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ori Ana A Bubble Memory Differential Detector By W.D. WYNN. (tanusript rested September 5, 1960) In the application of digital memories, significant variations ean ‘cur in the memory sensor analog respanser processed by detection circuitry t recover stored Binary data Across an ensemble of meraory devices of the same type, the variations are due to manufacturing deviations, and for « particular device, they accur ith changes tn ‘operating conditions, With magnetic bubble memories, inary data tare recovered. by sensing the presence or absence of @ magnetic domain in each Bit interval. In general nonadaptive bubble detetor does not accommodate the variations (Bat may occur in the bubble ‘response when each decision is based onty on the sensor responce in (he current bit interval. Here we present aiferential detector design that is reasonably simple to implement ae one integrated creat ad {stolerant of signal amplitude drifts in bubble memory sensor outputs, 1. mTRODUCTION In typieal Feld -uccessed magnetic bubble memories (MM), stored binary data on memory chips aro detacted by procaing miivele-Lovel, signals that are measured differentially across a matebed pat of permalloy magnotoresstive censor strips." Mealy these senses are subjected to the came magnetic, electrical, and environmental cni- tions with the exception chat only one, the netive sonsor, iv subjected to H fields of traversing bubble domainn tn the drive field period ofa field-ncessel MmBt chip, by design at least one time interval exits, defined here as the detection interval (ot), in which the sensor differ: ‘ential response is zelatively fre of unavoidable signal interferences sich as omit function drive erosstall and sensor magnetoresstive tllching noite, Within che 01 a deteccable diference should exit ‘between recponses forthe buble nnd no-bubble cycles ofthe memory. yen after carefully choosing the bi undesirable sens reponse variations can sil occur. Variations within the nr that hive been of major concer in the ite of previous nonatlaplive amet detoctona are 495 the device processing varution in magnetovestive sensor response, the negative emnperature catfcient of this response, the phase shift bf bubble response tanetion that occurs with ehip manufacturing and thrive field changes, andthe adjacent symbol intruetion of bubbles in the surwor array. In a detector where the D is fixe and where the Individual bubble reeponse in the Dis compared to a fixed threshold theeo variations have ben intalerable "An write analytical parametric description ofthe fanily of sensor signals tat are observed for any Hr chip type has been intractable bectuse of the nonlinear elt effects of permalloy magnetaressive fensor arrays and a lack of tdele for satistesl data about the ‘manufacturing variations for arma devices. Figure | shows qualitative trample of differenced sensor reqponoes within apr chat may exist for {chip (ype when the manufacturing and operational variations are Considre Fach signal pare show in the ifr some particular chip ff an ensemble of the same chip ype at particular operating eondi- thom Allsignls ave been referenced coa voltage V al he tart of the pi.The amplitude and time seals for ig. 1 ure representative of fed- [coeaed nM with eld drive frequencies inthe 100-eilz range. Not only ean response amplitade change with opersting conditions and felected chip, but azo the ahapo of the desired bubble response and {ncersymbol interference contributions ay vary. ‘Detectors could be dasignod that adept to variations inthe bubble ‘and no bubble zeponaee whore such Variations preclude a xe thresh ‘ld decector. For extaple, detector amplificr gris rould be tempers tive compensated fo track the temperatare sensi of the magne toresiave somor. Also the phase of the Dr could be adaptive, and Aletection thresholds could be adjusted dynamically from responses in tu known data pattem. Unfortunately, adaptive bubble detectors have tlisedvantages auch a increase einitcomplesty inereared cost, and tystem limitations such a detector adaptation time before reading dca censor sponse for aiuemory op men “ 486 THEBELL SYSTEM TECHNICAL JOURNAL, APHIL 1981 We dusribe a diferentil detection approach, and circuit realiza tion of i, which are insensitive to some signal amplitude, phase and shape ‘aristions ip the Dl, if these wariationa occur slowly when ‘compared wieh the memory drive Held period. Tho detector is based fon cho hypothesis chat some signal function, such as peak-to-peak ‘transition or the signal energy, is available in a DY avch that the ‘minimum fonctional change between unlike daca pars is gretor than the muximum functional change between lke data pirs in memory Aecection cycles that are contiguous. This condition must hold over ‘the range of manofacturing and envivonmencal changes inthe sensor outputs forthe chip ensemble, inclding noise and interaembol ncar- ference effect. "This proposed deceetor can be developed as a small invxpensive 16 Ls ealy incorporated in ema irae desig, and it iscompilte with mecuory syste requirements. 2.1 Signal ond wmtarteronce characteveice of memory ovtout ‘As indicated by the qualitative results shown in Fig. 1 the dynuecic ‘ange of bubble and no-hubhle responses inthe nr aye significant ders the resined operating range for nn enaembe of Mn chip pe. However, fora given mot chip und stationary operating environment, Te responsissfor two dala 1's (bubles or for wo data On successive p's difer only through random noise and intersymbol inlrference tan ete, "Sensor rerponzee for bubhle and no-hubble caverele depend on the smomory tensor design. the thip technology, memory rmanufueturing quality contre, and eperning environment. Typiealy, che bobble Tesponse transitions of the wan sensor in the ni have been in the 2- nV to 2i-m¥ mange, with 4 snore rescrcted dynamic range over the fensemble and operating mange of any particular memory type, Al. ‘though uni-to-unitvarinions may exit in both bubble and no-bubble responses, an indicated by Tig. 1. 8 typical momory product gives & minimum Urunsition diflerence of about 2 mV? between the (wo re mee the ‘When the phase of the ovis properly selected relative to sporadic vandom interference, auch aa mamercresictive switching aoise from. the sensor or control signal ross tall, residual noise within Ube nt becomes second-order lft, For raaray he nparcant equency ‘band of bubble signa a he Dy ix blow 10 f, where i the wae Hele ddnve frequency. Fur abu drive Trequerie of ler thar 200 KH, the ‘gn proving bund fora detootar i then lee than 2 MEH. For a 2 (MPs noise equivalent banalth fn a detector preamplifier, the total rrulom sie wid tha tia Jess than. BDV rma referred ¢o the ‘output contaces of the MRM chip mnrwor. Thit includes earaom toe 18 BUBBLE MEMORY DIFFERENTIAL ETECTOR 407 ‘fom the mei senaore under ative memory contions, the sensor bias cumene source, and the detector electronics, Fringe field souping of bubble domains cun exist wcrosselemenis in 1 aenaoe array. This effect in obverved as st in the Dt response Tike the desired Dr responce, the 1st deperus on the sensor array design and the memory operaling conditions, but iis pet well understood FHecaute of che nonlinear magaece interaction of permalloy verse arvaya and bubble domsing, accorace medeling and analysis of 51 ws ‘well asthe desired bubble respenge have heen intractable. Ta memory ‘esq, arays have bean selected to reduce the 18, aswell ws random noite, co acceptable leveli relative to the minimum difference between ola bubble and no-bubbla transitions in the Px, Thix condition ie Indicaved by Fig 22 fr contiguous bis for tw signal ample camen Ist is severe for « mecoory ces, dhs devign can be changed or signal equalization can be considered for the detector.” However, Inecase of the variable nature of ist as wll as tho desced signal's shape in the my an equalizer ahould be adaptive, For typloal useful Dt fdorations of afew ys and very short setup times of Uhe memory system, sulaptive equalizacion of 11 isnot cost effective in a bubble roemory detector. By carefal sensor design and proctssing control for the memory, i hus boon practical (o hold ter and noise to level that perm: diferentil detstion with probability of error Py -< 10" with ut using st equalization, 22 Date precoding end atorotiat recovery fram tre MBM ‘igure 1 indices that variations in he albolulelovel, phases, and shapes of bubble and no-bubble responsis along will x close match of like aymbol responses in contiguous Dis make differential amplitude detection alizetivw Tor fld-asoened axe. This it analogous to di fecentil phage election wed im communication channela® If the Pinay dete "1" i stored in the toni chip as a bubble-to-nb-bubble ‘transition, or the reverse transition in contiguous memory cycles, thee recovering daca from memory is possible when the absolute values of the greatest DI reqponse diferenee for pare of Tike symbole are Tent than the amallese aback values of differences for unlke symbol transitions. Figure 2b summarizos these proroyuiste conditions for ‘owas difference detection. Direct difference encoting of tw original data has the potential for ropoentingerrorsince «single error lesion ean real in Uns le (f phase reference and cause ambiguity in the detector. This problem ‘seliminatad by a simple preeading step for the data prior to reeording. "The pending doer not nedues the data throughput or torage capacity of the memory ‘An cviginal data sequence defined by (as), where ae € (0,1) for & 489 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1991 ya aco (mo pre ufone arakp 30 moana mao sadenen inte Linke aie Selene ee a 1A BURGLE MEMORY DIFFERENTIAL DETECTOR 489 0, is to be recorded as tamsitions between bubble and no-bubile Sstatos in consecutive field cycles in a way that eliminates propagating election errors through loa of reference. This is accomplished by precoding (as) into an equivalent sequence (B,), whe Gx € (0, 1} for [b= O,by the algorithm by= aD ben where ba=0, o snd denotes mdulo 2addivion, Then (bis recorded in the memory With a bubble state for by = 1 and a nodbubble sate for by = 0.'The precoding in (1) ia implemented with «shift register and an BX-OR logic stage inthe memory recording ctcuit. If, in response toy, a functional His available from the response in the pr and « decision of bubble or no-bubble in cycle isto be based om By, difference tection ix performed as foes. Tn the Bt data cycle the analog difernce C Be ~ Bans i formed. Wealy, there are three possible analog levels fr Cy carmeaponding tthe fur permutations of (2, Be), and a ternary decision on Cy resulta in a Ahroe-state logic output ex © (-1, 0, +). In terms of the binary sequence {e), a ar ei From expression (1), Ube movin 2 wsblrvetion ofp: from bath sides of O) giver (erdonts = (On~ Be shants= a wi {follows then thar {ay} ia available by simple logic eperations on the temary sequence {ec} obeained from (Ci) without the problem of corr propagation due to phase webigully ia deterlion eeror noes Th practice, the diferential detector outputs are excmates of the components of (ey) that are based on the available analog differences (Cx). Then the detector outputa form the estimate sequance {é,) and ‘the estimate of the orginal data sequence is 16d Bani), ABO. ” tin che Ath eycle there no deluction wer, 4 = 6 al eno dy ay. Figure 9 gives an example of the operalions diuwed in thie fection, where the functionals {B,) are the positive peaks of de ‘restored bubble and no-bubble responses in the detection intervals 23 Cholee of hnctiona B, ‘As discussed above, many varitions can occur in the sensor r0- sponces from a given type of mum chip which offect the shape, amplitude, neise content, and position of the responses in a DL To successfully detect diferences, any signa processing ofthe responses 490 THE BELL SYSTEM TECH®ICAL JOURNAL, APRIL 1981 Pig. Sia erection eae leading ta» sequence (Hs) mus sll in namnoverlning amplitude Janne forthe ctference functionals (Cag ehown in Fig 2, (a epile ‘of dunkerent variations in buble sensor response. A diver election ‘ot practical approaches in signal processing can be considered to obtain {a} from pr resprnses, amples of proctesing over the Wt for 2 ate -peak aequiiion, syutring-wich-inepration after de rear, rd larity ofthe nominal ble eqponse, and peak ac¥ixton Aer de restore Tach approsch haa advantages und divcvantages in detection error peeforntane and fetwctor cgeuit complesity fo Feld acosated not signals aa undertood to dake. Becaue f the Aelty 1 BUBBLE MEMORY DIFFERENTIAL DETECTOR 491 in modeling the statitenl charnterstica ofthe sm signal and inter. ference components, Ube anwlytien solution for @ useful optimum detoctor has boen intractable, The eelection of any detector, andy in particular any sanal processing approach te obtain {4} for the tlifernvinl deuecur ears Tere, has heen based primatily on the berlormance onerved in waperinenta with a representative ensemble ‘of doviens for each one design. The optimum ebuice of « deiner ix Complicated aldklonally by practical considerations of ite ereult re lation, cost, and Hlaibility of eperation. Section 2 describes a citferental detactor Uhl has heen aacessfut vith signal characteristics of presen bubble ernores, Thinrealiation ‘res poak detoctin of de zostored signals i the 0 to form (Bs). We Tne tout that signal equalization co reduce 1 and special noise Titering are not necessary in this detector for the bubble memory families of increst, 24 Detection erors and monitoring In Seotion 2.2 Une detected dain sequence {aah = (Gleato) o sche dis the temary logic aaquonce obtained by a bipolar threshold omparizon of Cr = 2 ~ Be. With (he cxoeption of nolse ond intergymbel effects in the gentcation wf (By), unly io values occur for Dy under tho contin of wlalic operation between memory detootion eyelen Then of the nine combinations of c. ex, the Bais (1) wad (1, —1) can oenr anly chrough dacoction error. The soqucnee {é,] can be monilored or che disallowed pattems ag asimple taror chu, ht this is not complete error test? Detection eons Thal yield valid o, ones pain cannot be deteeced by this simple tworioring I yenoea, an ertor within the memory chat xomlts im 9 ecovered bubble eequence other than the (0) hal. was rece fannot be devoctod with this pitiern mrnioring. The allered bubble Snjuence in the stsu appears at the detector as « differnt (¢.) sequence but one that could be generated by on admissible (a. rmeinory sequsnes ‘ince the diference detector ronsidened heve involves the threshold comparison uf C, = B.~ Bs, i would seem chat a deoetion error in recovery of a) = [exdneas cated by a disturbance in acquiring 0 Bs ‘ould be highly corslated with a dececton arur in tues = (eerDonts tice C1 = Bhny— By This apparent eoupling oss regards ofthe ite Torwing tenaved in Section 22 that ie used for prevencing fervor strings caused by detector rferemoe Tous Whether fi more probable in difference detection wo have an eor pie given thac one Error ceuurs will dept on the method of generating functional Br 492 THE BELL SYSTEM TECHNICAL JOUTINAL, APRIL 1981 and the interference that eases detection errors. It is known that in the came of additive Gaussian noise and matched filtering of signals ‘without intersymbol interference, differential detection docs not en Inanee the probability af double errs given single errors” [A PARTICULAR DETECTOR CIRCUIT REALIZATION "The circuit shown in Fig. performs the functions discussed in ‘Section 22, including forming the functional Ck = By ~ Ba roquized in dliflerence detection. The aystam characteristics outlinod below are compatible with avilable monaithic bipolar 10 technology. Spoed requirements implied by memory Geld frequencies up to 200 kllz seem ‘to make bipolae technology esantial at present for any analog differ lence detector type ofc. The realization of the functional Cie sple itrequires a capacitor, a clamping itch, and a bulferarplifier. Then ‘tg obtain Git is not necessary to construct a true sample-and-hold for ‘Buy which isan advantage inbipotarctcit integration. Alleapacitore ‘canbe integrated an the complete hubble detoctar implemented as a single inexpensive 1c wih three synckzonous input logic controls igure 6 show the key vollage waveforms forthe ciruit for the example data sequence (as) in Fix. 3. To simplify the operating flescription, any voltage offtts in swivches SW1, SW, and SW3 are reglected as are the droop effec due to leakage current fom capec- itors Cz, Cs, od C, when these rein their hold states. Adverse effects fetch offet in an ¢ con of te creltcan be eliminated by component trims during manufacture, At pointa inthe cizeuit where Teakaye currents from capactons are critical to performance, bipolar fol effec. transistr (uursr) amplifier isolation is wsuned these are shown in Fig. 4, HIFET ampiers have bias requirementa on the order of [nA within a circuit ambient range of [-10°C, + 70°C] “The dat cycle peed in Fig. ia 7 ff where fis che sume drive fhe fequeney. ‘The signal detovtion interval (on ie), modulo T- ‘The otber intervals of intent ae [4], modulo T, where switch ‘SW is cloned, al ff], modulo 7, where Cy ia dicharged through 'SW9, The munis aperating field frequency islimited bythe bubble ‘memory technology. ‘The period 1 eousiderul for the differential eector design isin the ange [5 us, 40 Th eaeh Dt, the Hy acquired is the maximum of the peak estcher input alls) + Vn for FE [tte] module 7, where ea) isthe incal condition of the voltage across Cy at. the start of the nr, and V; is the avitch voltage reference. The value of alt) a function of e(t) for €= fy, ma well ax the Mer action Iowied by the Rie. Co, Ry cavul sehin 8W1 ixcloed. The noise that 15 BUBBLE MEMORY DIFFERENTIAL DETECTOR 493 404 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1881