Frequency tuning loop for VCOs

FREQUENCY TUNING LOOP FOR VCOS B. Lina es-Ba anco’, A. Rod iguez-Vbquez2, E. Shchez-Sinencio’ and J. L. Hue as2 ’ Depa men o Elec ical Enginee ing Texas A&M Uni e si y, College S a ion, TX 77843-USA Dp o. de Diseiio Analbgico, Cen o Nacional de Mic oelec bnica, 41012-Se illa, Spain Abs ac -Fo a Vol age Con olled Oscilla o (VCO) he oscilla ing equency is con olled by an ex e nal inpu . The ela ionship be ween he oscilla ing equency and he inpu con ol ol - age is, in mos cases, expec ed o be linea and independen o pa asi ics, p ocess pa ame e s, empe a u e, and o he non-ideali i es. In his pape we in oduce a equency uning ci cui o VCOs so 4a he Anal ela ionship be ween oscilla ing equency and inpu con ol ol age is Axed and independen o he p e ious men ioned non-ideali ies. This uning loop is applied o an OTA-C sinusoidal VCO. Such an oscilla o has an ou pu equency-inpu ol age ela ionship ha depends on empe a u e, p ocess pa ame- e s, and e en ampli ude o he oscilla ions. I is shown ha by adding he uning loop all hese non-ideal dependences will be minimized. I. INTRODUCTION Vol age Con olled Oscilla o s a e key componen s in many signal p ocessing and communica ion sys ems [l-31, and a e also used o he uning o monoli hic il e s [4-91. They alsomay be used o ins umen a ion applica ions [2]. In many cases i is necessa y ha he ela ionship be ween he ac ual oscilla ing equency o he VCO and i s inpu con ol ol age is e y p ecise and does no change wi h empe a u e, p ocess pa ame e s o pa asi ics. Conside a ce ain VCO (sinusoidal, iangula , o squa e wa e,) ha has an “uncon olled” inpu ol age- ou pu equency cha ac e is ics. We will show ha by adding he equency uning loop desc ibed in his pa- pe he inpu ol ageou pu equency ela ionship can be made e y linea and comple ely independen o sec- onda y pa ame e s ( empe a u e, ampli ude, pa asi ics, and p ocess pa ame e s) wi hin he uning ange o he o iginal VCO. The uning loop p oposed in his pape is alid o any ype o VCO. We a e going o e i y i s ope a ion by applying i o an OTA-C sinusoidal VCO [lo]. Sinusoidal OTA-C VCOs ha e a nonlinea inpu ol ageou pu equency ela ionship ha depends on p ocess pa ame e s, empe a u e, pa asi ic componen s and e en ampli ude o he oscilla ions. By adding he p oposed uning loop we will show ha his ela ionship will become linea and independen o he p e iously men ioned pa ame e s. 11. THE FREQUENCY TUNING LOOP Figu e 1 shows he basic s uc u e o he equency uning loop [11,12]. co “OU? I Squa e- wa e Con e e F equency Di ide by N (Op ional) Fig. 1. Block Diag am o F equency Con ol Loop. The ou pu o he oscilla o (i no squa e) is con- e ed o a squa e wa e, di ided by N (op ional) and applied o a F equency- wVol age (FTV) con e e . Usually he maximum ope a ing equency o an oscil- la o is highe han he maximum possible equency in a F equency- o-Vol age Con e o . Fo his eason he inclusion o he equency di ide in Fig. 1 is many imes manda o y. As we will see in he nex sec ion, he FTV con- e e is a de ice such ha i in is he equency o i s inpu signal, i s ou pu ol age will be gi en by in REF Vz = n VBIAS- Whe e n is a posi i e in ege , VBlAS is a e e ence ol - age and REF he equency o a e e ence squa e wa e CH 30064/91/0000 - 2617 $1.00 C IEEE signal (like a c ys al oscilla o ). I is he ou pu e- quency o he VCO and a equency di ide is used wi h a a e N, hen in = /N. I p-' is he ime cons an o he in eg a o , i s ou pu ol age will be gi en by: V = ~( co - z) (2) On he o he hand, o a CMOS OTA-C sinusoidal os- cilla o we ha e 1131, (3) K = e = -(V - ih) 2* c 2TC whe e K and V h a e gi en by he opology and an- sis o geome ies o he OTAs used as well as by he p ocess pa ame e s. Sol ing equa ions (l), (2) and (3) yields, Vco - VBIAS- N REF which has a s able s a iona y poin (!$ < 0) cha ac- e ized by N Vco = REF- ~VBIAS (5) The ela ion be ween (ou pu equency) and V,, (in- pu ol age) is shown in Fig. 2. No e ha i is linea and i depends only on REF, VBIAS, n and N. In he nex Sec ion we will gi e he implemen a ion de ails o he FTV con e e , which is he key compo- nen o he equency uning loop. , Fig. 2. Fkequency s con ol ol age ela ionship o V,, wi h equency uning loop. 111. The FTV Con e e Conside he ci cui eo Fig. 3. I con ains a monos- able ha p oduces a pulse o du a ion o = 1 Du ing his pulse, capaci o C is cha ged by a cu en go (~~VBIAS - z). The es o he ime i is discha ged by goV,. The wa e o m ha esul s a he capaci o u( ) is shown in Fig. 4. Fo each cycle o u( ), Vz( ) is he peak alue o u( ) o he p e ious cycle. Two consecu i e peak alues a e ela ed by 2 REF ' V,( +nT) = V,( )-g"Vz(NT- o)+g"(2n ~~~s- z) , whe e T = 1/ . I is cons an and he FTV con e e C C (6) eaches i s s eady s a e, hen Vz( + NT) = Vz( ), i.e., A s abili y condi ion o he s eady s a e can be ob- ained by w i ing (6) in he Z-domain and making he poles o be inside he uni y ci cle. The esul ing condi- ion is [E], (8) Ngo '2C A con enien IC implemen a ion o his FTV con e e is shown in Fig. 5. To imp o e he ime esponse, he swi ch o Fig. 3 has been eplaced by a di e en ial pai and cu en mi o s. The cu en sou ces a e imple- men ed wi h linea ized OTAs. o = , --I+ MONOSTABLE N I 1 4 De ec o Uj , Peak &- - - Fig. 3. Fkequency- o-Vol age Con e e - u( ) Fig. 4. Time wa e o ms o U( ) inside he FTV Con e e , and o Vx( ), i s ou pu . s 1. --cHc Ju-l- V, I J Fig. 5. CMOS Ci cui Implemen a ion o FTV Con e e 2618 The monos able has o p o ide a pulse o du a ion o = 1/2 REF. such a ci cui is shown in Fig. 6. The du a ion o he ou pu pulse o is gi en by he ime i akes o cha ge capaci o CI om OV o VREF by he cu en IREF. VREF is gi en by he ol age o which Cz is cha ged, s a ing a OV by a cu en IREF du ing a ime o = 1/2 REF. The e o e, i he wo ci cui s enclosed by b oken lines in Fig. 6 a e pe ec ly ma ched, he du a ion o he monos able ou pu pulse will be o = 1/2 REF, independen o IREF and cl = cz = c. I he e is a misma ch be ween he wo ci cui s he wid h o he monos able ou pu pulse will no be equal o hal he pe iod o e e ence signal REF. Howe e , besides his, he ope a ion and cha ac e is ics o he uning loop s ill emain basically in a ian . --------------- _L ICIl I1 I- - - a ____-- M. Chcd ChUi . - c--c I. Fig. 6. Block Diag am o Monos able wi h Ou pu Pulse Wid h Con olled by an Ex e nal Squa e Wa e Signal o P ecise F equency. IV. EXPERIMENTAL RESULTS In o de o es heop oposed equency uning loop s a egy we used a sinusoidal OTA-C oscilla o [10,12]. I s equency dependence is shown in Fig. 7. No e ha he ela ionship be ween ou pu equency and inpu ol age (Bias Vol age) is nonlinea and changes wi h he ampli ude o he oscilla ions. A disc e e equency uning loop was implemen ed a ound he p e ious CMOS OTA-C oscilla o . Since he ou pu equency gdes o e 13MHz, a as compa a- o (CM710C) was used o con e he sinusoid o a squa e wa e signal. Fo he monos able a con en ional 555 ime was used. In his case, he wid h o i s ou pu pulse is no con olled by an ex e nal equency e e - ence, bu by a p ecise RC cons an . Also, since his ci cui has e y bad ime esponse (p opaga ion imes a e a ound lOOns he ou pu o he oscilla o had o be di ided by 32. The esul ing ou pu equency-inpu ol age ela ionship o he VCO wi h equency uning loop is shown in Fig. 8. This cha ac e is ic is he same o di e en alues o he oscilla ing ampli udes, o di - .O Bias Vd og. ( ol s) Fig. 7. F equency s Bias Vol age o sinusoidal OTA-C Oscilla o wi hou equency uning loop. e en empe a u es and o di e en IC chips. A his ime we do no ha e a ailable a comple e IC p o o ype o quan i y he deg ee o linea i y, which is highly de- penden on he implemen a ion. 14.0' 13.0. 12.0, 11.0 CI Y E" '"5 8. ;:5 1 6.0 5.0- 4.0. 3.0. 2.0 + 1:O 1:1 1:2 1.3 114 115 1.6 1.7 F oquoncy Con ol V&ap (Vdb) Fig. 8. Measu ed Oscilla ion F equency s F equency Con ol Vol age o VCO wi h F equency Tuning Loop. V. CONCLUSIONS A equency uning loop o VCOs is p esen ed ha compensa es o empe a u e, p ocess pa ame e s, am- pli ude, and pa asi ics in luence in he o iginal ou pu equency-inpu ol age ela ionship. This uning loop can be applied o any ype o VCO. I s iabili y has been demons a ed, by applying i o a sinusoidal OTA-C VCO. The uning loop was cons uc ed using a b ead- boa d p o o ype, bu i can be in eg a ed oge he wi h he oscilla o in he same IC die [14]. 2619 VI. REFERENCES [ 11 K.K. Cla ke and D.J. Hess, “Communica ion Ci - cui s: Analysis and Design”, Addison Wesley 1978. 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