Charge-Pump Phase-Locked Loops

Transcription

1 Phase-Locked Loos Charge-Pum Phase-Locked Loos Ching-Yuan Yang National Chung-Hsing University Deartment of Electrical Engineering

2 Concetual oeration of a hase-frequency detector (PFD) PFD 5- Ching-Yuan Yang / EE, NCHU

3 Phase detector : PFD three-state PD A B A B A A State State State Q A Q A A Q A = Q B = 0 Q A = 0 Q B = 0 Q A = 0 Q B = B Q B Q B B B A A State diagram B Q A B Q A Q B Q B Timing diagram 5- Ching-Yuan Yang / EE, NCHU

4 - mlementation of PFD nut-outut characteristic: PFD followed by low-ass filters: 5-3 Ching-Yuan Yang / EE, NCHU

5 - Phase detector : PFD When locked, the hase difference is 0 degree Outut voltage indeendent on the inut signal amlitudes Outut voltage indeendent on the inut duty cycles Wide linear range (Wide lock range) Discriminate frequency difference Use caully for Data/Clock Recovery PLL - Hybrid PLL (Analog PLL + Digital PLL) Dead Zone roblem Due to finite gate delay ntroduce large jitter or oor hase noise 5-4 Ching-Yuan Yang / EE, NCHU

6 Charge-um PLL Phase/Frequency Detector Charge Pum Loo Filter VCO Vi R V U D i R,C Vcont Kv Vout Why charge um PLL? Advantages No active comonent for zero steady state hase error Large frequency and hase cature range Digital outut (full CMOS swing) Simle and robust design Discrete time analysis Disadvantages Slow comaring with analog PLL May create dead zone roblem Noisy 5-5 Ching-Yuan Yang / EE, NCHU

7 Addition of zero to charge-um PLL ( nd order) PFD V DD P D Q u S V in rst i e Loo Filter V cont VCO V out D rst Q dn S R C V DD P Oen-loo transfer function out P KVCO ( s) R a zero at s z = /(R C ). in Cs s oen Closed-loo transfer function H( s) n P K C VCO R CK P VCO s P K C P K VCO VCO R s P C and decay time constant R C s K VCO 4 n R K VCO 5-6 Ching-Yuan Yang / EE, NCHU

8 Comensated tye PLL PFD V DD P D Q u S V in rst i e Loo Filter V cont VCO V out i e P t D rst Q dn S R C Vcont P V DD P t Critical drawback : Since the charge um drives the series combination of R P and C P, each time a current is injected into the loo filter, the control voltage exeriences a large jum. n lock condition, the mismatches between and and charge injection and clock feedthrough of S and S introduce voltage jums in V cont. The resulting rile severely disturbs the VCO, corruting the outut hase. 5-7 Ching-Yuan Yang / EE, NCHU

9 PLL frequency synthesizer using dividers n the locked state: F VCO = N F R, N F VCO / N = F X / R = F R 5-8 Ching-Yuan Yang / EE, NCHU

10 Linear model of 3 rd -order PLLs with nd -order loo filter PFD/CP Loo Filter VCO e div K PD Z LF (s) K vco s o Main Divider N PLL hase transfer functions: Forward-loo gain Reverse-loo gain Oen-loo gain Closed-loo gain o K PDZ LF ( s) K G( s) e s div ( s) o N div K PDZ LF ( s) K ( s) G( s) e Ns o G( s) ( s) G( s) vco vco 5-9 Ching-Yuan Yang / EE, NCHU

11 - nd -order assive filter i e R C V cont Z LF ( s) k s s z s k s s z s b z C Z LF ( s) s R C C sr C s( C C ) s( C C s z )( s ) i e Parameters Loo Filter Loo Filter V cont z R C R (C + C ) R C C C R R C C C C k b z b b C C C C C C Z LF ( s) sr ( C C) sc ( sr C ) 5-0 Ching-Yuan Yang / EE, NCHU

12 - Oen-loo bandwidth c and hase margin m Oen-loo gain K Z ( s) K K div PD LF vco P vco ( s) G( s) e Ns N s K P vco ( s) G( s) s j N k j z j k s z s () tan() tan() 80 z Gain Higher P K VCO Phase 0 db c z Lower P K VCO c c m,max : d z 0 d ( ) ( ) c z z m,max Bode lot of oen loo resonse 80 o and m,max tan z z tan b b 5- Ching-Yuan Yang / EE, NCHU

13 f the loo bandwidth c and the hase margin m are secified, we have b tanm m 0 O 30 O 40 O 45 O 50 O 55 O 60 O 70 O 80 O b cosm sqrt(b) and For loo filter : P Kvco b P Kvco C c R R N b N C C N b R c P Kvco b C z z and C R R For loo filter : P Kvco b P Kvco C C c R R N b N C R C N b P Kvco b z and R c C z R 5- Ching-Yuan Yang / EE, NCHU

14 Active loo filter imlementation C C i e R V cont The active loo filter is often used when the charge-um outut can not directly rovide the required voltage range for tuning of the VCO. Such voltages are incomatible with charge-ums built in standard C technologies, so that a (artly external) active loo filter is then used to isolate the charge-um outut from the VCO tuning inut, and to generate the high tuning voltages. 5-3 Ching-Yuan Yang / EE, NCHU

15 Design flow of 3 rd -order PLLs Determine K VCO. Determine the nominal value of N according to the system to be alied to. Deending on the desired noise and transient erformance, determine the loo bandwidth C. Select P, to meet the reasonable trade-off between the value the filter comonents (i.e., chi area) and the um current. Select the required hase margin m or b. With K VCO, N, C, P and b determined, calculate R. With z and determine by C and b, calculate C and C. 5-4 Ching-Yuan Yang / EE, NCHU

16 Multi-ath charge-um filter (/3) i e i e V C a V a V cont V b R b C b V cont V b V a z c log( ) i e e ie e The loo filter transfer function is Vcont () s K dz LF () s Rb e() s sca sc b scb Cb Ca sca srbcb Rb Cb Ca RbCa z b b R C Kvco Kvco R N C N c b a z The caacitor C a can be multilied by n / ( ). J. Craninckx, EEE JSSC, Dec Ching-Yuan Yang / EE, NCHU

17 Multi-ath charge-um filter (/3) The oen-loo transfer function is K f KVCO s z G() s N s s C b where K f Rb nc a c Cb nc a R K N b VCO c is larger than the traditional one. i e e ie e The filter transfer function is V srb nca Cb cont F() s i s nr C C snc nc e b a b a Rb Cb nca nrbca b R C a z C b F s V i cont b a () e sr nc snc a n The closed-loo transfer function is PKVCO RbnCa s G() s N nca H () s () G s K R K s s N NnC K P VCO b P VCO R nc K P VCO b P a VCO n nca b a 5-6 Ching-Yuan Yang / EE, NCHU

18 Multi-ath charge-um filter (3/3) The oen-loo transfer function is G() s where c K f KVCO s z N s s C b K f n Rb C a Cb nc a n R K N b VCO c is larger than the traditional one. i e e ie e The filter transfer function is V srb nca Cb cont F() s i s nr C C snc nc e b a b a Rb Cb nca nrbca b R C a z C b F s V i cont b a () e sr nc snc a n The closed-loo transfer function is PKVCO nrbca s G() s N Ca H () s () G s K nr K s s N NC K P VCO b P VCO nr C K P VCO b P a VCO n Ca b a 5-7 Ching-Yuan Yang / EE, NCHU

19 Nonideal Effects in PLLs 5-8 Ching-Yuan Yang / EE, NCHU

20 PFD and charge-um filter V DD PFD P Owing to the finite risetime and falltime V D Q rst u i e resulting from the caacitance seen at the nodes, the ulse may not find enough time to reach a logical high level, failing to turn V div rst D Q dn on the charge um switches. For 0, the charge um injects no current. V DD P The loo gain dros to zero and the outut hase is not locked. i e The PFD/CP suffers from a dead zone equal to 0 around = 0. Jitter resulting from the dead zone e Dead Zone 5-9 Ching-Yuan Yang / EE, NCHU

21 Reference surs V DD D PFD Q u P Periodic disturbance of VCO control line due to charge um activity: V rst i e i e ( t ) P c P c n cos( nf t ) rst DC Sectral comonents V div D Q dn Main effects which generate erence surious breakthrough: V DD P leakage current in the loo filter, V skew between u and down (dn) signals, V div u dn mismatch in the charge u and down current sources, Charge sharing. t 5-0 Ching-Yuan Yang / EE, NCHU

22 - Effect of leakage current leak i e c Locking osition e Sources of leakage currents: the caacitor of loo filter, the inut of VCO, the charge-um outut, the inut biasing current of the o-am, when active loo filter configuration is used. The duty cycle of the charge-um outut is i e P c leak c leak P The amlitude of charge-um outut: i e ( t ) leak leak n cos( nf The sectral comonent are twice the value of leak. Not deendent on the nominal charge-um current P. t ) 5- Ching-Yuan Yang / EE, NCHU

23 Link the leakage current to the magnitude of the surious comonents at the outut of the VCO: Phase deviation ( n P f f ( n f ) n f V rile ) V ( n f ) leak Z LF ( jnf rile ( n n f f Each of baseband modulation frequencies nf generates two RF surious signals at offset frequencies nf from the carrier f LO. The amlitude of each surious signal P ( n f ) leak Z LF ( jnf ) Kvco ASP ( flo n f ) ALO ALO n f A SP A A LO ) K vco ( flo n f ) leak Z LF ( jnf ) K A n f SP LO dbc ( n f 0log ) 0log leak Z LF vco leak Z ( jnf n f LF n 5- Ching-Yuan Yang / EE, NCHU ) ( jnf ) K f vco [dbc] The relative amlitude of the surious signal is not deendent on the value of loo bandwidth or on the nominal charge-um current P. Theoretically, if leak = 0 there are no erence surs in the outut. ) K vco

24 - Effect of skew between u and down signals 5-3 Ching-Yuan Yang / EE, NCHU

25 - Effect of mismatch in the charge-um current sources PFD V DD V b M 4 D Q u u M V rst u i e V cont rst V DD dn C V div D Q dn dn M V DD V b M 3 skew suression u dn u dn e u u dn dn i e i e V cont t V cont t 5-4 Ching-Yuan Yang / EE, NCHU

26 - Effect of mismatch in the charge-um current sources For the loo to remain locked, the average value of V cont must remain constant. The PLL theore creates a hase error between the inut and the outut such that the net current injected by the charge um in every cycle is zero. The control voltage still exeriences a eriodic rile. Owing to the low outut imedance of short-channel MOSFETs, the current mismatch varies with the outut voltage. The clock feedthrough and charge injection mismatch between M and M further increases both the hase error and the rile. The magnitude of the sectral comonents of the rile voltage due to currentsource mismatch can be found V mismatch ( n f ) out ( n f ) Z LF ( jnf ) A s ( n f ) out ( n f ) Z LF ( jnf A 0log LO n f dbc ) K vco 5-5 Ching-Yuan Yang / EE, NCHU

27 - Effect of charge sharing V DD V DD V b M 4 C Y V b M 4 C Y Y Y V Y S S V cont V cont V cont X S C P X S C P V X t V b M 3 C X V b M 3 C X Charge sharing between C P and caacitances at X and Y: S and S are off, allowing M 3 to discharge X to ground and M 4 to charge Y to V DD. At the next hase comarison instant, both S and S turn on, V X rises, V Y falls, and V X V Y V cont. f the hase error is zero and D3 = D3, does V cont remain constant after the switches turn on? Even if C X = C Y, the change in V X is not equal to that in V Y. 5-6 Ching-Yuan Yang / EE, NCHU

28 - Effect of charge sharing V DD Bootstraing X and Y to minimize charge sharing: P When S and S turn off, S 3 and S 4 turn on, Y S S 4 + allowing the unity-gain amlifier to hold nodes X and Y at a otential equal to V cont. S C P V cont At the next hase comarison instant, S and S turn on, S 3 and S 4 turn off, and V X and V Y begin with a value equal to V cont. X S 3 The ideal is to in V X and V Y to V cont after hase P comarison is finished. Thus, no charge sharing occurs between C P and the caacitances at X and Y. 5-7 Ching-Yuan Yang / EE, NCHU