthm wth thrd order sgma-delta modulator s proposed for applcaton n sgma-delta fractonal-n frequency syntheszer for WMAX. In addton, ths paper dscusses the mpact of msmatch between Dgtal-to-Analog convertor (DAC) unt elements. The smulaton results show the effectveness of the DWA technque n reducton of spurs, also DWA technque proves ts ablty to solve DAC unt elements msmatch. The fractonal spur nose s converted nto a broadband. Quantzaton nose wth DWA s mproved by 10 db f the element msmatch 0.01 and 5 db f the element msmatch 0.08 wth 8 bt PFD/DAC. Matlab (V.7) program s used for smulaton. Key words: WMAX, Frequency syntheszer, DAC msmatch, DWA technque. Introducton WMAX (Worldwde Interoperablty for Mcrowave Access) also referred to as IEEE , s a broadband technology that provdes wreless servces connectvty to fxed, nomadc, portable, and moble users [1]. However the present standards, such as wreless fdelty (W-F), fal to provde suffcent hgh data rate, long range and low deployments costs, whle WMAX overcomes the above problems [2]. However, common to all wreless communcaton system s the frequency syntheszer (FS). The frequency syntheszer s the heart and the most crtcal part of these systems [3].The number of frequences generated, spurous levels and the swtchng rate are the man quanttes that determne system capabltes of the syntheszer tself. Recently, syntheszers used sgma-delta modulator. Ths delta-sgma modulator uses oversamplng and nose shapng to reduce the phase nose [4,5]. The quantzaton nose-bandwdth tradeoff assocated wth ΣΔ fractonal-n synthess can be removed f quantzaton nose can be reduced. Fg.(1) shows a block dagram of a Σ fractonal-n PLL frequency syntheszer. The key advantage of ths proposed method s that the crcutry that njects quantzaton nose nto the loop (the PFD and charge-pump) s combned wth the cancellaton sgnal by DAC to create an nherent gan match between the two sgnals. The resultng msmatch compensated PFD/DAC syntheszer s capable of dramatcally reducng quantzaton nduced phase nose [6, 7], as shown n Fg. (2). The PFD/DAC block whch s a combnaton of PFD and DAC n one sngle element s used to overcome non-dealtes between them, as shown n Fg. (1). The man source of msmatch n the charge-pump porton of the PFD/DAC s the msmatch between the DAC unt elements, for ths reason, a DAC msmatch compensaton block s used [6], as shown n Fg. (2). Thermometer decoder and data weght averagng (DWA) crcut, are used to shape msmatch nose to hgh frequences [8]. DWA Algorthm Msmatch n the DAC unt elements causes dstorton n the sgnal band whch reduces the obtaned sgnal to nose plus dstorton rato (SNDR). To solve ths problem, DAC uses dynamc element matchng (DEM) technques. The functonal prncple of all DEM technques s to transfer dstorton that occurs due to the fxed step-sze errors nto a nose sgnal that s spread over the whole frequency band up to half of the samplng frequency, then shapng to hgh frequency [9]. The components of the quantzaton nose and the DAC nose outsde of the sgnal band n Fg. (3a) wll be removed by the flter but much of the DAC nose wll be n band. Usng nose-shapng DEM technques results n DAC nose that pushes DAC nose outsde the sgnal

2 H. T. Zboon band as llustrated n Fg. (3b). There are several DEM technques whch have been publshed (Barrel-Shft, Indvdual Level Averagng (ILA), and Data Weghted Averagng (DWA) algorthms) [9]. Data Weghted Averagng DWA [9,10] s the most popular DEM technque, because of ts smple mplementaton and effcency. The basc DWA for a 4-bt DAC [6,8] s llustrated n Fg.(4). Sgma-delta modulator output codes denoted as y(n), range from one to four unt ( U 1, U 2, U3, U 4 ), as shown n Fg. (4). The counter s used to create ponter (ptr(n)) that shows the pont of frst unused unt element, IM(ptr(n)), referred to as ntegral msmatch, s the accumulaton of the element msmatch error. The code s converted to thermometer code ( 2 B 1), and a logarthmc shfter s used to move the orgn of the thermometer code to the new poston. The algorthm cycles through the DAC elements by sequentally selectng the elements based are upon the nput data, as shown n Fg. (5). Mathematcally, DAC msmatch nose ( N DAC ) can be defned as a functon of the IM(ptr(n)) n Z-doman [8]: 1 N dac ( z) (1 z ) IM(ptr(z))... (1) 1 Where ( 1 z ) s a frst order hgh-pass fltered of the ntegral msmatch error generated and Where IM(ptr(n)) = ptr( n) 1 ( 0 ptr 1 U U ) /( U )..(2) mean mean = e... (3) 0 U mean s the average value of the DAC 1 N 1 0 unt elements, U mean U, U s the N th DAC unt element value, e s the DAC msmatch error of th DAC unt element, and N s the number of total DAC unt. The flow chart of DWA algorthm s shown n Fg. (6). elements results n a gan msmatch between the quantzaton nose sgnal and cancellaton sgnal. In order to match the unt elements, the data weghted averagng (DWA) algorthm s used. Wthout DWA enabled, the unt element msmatch leads to a large amount of fractonal spur feed-through. Fgs. (7) and (8) present the results of smulaton for several values of DAC unt element msmatch ( ) wth dfferent mult-bt PFD/DAC. These smulaton results show the mpact of ncrease n msmatch value ( =0.01 and =0.08) between DAC unt elements on overall performance of PLL, whch causes ncrease n the phase nose and large amount of fractonal spur feed-through at the output spectrum of PLL. The mnmum value of result of low spur appears at the output, as shown n Fg. (7). As the msmatch value ( ), between DAC unt elements, s ncreased, more sgnfcant spurs result, and adversely affect syntheszer performance, as shown n Fg. (8). In order to evaluate the mpact of ncrease n DAC unt elements of current sources msmatch on PFD/DAC performance, a sute of smulatons was run wth the PFD/DAC resoluton set to 12 bt, as shown n Fg. (9). Smulaton results show that the hgh spurs appear at the output of PFD/DAC block, and result n hgh contents of spurs. Conclusons Msmatch between DAC elements s studed and shows that the spurous levels ncrease wth ncreasng the msmatch between DAC elements. It was found that DWA algorthm proves ts ablty to solve ths problem. The net mprovement n quantzaton nose s obtaned (- 10 db) f = 0.01 and (-5 db) f = 0.08 wth 8 bt. However, the quantzaton nose wth DWA s mproved by ( 8 db) f = 0.01 and (- 3 db) f = 0.08 wth 12 bt PFD/DAC. Therefore DWA technque s proposed to be used n sgma-delta fractonal-n frequency syntheszer for WMAX system. Smulaton Results B The PFD/DAC s composed of 2 unt elements, where B s the number of bts n the PFD/DAC. Msmatch between the unt

3 Journal of Al-Nahran Unversty Vol.12 (2), June, 2009, pp Scence f Re f PFD/DAC 8-bt Loop flter (Passve flter) VCO f out N /( N 1) B Accumulator Fracton part R resdue Integer part Dgtal MASH Σ Fg. (1) : Block dagram of PFD/DAC frequency syntheszer. f Re f f Dv Clock Delay Dv0 Dv 1 Tmng msmatch compensaton and resynchronzaton block PFD UP 0 1 Charge pump To loop flter Swap B 2 1 dac From dgtal Σ modulator B DAC msmatch shapng block DAC current sources Fg. (2) : PFD/DAC block dagram. (a) Fg. (3): Typcal power spectral denstes at the output of the DAC (a) wthout DEM (b) wth DEM algorthm. (b)

4 H. T. Zboon Σ DAC Input 2-bt Bnary to thermometer decoder Logshfter U1 U2 U3 dac U4 Ponter Clock Counter Fg. (4) : DWA Crcut. Fg. (5) : DWA operaton.

5 Journal of Al-Nahran Unversty Vol.12 (2), June, 2009, pp Scence Start Inputs: The modulator order, DAC msmatch value ( ), and the number of unt elements (U) Unt elements requres an (U+1) level quantzer K=1: length (U) Calculate quantzer output as a functon of DAC element value Increase the ponter ptr(k+1)=ptr(k)+1 K=U No Yes Plot output power spectrum End Fg. (6): The flow chart of the DWA algorthm.

6 H. T. Zboon Fg. (7): Smulaton showng the mpact of unt element msmatch ( = 0.01) wth PFD/DAC-8 bt. Fg. (8): Smulaton showng the mpact of unt element msmatch ( = 0.08) wth PFD/DAC-8 bt.

7 Journal of Al-Nahran Unversty Vol.12 (2), June, 2009, pp Scence a b Fg. (9): Smulaton showng the mpact DWA on the quantzaton nose spectrum for PFD/DAC-12 bt (a) = 0.01 (b) =

8 H. T. Zboon References [1]. S. Fl, ''Fxed, Nomadc, Portable and Moble Applcatons for and e WMAX Network'', WMAX Forum, November 2005, [2]. Y. Tan and I. Chu, ''Seamless Moble Lfestyle Through WMAX'', Motorola, Inc., and De Paul Unversty, [3]. Dxon, ''Spread Spectrum Systems'', John Wley & Sons, Inc., [4]. K. Shu and E. S. Snenco, ''CMOS PLL Syntheszers: Analyss and Desgn'', Sprnger, Inc., [5]. B. G. Goldberg, ''Dgtal Frequency Synthess Demystfed: DDS and Fractonal-N PLLs'', LLH Technology Publshng, [6]. S. E. Mennger and M. H. Perrott, "A Fractonal-N Frequency Syntheszer Archtecture Utlzng a Msmatch Compensated PFD/DAC Structure for Reduced Quantzaton-Induced Phase Nose", IEEE Transactons on Crcuts and Systems-II: Analog and Dgtal Sgnal Processng, Vol. 50, No. 11, page: 8-849, November [7]. S. E. Mennger, ''Desgn of a Wdeband Fractonal-N Frequency Syntheszer Usng CppSm", May 2005, s.html. [8]. K.Chen and T. Kuo,'' An Improved Technque for Reducng Baseband Tones n Sgma Delta Modulators Employng Data Weghted Averagng Algorthm Wthout Addng Dther'', IEEE Transactons on Crcuts and Systems-II: Analog and Dgtal Sgnal Processng, Vol. 46, No. 1, January 1999, pp [9]. E. Bach, ''Multbt oversamplng D/A converters usng dynamc element matchng methods'', Semens Corp., Techncal Report: Sysconv , [10]. Z. Zhang and G. C. Temes, "A Segmented Data-Weghted-Averagng Technque", IEEE press, Center for Desgn of Analog-Dgtal Integrated Crcuts (CDADIC), 2007, pp الخالصة ذقتمممست دامممترز ي ر س ذيممم ممم يتضمممذه اممم ( DWA )ذه زسج ال ى ذع ذضذه ) Sgma-delta (ذمه تمسزز ذمه نمم ع Sgma-( لث لم ت اميه رز ذسرم زسجم WMAX.إضمث إ مى )Delta Fractonal-N ذنظ ذم ممميه عنث مممس ذ ممم ممم تمممي ذنثة ممم ذ مممري عمممزي ت يممم.(DAC) نتمممثال ذ ثرمممثل ينممم مممى رذيممم ال مممثسل سةذيممم DWA ممت تقييمم ذاممت د تممسزز لي ممت رمم عث يمم تقنيمم ممميه عنث مممس.(DAC) تغيممم عيمممى ذ مممري عمممزي ت يممم عسيضم. ذم مى رامسي تمي ت ييم ذامت د ض ضمث رذي ذقز س) 01-( db عنمزذث ترم ه تي ت ايه ض ضث ذقمممز س )-5dB( عنمممزذث يرممم ه )0.01( ةيذممم عمممزي ت يممم )0.08(. تمي امترز ي سنمثذل Matlab ةيذم عمزي ت يم (7.V) ت ذ ثرثل. 011