Using SigLab for Production Line Audio Test

Transcription

1 APPLICATION NOTE Using SigLab for Production Lin Audio Tst SigLab is idal for charactrizing audio componnts. Both its input and output subsystms hav low nois, low distortion and low cross talk. SigLab's masurmnt prformanc xcds th tsting rquirmnts of most any consumr audio dvic. Its thr intrnal DSP procssors and its SCSI intrfac insurs maximum masurmnt throughput. This not covrs th us of SigLab in making svral common audio masurmnts. Ovrviw Th following ar thr typical audio masurmnts on a stro amplifir that will b discussd. channl to channl cross talk harmonic distortion signal to nois ratio Othr masurmnts that can b mad with SigLab includ intrmodulation distortion; gain and phas rspons; and stp rspons. Masurmnt Goals Th objctiv of any automatd tst is to rapidly dtrmin th quality of a particular product or subassmbly. Ovrtsting as wll as undrtsting is undsirabl. In most cass, tst spd is as important a factor as any. Th goals of th following tst scnario ar: simpl tst adaptr (no switchs/rlays) all tsting compltd in undr 1 scond amnabl to full automation tst quality xcds amplifir rquirmnts Tst Stup Intrfacing th Amplifir to SigLab Th tsting of an amplifir rquirs carful attntion to th sourc and load signal lvls and impdanc charactristics. In addition th load impdanc must b capabl of dissipating th powr gnratd by th amplifir. SigLab's output gnrators can gnrat a 20 volt pak-pak signal bhind a 50 ohm impdanc. If high snsitivity phono inputs ar bing tstd, an attnuator should b usd. If a sourc impdanc of gratr than 50 ohms is dsird, simply add th rquird rsistanc in sris with th signal conductor from SigLab's output. SigLab's input subsystm is capabl of handling 20 volt pak-pak signals. Thrfor, for powr amplifirs, a load rsistor plus an attnuator is rquird to bring th maximum signal lvl within SigLab's input rang. To obtain th bst masurmnt dynamic rang, th attnuator should bring th maximum xpctd lvl down to 5 to 7 volts pak Load Attnuator Circuit (LAC) 8 8 SigLab OK Right +20dB +20dB Lft Modl Powr Tap/CD Input Tap/CD Input Amp Undr Tst Figur 1 - Amplifir Tst Stup Th amplifir undr tst (nclosd by th brokn lin) and SigLab ar connctd by cabls, a combination load and an attnuator circuit, th LAC as shown in Figur 1. Th input to th amplifir is connctd dirctly to th SigLab output channls providing a 50 ohm sourc impdanc with no attnuation. On Application Not 8.1 Using SigLab for Production Lin Audio Tst 1 11/12/96SLAP8_1

2 Th attnuator on th amplifir's output was chosn to b approximatly 10:1 or 20 db. This is actually not optimal sinc, givn th maximum powr output of th amplifir undr tst, a 2:1 ratio would hav bn mor than sufficint to bring th signal lvls within SigLab's input rang. Howvr, th 10:1 allows an amplifir gain (btwn CD/Tap input and th output) to b about 20 db for th tsting. Th rsulting amplifir and LAC thn hav an ovrall gain of about 0 db. This allows th amplifir and LAC to b rplacd by two cabls to dmonstrat th prformanc of SigLab masuring an "idal" amplifir with no nois, distortion, or cross talk. Control Sttings Tabl 1 summarizs th control sttings on th dvic undr tst. Th actual dvic was a Radio Shack STA-20 rcivr. This not focuss on tsting th amplifir portion only. Control Powr Ton Balanc Loudnss Volum Input Slctor Equalization Stting On Cntrd Cntrd Off (out) +20 db gain CD/Tap Off Tabl 1 - Control Sttings Baslin Masurmnts Bfor dsigning th automatd masurmnt systm, som masurmnts wr mad on th amplifir. Frquncy Rspons Frquncy rspons is a typical masurmnt mad on amplifirs. Th ntwork analyzr virtual instrumnt was usd to mak th masurmnt shown in Figur 2. M a g n i t u d d B Magnitud Phas Hrtz Figur 2 - Frquncy Rspons It is intrsting to not that vn with th controls st to th ostnsibly flat position, th magnitud and phas rspons ar far from flat. Th amplifir gain was st to +20 db at th low frquncy nd. Cross Talk Th ntwork analyzr also was usd to masur channl-to-channl cross talk ovr th 20 khz rang. Th xcitation was fd to th lft channl and th transfr function was masurd btwn th lft and right outputs. Th transfr function provids an asy and accurat way to gt th cross talk as a function of frquncy. Figur 3 shows th rsults of this masurmnt. At 1000 Hz th lft-to-right cross talk is about -54 db and at Hz it dgrads to -34 db. For th crosstalk masurmnt, th amplifir s right channl must not rciv any input. As shown in Figur 1, this is satisfid by driving it with SigLab s channl 2 output st to zro amplitud, sinc SigLab s output impdanc is low (50 ohms). V/V db Right Out / Lft Out Hz P h a s d g r s 2 Application Not 8.1 Using SigLab for Production Lin Audio Tst 11/12/96 Slap8_1

3 Figur 3 - Cross Talk, Lft Channl to Right Svral mthods of implmnting this masurmnt ar availabl. In this xampl, rsults ar only dsird at 1 khz and 10 khz. Tst tim is minimizd by stting up an automatd tst to produc tons and mak masurmnts only at ton frquncis Right Out/Frquncy db (0dB=1 Volts rms) Harmonic Distortion Nxt, harmonic distortion was masurd on th right channl. Th function gnrator virtual instrumnt was usd to provid a 1000 Hz sin wav to th amplifir. Th powr spctrum (in db Vrms) of th amplifir output is shown in Figur 4. Notic, th scond harmonic is on th ordr of 60 db down from th fundamntal. Th scond harmonic dominats th total harmonic distortion prformanc masurmnt. db Right Out/Frquncy 0dB=1 Volts rms KHz. Figur 4 - Harmonic Distortion Nois Finally, th nois from th amplifir with no input signal was masurd. Th rsults ar shown in Figur 5. Typically in most amplifirs, th low frquncy spctrum is dominatd by 1/f nois. Incrasd frquncy rsolution would provid dtails of th low frquncy nois, but this is not rally ncssary for computing a signal to nois ratio from th data KHz. Figur 5 - Amplifir Nois Spctrum To comput th signal to nois ratio, th powr in all th spctral lins, xcluding dc, is summd. Th ratio of th nois powr and th amplifir s maximum ratd output powr is thn computd and transformd to a dcibl rprsntation. Th computation must also account for th attnuator in th LAC. Automatd Masurmnts Th SigLab-MATLAB combination is idal for masurmnt automation. SigLab has automatd vrsions of its virtual instrumnts (S th Application Not ntitld Automating Masurmnts with SigLab) that tak th pain out of masurmnt automation. Whn ncssary, th masurmnt conditions can also b controlld from within MATLAB by using calls to siglab.dll. A Simpl Amplifir Tst GUI Figur 6 shows a simpl application, writtn in MATLAB, to tst crosstalk, total harmonic distortion and signal to nois ratio. Application Not 8.1 Using SigLab for Production Lin Audio Tst 3 11/12/96SLAP8_1

4 outputs wr simply connctd to th SigLab inputs. Nothing ls is changd. Ths cabls thn bcam th amplifir undr tst. Figur 6 - Simpl GUI for Automatd Amplifir Tst. Th oprator intrfac is simpl, containing only thr push buttons and a txt ara to display tst rsults. Th Toggl Plots button turns graphs on for dbugging and dmonstration; or off for maximum throughput. Th Tst Rsults Th printd rsults in Figur 6 agr with th prvious masurmnts. Th tst tim rport is 0.94 sconds, which satisfis th scond goal of complting all tsts undr 1 scond. Th "Goldn Amplifir" As prviously mntiond, a 20 db attnuator maks it simpl to analyz a prfct amplifir. For this cas, th actual amplifir and LAC wr rmovd and th SigLab Figur 7 - Masurmnt Rsiduals Th rsults of ths masurmnts ar shown in Figur 7. Not, th rsidual nois and distortion lvls du to SigLab ar far smallr (about 20 db) than a typical ral audio amplifir. Th Masurmnt Algorithms. Th following sctions provid a dscription and listings of th masurmnts and calculations don in MATLAB to implmnt th thr tsts. A dscription of th dsign procss is also providd. 4 Application Not 8.1 Using SigLab for Production Lin Audio Tst 11/12/96 Slap8_1

5 % cross talk cod fragmnt from m-fil at2.m sin_frq = [1000,10000]; % us two ton option... rms_lvl = 0.6; % xcitation lvl siglab('outlvl',1,1.414*rms_lvl,'offst',0); % st lvl, thn th function siglab('outsin',1,sin_frq(1),sin_frq(2),1); % two tons, sam amplitud siglab('outlvl',2,0,'offst',0); % shut down othr gnrator siglab('outsin',2,10); % shut down othr gnrator % mak a spctrum masurmnt using stup stat in fil xtlk.vsa [AspcDat1,Fvc, WinPwrCor, ChanStat]=vsa_auto('mas2out','xtlk.vsa','nw'); % comput cross talk at th two givn frquncis... df = Fvc(2)-Fvc(1); indx_1 = 1+sin_frq(1)/df; indx_2 = 1+sin_frq(2)/df; xtlkl_r = 10*log10(AspcDat1([indx_1,indx_2],2)./AspcDat1([indx_1,indx_2],1)); siglab('outlvl',2,1.414*rms_lvl,'offst',0); siglab('outsin',2,sin_frq(1),sin_frq(2),1); % two tons, sam amplitud siglab('outlvl',1,0,'offst',0); % shut down othr gnrator siglab('outsin',1,10); % shut down othr gnrator % st gains to optimiz R to L xtalk siglab('inpgain',1,0.078,'ac'); siglab('inpgain',2,1.2,'ac'); % using th 'rpat' mod has two ffcts: % 1) it savs som tim sinc stup info is not rsnt % 2) it dos not ovrwrit th gain stting just st [AspcDat2,Fvc, WinPwrCor, ChanStat]= vsa_auto('mas2out','xtlk.vsa','rpat'); xtlkr_l = 10*log10(AspcDat2([indx_1,indx_2],1)./AspcDat2([indx_1,indx_2],2)); Listing 1 - Cross Talk Masurmnt Cross Talk Listing 1 shows th dtails of th cross talk masurmnt. First, th cross talk btwn th lft-to-th-right channl is masurd. Th output gnrator connctd to th lft channl is st to provid two tons at 1000 and Hz, whil th othr gnrator is turnd off. Th spctrum of both th amplifir lft and right output channls is masurd. Idally, thr would b no output on th right channl. Th ratio of th lft channl to right channl powr is computd at th two ton frquncis and convrtd to db to produc th cross talk masurmnt. Nxt, SigLab's output channl sttings ar rvrsd to masur th right to lft amplifir cross talk. A shortcut is utilizd to improv th masurmnt tim rathr than rst th ntir SigLab front nd, calls ar mad dirctly to th siglab.dll to swap th input subsystm gain sttings. Thos ar th only input paramtrs that nd to b changd. Th "rpat" fatur of th vsa_auto.m allows a rptition of th prvious masurmnt without snding nw stup information to SigLab. Application Not 8.1 Using SigLab for Production Lin Audio Tst 5 11/12/96SLAP8_1

6 % Harmonic Distortion cod fragmnt from at1.m sin_frq = 1000; rms_lvl = 0.707; siglab ('outlvl,'1,1.414*rms_lvl,'offst',0); siglab('outsin',1,sin_frq); siglab('outlvl',2,0,'offst',0); siglab('outsin',2,sin_frq); % prform 2 sparat masurmnts for THD sinc finit amplifir cross talk % causs masurmnt variation which is a function of th rlativ % (and arbitrary) phass of th sin xcitations [AspcDat1,Fvc,WinPwrCor, ChanStat]=vsa_auto('mas2out','hd12.vsa','nw'); ovld_1 = ChanStat(1,ovld_i); % log ovrload info df=fvc(2)-fvc(1); i_fund = 1+sin_frq/df; i_scond = 1+2*sin_frq/df; % for THD, pick out only th harmonics.assum othr nois pickd up % in nois masurmnts, attnuation factor dos not mattr hr. thd1_db = 10*log10(sum(AspcDat1(i_scond:(i_fund-1):lngth(AspcDat1),1)),..../AspcDat1(i_fund,1)); %th ATTEN variabl is to th attnuator, us th LAC pwr1_fund = (AspcDat1(i_fund,1)/Load_rs)/(Attn^2); siglab('outlvl',2,1.414*rms_lvl,'offst',0); siglab('outlvl',1,0,'offst',0); % not: although 2 Aspc masurmnts ar bing mad, and only on usd, % it is fastr to mak th 2 masurmnts than it is to rspc stup of % thrfor, th masurmnt dfind by hd12.vsa is 'rpatd'. [AspcDat2,Fvc, WinPwrCor, ChanStat]=vsa_auto('mas2out','hd12.vsa','rpat'); ovld_2 = ChanStat(2,ovld_i); % log ovrload info thd2_db = 10*log10(sum(AspcDat2(i_scond:(i_fund-1):lngth(AspcDat2),2)),..../AspcDat2(i_fund,2)); pwr2_fund = (AspcDat2(i_fund,2)/Load_rs)/(Attn^2); Listing 2 - Distortion Masurmnt Harmonic Distortion Du to th xistnc of amplifir cross talk, th harmonic distortion masurmnt must also b mad sparatly for ach channl. Listing 2 shows th cod fragmnt implmnting th distortion masurmnt. SigLab s output gnrator producs a singl sin wav at 1000 Hz at a spcifid amplitud. Nxt, th powr spctrum of th amplifir output is masurd. To comput th total harmonic distortion, th powr at spctral componnts lying xactly on harmonics is summd and thn th ratio of this harmonic powr and th fundamntal powr is takn. Th rsults ar xprssd in db. Th amplifir output powr is also of intrst, and is computd from th powr in th fundamntal ton. Th LAC attnuation factor and load rsistor must b known to mak this calculation. Again th vsa_auto.m rpat function is usd to masur th distortion of th right channl to sav masurmnt tim. No changs ndd for th input stup, just th output gnrator stats ar swappd. 6 Application Not 8.1 Using SigLab for Production Lin Audio Tst 11/12/96 Slap8_1

7 % Signal to Nois Ration masurmnt and calculation % ******************************************************************** % amplifir and intrfac paramtrs Pout_max = 5; % 5 watts assumd max rms output Load_rs = 8; % load rsistanc Attn = 47/(510+47); % attnuator aftr load rsistor % *************************************************************** st(hat2_([qpb_i,tstpb_i]),'nabl','off'); % disabl quit,tst pbs whil tst in progrss % nois floor masurmnt % st both siglab outputs to 0 sin_frq = 10; rms_lvl = 0.0; for i=1:2 siglab('outsin',i,sin_frq); siglab('outlvl',i,1.414*rms_lvl,'offst',0); nd; [AspcDat1,Fvc,WinPwrCor, ChanStat]=vsa_auto('mas2out','nois1.vsa','nw'); % sum oprator sums lmnts in columns. 1 column pr channl % ignor dc bias, rmov by starting at lmnt #2 nois_pwr = sum(aspcdat1(2:lngth(aspcdat1),:))/load_rs; % b sur to includ attnuation factor in th calculations snr_db = 10*log10(Pout_max*nois_pwr.^(-1))+20*log10(Attn) Listing 3 - Signal to Nois Ratio Signal to Nois Ratio First, th nois powr output of th amplifir is masurd with no input signal. Thn th signal to nois ratio is computd by taking th ratio of this powr to th maximum th amplifir can produc whil staying within its distortion spcifications. For this masurmnt both SigLab outputs ar turnd off and th lft and right channl powr spctrums ar masurd simultanously. Th maximum amplifir output, load rsistor valu, and attnuation factor of th LAC must b known for this calculation. Conclusion SigLab maks quick and accurat automatd masurmnts of ky audio amplifir paramtrs. A simpl MATLAB GUI was cratd to run th tsts, display th rsults, and track th masurmnt tim. Th rsults can asily b compard to tst limits for sorting or go/no-go tsting. Th SigLab-MATLAB combination is idal for production-orintd audio tsting. For mor information contact: Spctral Dynamics 1010 Timothy Driv San Jos, CA Phon: (408) Fax: (408) siglabsupport@sd-star.com Spctral Dynamics, Inc. SigLab is a tradmark of Spctral Dymanics, Inc. MATLAB is a rgistrd tradmark and Handl Graphics is a tradmark of Th MathWorks, Incorporatd. Othr product and trad nams ar tradmarks or rgistrd tradmarks of thir rspctiv holdrs. Printd in U.S.A.