Short Course On Phase-Locked Loops and Their Applications Day 4, AM Lecture. Digital Frequency Synthesizers

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1 Short Course On Phase-Locked Loops and Their Applications Day 4, AM Lecture Digital Frequency Synthesizers Michael Perrott August 4, 2008 Copyright 2008 by Michael H. Perrott All rights reserved.

2 Why Are Digital Phase-Locked Loops Interesting? PLLs are needed for a wide range of applications - Communication systems (both wireless and wireline) - Digital processors (to achieve GHz clocks) Performance is important - Phase noise can limit wireless transceiver performance - Jitter can be a problem for digital processors The standard analog PLL implementation is problematic in many applications - Analog building blocks on a mostly digital chip pose design and verification challenges - The cost of implementation is becoming too high Can digital phase-locked loops offer excellent performance with a lower cost of implementation? 2

3 Integer-N Frequency Synthesizers div(t) e(t) v(t) F out = N F ref div(t) Phase Detect e(t) Analog v(t) out(t) Loop Filter Divider VCO Sepe and Johnston US Patent (968) Use digital counter structure to divide VCO frequency - Constraint: must divide by integer values Use PLL to synchronize reference and divider output Output frequency is digitally controlled N 3

4 Fractional-N Frequency Synthesizers div(t) e(t) v(t) F out = M.F F ref div(t) Phase Detect e(t) Analog v(t) out(t) Loop Filter Divider VCO N sd [k] Σ Δ Modulator N[k] M.F Dither divide value to achieve fractional divide values - PLL loop filter smooths the resulting variations Very high frequency resolution is achieved 4

5 The Issue of Quantization Noise div(t) e(t) v(t) F out = M.F F ref div(t) Phase Detect e(t) Analog v(t) out(t) Loop Filter Divider VCO Limits PLL bandwidth Increases linearity requirements of phase detector N sd [k] Σ Δ Modulator N[k] M.F Σ Δ Quantization Noise f 5

6 Striving for a Better PLL Implementation

7 Analog Phase Detection div(t) reset D Q D Q Reg error(t) phase error div(t) error(t) Phase Detect Analog Loop Filter out(t) div(t) Divider VCO Pulse width is formed according to phase difference between two signals Average of pulsed waveform is applied to VCO input 7

8 Tradeoffs of Analog Approach div(t) error(t) Phase Detector Signals Average of error(t) Phase Detector Characteristic phase error Phase Detect Analog Loop Filter out(t) div(t) Divider VCO Benefit: average of pulsed output is a continuous, linear function of phase error Issue: analog loop filter implementation is undesirable 8

9 Issues with Analog Loop Filter error(t) Charge Pump I cp V out C int Phase Detect Analog Loop Filter out(t) Divider VCO Charge pump: output resistance, mismatch Filter caps: leakage current, large area 9

10 Going Digital Phase Detect Analog Loop Filter out(t) Divider VCO Time -to- Digital Digital Loop Filter Divider DCO out(t) Staszewski et. al., TCAS II, Nov 2003 Digital loop filter: compact area, insensitive to leakage Challenges: - Time-to-Digital Converter (TDC) - Digitally-Controlled Oscillator (DCO) 0

11 Time-to-Digital Conversion

12 Classical Time-to-Digital Converter div(t) D Q Reg Delay Delay D Q Reg Delay D Q Reg e[k] div(t) Delay 0 0 e[k] div(t) Time -to- Digital Digital Loop Filter Divider DCO out(t) Resolution set by a Single Delay Chain structure - Phase error is measured with delays and registers Corresponds to a flash architecture 2

13 Impact of Limited Resolution and Delay Mismatch div(t) Delay varies due to mismatch 0 0 e[k] detector output Phase Detector Characteristic phase error div(t) Time -to- Digital Digital Loop Filter Divider DCO out(t) Integer-N PLL - Limit cycles due to limited resolution (unless high ref noise) Fractional-N PLL - Fractional spurs due to non-linearity from delay mismatch 3

14 Modeling of TDC Phase Detector Characteristic quantization error detector output Δt del time error phase error[k] T 2π t q [k] TDC Gain Δt del e[k] T reference period div(t) Time -to- Digital Digital Loop Filter Divider DCO out(t) Phase error converted to time error by scale factor: T/2π TDC introduces quantization error: t q [k] TDC gain set by average delay per step: Δt del 4

15 How Do We Improve Performance? Two Key Issues: TDC resolution Mismatch

16 Improve Resolution with Vernier Delay Technique div(t) D Q Reg Delay Delay D Q Reg Delay D Q Reg e[k] div(t) Delay 0 0 e[k] div(t) Vernier Delay Delay Delay D Q D Q D Q Reg Reg Reg Delay2 Delay2 Delay2 e[k] div(t) Delay 0 0 e[k] Effective resolution: Delay-Delay2 Delay2 6

17 Issues with Vernier Approach Mismatch issues are more severe than the single delay chain TDC - Reduced delay is formed as difference of two delays Large measurement range requires large area - Initial PLL frequency acquisition may require a large range div(t) Vernier Delay Delay Delay D Q D Q D Q Reg Reg Reg Delay2 Delay2 Delay2 e[k] div(t) Delay 0 0 e[k] Effective resolution: Delay-Delay2 Delay2 7

18 Two-Step TDC Architecture Allows Area Reduction Single Delay Chain Vernier Delay Delay Delay div(t) Delay Delay Delay Mux D Q D Q D Q D Q Reg D Q Reg D Q Reg Reg Reg Reg Ramakrishnan, Balsara VLSID 06 Logic Single delay chain provides coarse resolution (Folded) Vernier provides fine resolution Coarse e[k] Delay Delay2 Delay2 Delay2 Delay - Delay2 Fine e[k] 8

19 Two-Step TDC Using Time Amplification Single Delay Chain Time Amplifier Single Delay Chain div(t) Delay Delay Delay Delay Delay Delay Mux D Q D Q D Q D Q D Q D Q Reg Reg Reg Reg Reg Reg Simplified view of: Lee, Abidi VLSI 2007 Logic Coarse e[k] Delay Fine e[k] Single delay chain provides coarse and fine resolution Time amplification is used to improve resolution Delay Amplification of Time 9

20 Leveraging Metastability to Create a Time Amplifier in(t) Time Amplifier out(t) in(t) Δt in D Q Latch out(t) Δt in in(t) in(t) out(t) out(t) Δt out Δt out Simplified view of: Abas, et al., Electronic Letters, Nov 2002 (note that actual implementation uses SR latch) Metastability leads to progressively slower output transitions as setup time on latch is encroached upon - Time difference at input is amplified at output 20

21 Interpolating time-to-digital converter Tq Start Delay Delay Delay Start Stop Registers T stop Out 0 Out Henzler et al., ISSCC 2008 Stop Tin Interpolate between edges to achieve fine resolution Cyclic approach can also be used for large range 2

22 An Oscillator-Based TDC V dd Ring Oscillator div(t) Osc(t) Phase Error[] Phase Error[2] div(t) Reset Logic Counter Register Count[k] e[k] Count[k] e[k] 3 3 Output e[k] corresponds to the number of oscillator edges that occur during the measurement time window Advantages - Extremely large range can be achieved with compact area - Quantization noise is scrambled across measurements 22

23 A Closer Look at Quantization Noise Scrambling V dd Ring Oscillator div(t) Osc(t) Phase Error[] Phase Error[2] div(t) Reset Logic Counter Register Count[k] e[k] Count[k] Quant. Error[k] e[k] -q[0] q[] -q[2] 3 3 Quantization error occurs at beginning and end of each measurement interval As a rough approximation, assume error is uncorrelated between measurements q[3] - Averaging of measurements improves effective resolution 23

24 Deterministic quantizer error vs. scrambled error Deterministic TDC do not provide inherent scrambling For oversampling benefit, TDC error must be scrambled! Some systems provide input scrambling (ΔΣ fractional-n PLL), while some others do not (integer-n PLL) 24

25 Proposed GRO TDC Structure

26 A Gated Ring Oscillator (GRO) TDC Ring Oscillator Enable div(t) Osc(t) Phase Error[] Phase Error[2] div(t) Reset Logic Counter Register Count[k] e[k] Count[k] Quant. Error[k] e[k] -q[0] q[] -q[] 3 4 Enable ring oscillator only during measurement intervals - Hold the state of the oscillator between measurements Quantization error becomes first order noise shaped! - e[k] = Phase Error[k] + q[k] q[k-] - Averaging dramatically improves resolution! q[2] 26

27 Improve Resolution By Using All Oscillator Phases Phase Error[] Phase Error[2] Ring Oscillator Enable div(t) Reset Counters Osc. Phases(t) Logic div(t) Register Count[k] e[k] Helal, Straayer, Wei, Perrott VLSI 2007 Count[k] Quant. q[] q[2] Error[k] -q[0] -q[] e[k] 0 Raw resolution is set by inverter delay Effective resolution is dramatically improved by averaging 27

28 GRO TDC Also Shapes Delay Mismatch Measurement Enable Measurement 2 Enable Measurement 3 Enable Measurement 4 Enable Barrel shifting occurs through delay elements across different measurements - Mismatch between delay elements is first order shaped! 28

29 Simple gated ring oscillator inverter-based core Enabled Ring Oscillator Disabled Ring Oscillator (a) (b) Gate the oscillator by switching the inverter cores to the Enable Vo n- Delay Element Enable M 4 power supply Vo 5 Vo n Vo i- M 3 Vo i Vo 4 Vo M 2 Vo 3 Vo 2 Enable M 29

30 GRO Prototype En Dis 5 Stage Gated Ring Oscillator S Q R enable(t) enable enable Straayer, Perrott Logic error[k] GRO implemented as a custom 0.3 μm CMOS IC 30

31 Measured GRO Results Confirm Noise Shaping Variable Delay 5 Stage Gated Ring Oscillator S Q R enable(t) enable enable Input variable delay signal Harmonics due to nonlinearity of variable delay Logic error[k] Amplitude (db) Frequency (MHz) Noise shaped quant. noise 3

32 Measured deadzone behavior of inverter-based GRO Deadzones were caused by errors in gating the oscillator GRO injection locked to an integer ratio of F S Behavior occurred for almost all integer boundaries, and some fractional values as well Noise shaping benefit was limited by this gating error 32

33 The issue of gating non-idealities GRO Phase Original phase trajectory Original phase + T disable Actual phase T skew Enable T disable T in Oscillator does not stop and start instantly Actual phase trajectory deviates from ideal trajectory by a time defined as T skew 33

34 Interrupted transition causes charge redistribution Enable Rinv Enable Rsw V ss V o V d Vo Cp Vd Cd V d V o V dd Rsw Enable Vd Rinv Cd Vo Cp Charge redistribution depends on when the transition is stopped Positive and negative transitions are not perfectly symmetric Gating skew (T skew ) then depends on GRO phase (θ GRO ) when Enable transitions low (a) (b) 34

35 Cartoon depicting the error from individual stages Transitions Delay Element Output Voltages Delay Element Skew Error Up Down Up Down Only one stage in transition at a time T skew is the sum of error from each of the individual stages Periodic with 2T q due to positive and negative transition asymmetry 4 Total Skew 2N- 0 2 GRO Phase State 3 35

36 Next Generation GRO: Multi-path oscillator concept Single Input Single Output Multiple Inputs Single Output Use multiple inputs for each delay element instead of one Allow each stage to optimally begin its transition based on information from the entire GRO phase state Key design issue is to ensure primary mode of oscillation 36

37 Multi-path inverter core Lee, Kim, Lee JSSC 997 Mohan, et. al., CICC

38 Proposed multi-path gated ring oscillator Hsu, Straayer, Perrott ISSCC 2008 Oscillation frequency near 2GHz with 47 stages Reduces effective delay per stage by a factor of 5-6! Represents a factor of 2-3 improvement compared to previous multi-path oscillators 38

39 A simple measurement approach Enable N-Stage Gated Ring Oscillator Start Reset Logic Counters Stop Count[k] Register e[k] Helal, Straayer, Perrott VLSI counters per stage * 47 stages = 94 counters each at 2GHz Power consumption for these counters is unreasonable Need a more efficient way to measure the multi-path GRO 39

40 Phase-based measurement for a simple GRO Simple logic provides map from GRO output state to phase Transition sequence is predictable, unambiguous GRO Delay Stage Key: Logical Logical Quantized GRO Phase State 29 40

41 Accounting for phase wrapping Calculate phase from: - A single counter for coarse phase information - GRO output state for fine phase residual counter and N registers much more efficient 4

42 Accuracy considerations Counter and registers need to have the same state Cannot allow counters to double-count a single transition 42

43 De-glitch circuits to ensure accurate measurements 43

44 Key issue with scheme for an multi-path GRO GRO Delay Stage Key: Logical Unknown Logical Quantized GRO Phase State 93 More than one delay element output is logically uncertain Transition sequence is unpredictable and ambiguous Cannot map from entire GRO output state to phase 44

45 Restoring the predictable relationship Calculate phase contribution from each cell independently Transition sequence within each cell is now predictable 45

46 Prototype 0.3μm CMOS multi-path GRO-TDC Start Stop Timing Generation Enable 47-stage Gated Ring Oscillator Start Stop Enable CLK Z -47 State Register Measurement Cells CLK Adder Out Two implemented versions: - 8-bit, 500Msps - -bit, 00Msps version 2-2mW power consumption depending on input duty cycle 46

47 Measured noise-shaping of multi-path GRO Power Spectral Density (db ps 2 /Hz) Data collected at 50Msps More than 20dB of noise-shaping benefit 80fs rms integrated error from 2kHz-MHz Floor primarily limited by /f noise (up to 0.5-MHz) 65,536 pt. FFT (Hanning window + 20x averaging) Input of.2ps pp Noise of 80fs rms in MHz BW Frequency (Hz) (a) Ideal variance of 50-Msps quantizer with ps steps Filtered TDC Output TDC Output after MHz LPF.2ps Time (µs) (b) 47

48 Measured -bit range of multi-path GRO Raw TDC Output Time (µs) 48

49 Measured deadzone behavior for multi-path GRO Only deadzones for outputs that are multiples of 2N - 94, 88, 282, etc. - No deadzones for other even or odd integers, fractional output Size of deadzone is reduced by 0x 49

50 Revised gating skew cartoon for the multi-path GRO Transition Width Spans Entire Range of Input Connections Delay Element Output Voltages Delay Element Skew Error At least 3 stages in transition at a time - Most of the mismatch from positive and negative transitions is cancelled T skew is the average of error from each of the individual stages - GRO phase trajectory is determined by many stages, not just one Total Skew GRO Phase State 50

51 -bit GRO-TDC performance summary Sampling Frequency Raw delay resolution Effective resolution Integrated noise Dynamic range Power Area Technology <00 MHz 6ps 50Msps 80fs-rms, 2kHz-MHz 95dB, MHz BW 2.2-2mW (<4mW typical) 57 x 258μm 0.3μm CMOS 5

52 Summary of Time to Digital Conversion Key performance metrics are - Resolution: want low quantization noise - Mismatch: want high linearity - Power and area: want long battery life, low cost Many structures have been introduced - Classical, Vernier, Two-Step, Time Amplifiers, Re-cycling, Gated Ring Oscillator Comparable to ADCs but suffers from lack of time memory element - Cyclic conversion and pipeline structures have not been achieved A very promising research area! 52

53 Digitally-Controlled Oscillators

54 A Straightforward Approach for Achieving a DCO DAC Varactor Varactor Analog Control div(t) Time -to- Digital Digital Loop Filter Divider DCO out(t) Ferriss ISSCC 2007 Hsu ISSCC 2008 Use a DAC to control a conventional LC oscillator - Allows the use of an existing VCO within a digital PLL - Can be applied across a broad range of IC processes 54

55 A Much More Digital Implementation Varactor Varactor Digital Control div(t) Time -to- Digital Digital Loop Filter Divider DCO out(t) Staszewski et. al., TCAS II, Nov 2003 Adjust frequency in an LC oscillator by switching in a variable number of small capacitors - Most effective for CMOS processes of 0.3u and below 55

56 Leveraging Segmentation in Switched Capacitor DCO Coarse Control Fine Control Varactor Varactor Binary Array x 2x 4x 2 n x Unit Element Array x x x x Similar in design as segmented capacitor DAC structures - Binary array: efficient control, but may lack monotonicity - Unit element array: monotonic, but complex control Coarse and fine control segmentation of DCO - Coarse control: active only during initial frequency tuning (leverage binary array) - Fine control: controlled by PLL feedback (leverage unit element array to guarantee monotonicity) 56

57 Leveraging Dithering for Fine Control of DCO out(t) Varactor Divide-by-K Varactor Coarse Control Fine Control Initial Frequency Tuning Digital Σ Δ Modulator T in[k] Digital Loop Filter T c =T/M DCO TDC out Increase resolution by Σ Δ dithering of fine cap array Reduce noise from dithering by - Using small unit caps in the fine cap array - Increasing the dithering frequency (defined as /T c ) We will assume /T c = M/T (i.e. M times reference frequency) 57

58 Time-Domain Modeling of the DCO Digital Σ Δ Upsampler Modulator in[k] in sd [m] in q [m] M Σ Δ Zero-Order Hold 0 T c t in cap (t) K v Frequency to Phase f cap (t) Φ out (t) 2π in[k] in sd [m] in q [m] in cap (t) Φ out (t) k m m t t Input to the DCO is supplied by the loop filter - Clocked at /T (i.e., reference frequency) Switched capacitors are dithered by Σ Δ at a higher rate - Clocked at /T c = M/T - Held at a given setting for duration T c Fine cap element value determines K v of VCO T c T c - Units of K v are Hz/unit cap 58

59 Frequency Domain Modeling of DCO Digital Σ Δ Upsampler Modulator in[k] in sd [m] in q [m] M Σ Δ Zero-Order Hold 0 T c t in cap (t) K v Frequency to Phase f cap (t) Φ out (t) 2π q raw [k] Upsampler by M Digital Σ Δ Modulator H ntf (z) Zero-Order Hold Conversion to Phase in[k] Upsampler and zero-order hold correspond to discrete and continuous-time sinc functions, respectively Σ Δ has signal and noise transfer functions (H stf (z), H ntf (z)) 0 M /MT c f H stf (z) z=e j2πft c - Note: var(q raw [k]) = /2 (uniformly distributed from 0 to ) 0 T c /T c f 2πK v s s=j2πf Φ out (t) 59

60 Simplification of the DCO Model M T c Digital Σ Δ Modulator q raw [k] Upsampler by M H ntf (z) Zero-Order Hold Conversion to Phase in[k] 0 M /MT c f H stf (z) z=e j2πft c Focus on low frequencies for calculations to follow - Assume sinc functions are relatively flat at the low frequencies of interest /T c Upsampler is approximated as a gain of M 0 T c f 2πK v s s=j2πf Φ out (t) Zero-order hold is approximated as a gain of T c Assume H stf (z) = - True for Σ Δ structures such as MASH (ignoring delays) 60

61 Spectral Density Calculations CT CT x(t) H(f) y(t) DT DT x[k] H(e j2πft ) y[k] DT CT x[k] H(f) y(t) CT CT DT DT DT CT 6

62 Calculation of Quantization Noise from Cap Dithering Quantization Noise in[k] M q raw [k] H ntf (z) z=ej2πft c q[k] DT to CT spectral calculation: f T c 2πK v s s=j2πf Phase Noise f Φ out (t) - S qraw (f) = /2 since q raw [k] uniformly distributed from 0 to - H ntf (z) is often -z - (first order) or (-z - ) 2 (second order) 62

63 Example Calculation for DCO Quantization Noise Assumptions (Out freq = 3.6 GHz) - Dithering frequency is 200 MHz (i.e., /T c = 200e6) - Σ Δ has first order shaping (i.e., H ntf (z) = - z - ) - Fine cap array yields 2 khz/unit cap (i.e., K v = 2e3) At a frequency offset of f = 20 MHz: Below the phase noise (-53 dbc/hz at 20 MHz) in the example 63

64 Further Simplification of DCO Model Quantization Noise in[k] M f q raw [k] H ntf (z) z=ej2πft c q[k] T c 2πK v s Proper design of DCO will yield quantization noise that is below that of the intrinsic phase noise (set by tank Q, etc.) - Assume q[k] = 0 for simplified model Note that T = M T c s=j2πf in[k] Phase Noise f Φ out (t) DT-CT T DCO-Referred Noise S Φ n (f) 2πK v s s=j2πf Φ n (t) f Φ out (t) 64

65 Overall Digital PLL Model TDC DCO TDC-referred Noise S t q (ej2πft ) DCO-referred S Φ n (f) Noise -20 db/dec Φ ref [k] T 2π f t q [k] TDC Gain Δt del e[k] Loop Filter H(z) z=e j2πft DT-CT T 2πK v s f s=j2πf Φ n (t) Φ out (t) Φ div [k] Divider CT-DT N T TDC and DCO-referred noise influence overall phase noise according to associated transfer functions to output Calculations involve both discrete and continuous time 65

66 Key Transfer Functions Φ ref [k] T 2π t q [k] TDC Gain Δt del e[k] Loop Filter H(z) DT-CT T 2πK v s Φ n (t) Φ out (t) Φ div [k] z=e j2πft s=j2πf CT-DT N T TDC-referred noise DCO-referred noise 66

67 Introduce a Parameterizing Function Φ ref [k] T 2π t q [k] TDC Gain Δt del e[k] Loop Filter H(z) DT-CT T 2πK v s Φ n (t) Φ out (t) Φ div [k] z=e j2πft s=j2πf CT-DT N T Define open loop transfer function A(f) as: Define closed loop parameterizing function G(f) as: - Note: G(f) is a lowpass filter with DC gain = 67

68 Transfer Function Parameterization Calculations TDC-referred noise DCO-referred noise 68

69 Key Observations Φ ref [k] Φ div [k] T 2π t q [k] TDC-referred noise TDC Gain Δt del DCO-referred noise e[k] N Loop Filter H(z) z=e j2πft DT-CT T CT-DT T 2πK v s s=j2πf Lowpass with a DC gain of 2πN Φ n (t) Highpass with a high frequency gain of Φ out (t) How do we calculate the output phase noise? 69

70 Phase Noise Calculation TDC-referred Noise S t q (ej2πft ) f t q [k] DCO-referred Noise S Φ n (f) -20 db/dec f Φ n (t) TDC noise - DT to CT calculation - Dominates PLL phase noise at low frequency offsets dbc/hz 2πN G(f) -G(f) f o f o Φ out (t) 2πNG(f) 2 T St q (ej2πft) - G(f) 2 S Φ n (f) DCO noise - CT to CT calculation - Dominates PLL phase noise at high frequency offsets f o f 70

71 Impact of PLL Bandwidth TDC-referred Noise S t q (ej2πft ) f t q [k] DCO-referred Noise S Φ n (f) -20 db/dec f Φ n (t) PLL bandwidth dramatically influences relative impact of TDC and VCO noise 2πN G(f) -G(f) f o f o Φ out (t) Want high PLL bandwidth? Need low TDC Noise Low PLL Bandwidth High PLL Bandwidth dbc/hz TDC Noise f o DCO Noise f dbc/hz DCO Noise f o TDC Noise f 7

72 System Level Design

73 Closed Loop PLL Design Approach Closed-Loop Performance Specifications {f o, type, order} Open-Loop Design Approach Open-Loop Characteristics A(f) A(f) {K,f p,f z,...} G(f) = A(f) = A(f) +A(f) G(f) -G(f) Closed-Loop Transfer Function G(f) Proposed Closed Loop Design Approach Classical open loop approach - Indirectly design G(f) using bode plots of A(f) Proposed closed loop approach - Directly design G(f) by examining impact of its specifications on phase noise (and settling time) - Solve for A(f) that will achieve desired G(f) Lau and Perrott, DAC, June 2003 Implemented in PLL Design Assistant Software 73

74 Transfer Function Design using PLL Design Assistant PLL Design Assistant assumes continuous-time open loop transfer function A calc (s): Above parameters are calculated based on the desired closed loop PLL bandwidth, type, and order of rolloff (which specify G(s)) For 00 khz bandwidth, type = 2, 2 nd order rolloff, we have: - K = 3.0x0 0 - w p = 2π(53 khz) - w z = 2π(0 khz) 74

75 Continuous-Time Approximation of Digital PLL Φ ref [k] T 2π t q [k] TDC Gain Δt del e[k] Loop Filter H(z) DT-CT T 2πK v s Φ n (t) Φ out (t) Φ div [k] N z=e st CT-DT T s=j2πf At low frequencies (i.e., st << ), we can use the first order term of a Taylor series expansion to approximate Resulting continuous-time approximation of open loop transfer function of digital PLL: 75

76 Applying PLL Design Assistant to Digital PLL Design Given the continuous-time approximation of A(s), we then leverage the PLL Design Assistant calculation: - Also note that: Given the above, we obtain: 76

77 Simplified Form for Digital Loop Filter (Type II PLL) From previous slide: Simplified form with type = 2 (assume order = 2) - Where: Note: T dco = T/N * * Typically implemented by gain normalization circuit 77

78 Summary of Loop Filter Design PLL Design Assistant allows fast loop filter design Assumption: Type = 2, 2 nd order rolloff - Where: * * implemented by gain normalization circuit PLL Design Assistant provides the values of K, w p = 2πf p, w z = 2πf z 78

79 Example Digital Loop Filter Calculation Assumptions - Ref freq (/T) = 50 MHz, Out freq = 3.6 GHz (so N = 72) - Δt del = 20 ps, K v = 2 khz/unit cap - 00 khz bandwidth, Type = 2, 2 nd order rolloff 79

80 Overall PLL Noise Analysis

81 Calculation of TDC Noise Spectrum: Delay Chain TDC div(t) Δt del Δt del Δt del D Q Reg D Q Reg D Q Reg e[k] time error[k] t q [k] TDC Gain Δt del e[k] Under the assumption that quantization error is uniformly distributed across time interval Δt del : Key issue: quantization error may not be white for this TDC! - Use behavioral simulations to get a more accurate view /f noise may have impact S t q (ej2πft ) /f noise 2 Δt del 2 thermal noise f 8

82 Calculation of TDC Noise Spectrum: GRO TDC Enable t raw [k] div(t) Δt del Δt del Digital Logic e[k] Δt del GRO achieves noise shaping: time error[k] t q [k] -z - z=e j2πft TDC Gain Δt del e[k] S t q (ej2πft ) -e j2πft 2 2 Δt del 2 /f and thermal noise limit noise performance at low frequency offsets /f noise thermal noise f 82

83 Example Calculation for Delay Chain TDC Ref freq = /T = 50 MHz, Out freq = 3.6 GHz S t q (ej2πft ) f t q [k] 2 Δt del 2 Inverter delay = Δt del = 20 ps f o 2πN G(f) S Φ out (f) tdc 2πNG(f) 2 Δt del T 2 f o f 2 Note: G(f) = at low offset frequencies 83

84 Calculation of Noise Spectrum: Switched Cap DCO Varactor Varactor Digital Control Phase noise - Same as for conventional VCO (tank Q, etc.) Quantization noise from dithering - Design to be less than VCO phase noise Quantization Noise in[k] M f q raw [k] H ntf (z) z=ej2πft c q[k] T c 2πK v s s=j2πf Phase Noise f Φ out (t) 84

85 Evaluate Phase Noise with 500 khz PLL Bandwidth Key PLL parameters: - G(f): 500 khz BW, Type II, 2 nd order rolloff - TDC noise: dbc/hz - DCO noise: -53 dbc/hz at 20 MHz offset (3.6 GHz carrier) 85

86 Calculated Phase Noise Spectrum with 500 khz BW L(f) (dbc/hz) Output Phase Noise of Synthesizer Overall PLL Phase Noise TDC Noise GSM Mask (Referenced to 3.6 GHz carrier) DCO Noise Detector Noise VCO Noise Total Noise Frequency Offset (Hz) TDC noise too high for GSM mask with 500 khz PLL bandwidth 86

87 Change PLL Bandwidth to 00 khz Key PLL parameters: - G(f): 00 khz BW, Type = 2, 2 nd order rolloff - TDC noise: dbc/hz - DCO noise: -53 dbc/hz at 20 MHz offset (3.6 GHz carrier) 87

88 Calculated Phase Noise Spectrum with 00 khz BW Output Phase Noise of Synthesizer Detector Noise VCO Noise Total Noise L(f) (dbc/hz) Overall PLL Phase Noise TDC Noise DCO Noise GSM Mask (Referenced to 3.6 GHz carrier) Frequency Offset (Hz) GSM mask is met with 00 khz PLL bandwidth 88

89 Digital Fractional-N Synthesis

90 A First Glance at Fractional-N Signals (F out = 4.25F ref ) N[k] out(t) div(t) e[k] 5 4 div(t) Time -to- Digital e[k] Digital Loop Filter Divider DCO out(t) N[k] Constant divide value of N = 4 leads to frequency error - Phase error accumulates in unbounded manner 90

91 TI Approach to Fractional Division out(t) div(t) e[k] cnt[k] 5 4 Re-time signal Time -to- Digital Reg Reg e[k] Phase Unwrap cnt[k] div(t) Reg Digital Loop Filter count reset DCO out(t) Staszewski et. al., TCAS II, Nov 2003 Wrap e[k] by feeding delay chain in TDC with out(t) Counter provides information of when wrapping occurs 9

92 Key Issues Re-time signal Time -to- Digital Reg Reg e[k] Phase Unwrap cnt[k] div(t) Reg Digital Loop Filter count reset DCO out(t) Counter, re-timing register, and delay stages of TDC must operate at very high speeds - Power consumption can be an issue Calibration of TDC scale factor required to achieve proper unwrapping of e[k] - Can be achieved continuously with relative ease See Staszewski et. al, JSSC, Dec

93 Fractional-N Synthesizer Approach (F out = 4.25F ref ) N[k] out(t) div(t) e[k] div(t) Time -to- Digital Accum e[k] N[k] Digital Loop Filter Divider DCO out(t) Accumulator guides the swallowing of VCO cycles - Average divide value of N = 4.25 is achieved in this case 93

94 The Accumulator as a Phase Observer Accumulator residue corresponds to an estimate of the instantaneous phase error of the PLL - Fractional value (i.e., 0.25) yields the slope of the residue Carry out signal is asserted when the phase error deviation (i.e. residue) exceeds one VCO cycle - Carry out signal accurately predicts when a VCO cycle should be swallowed div(t) Time -to- Digital e[k] Digital Loop Filter Divider DCO out(t) N.Frac = 4.25 Accum Carry Out Residue Carry Out Frac =

95 Improve Dithering Using Sigma-Delta Modulation Provides improved noise performance over accumulator-based divide value dithering - Dramatic reduction of spurious noise - Noise shaping for improved in-band noise - Maintains bounded phase error signal Digital Σ Δ fractional-n synthesizer architecture is directly analogous to analog Σ Δ fractional-n synth. N sd [k] div(t) Time -to- Digital Σ Δ Modulator e[k] N[k] Digital Loop Filter Divider DCO out(t) Σ Δ Quantization Noise M.F f 95

96 Model of Digital Σ Δ Fractional-N PLL TDC Φ ref [k] Φ div [k] TDC-referred Noise S t q (ej2πft ) T 2π f t q [k] TDC Gain Δt del Divider Σ Δ Quantization Noise S q (e j2πft ) n[k] f e[k] N nom Loop Filter H(z) 2π z - -z - DCO z=e j2πft z=e j2πft DCO-referred S Φ n (f) Noise -20 db/dec DT-CT T CT-DT T 2πK v s f s=j2πf Φ n (t) Divider model is expanded to include the impact of divide value variations Φ out (t) 96

97 Transfer Function View of Digital Σ Δ Fractional-N PLL Σ Δ Quantization Noise S q (e j2πft ) n[k] 2π z - -z - dbc/hz TDC-referred Noise S t q (ej2πft ) f z=e j2πft T G(f) f o f o f t q [k] DCO-referred Noise S Φ n (f) -20 db/dec f Φ n (t) 2πN f nom G(f) -G(f) o f o T f Φ out (t) 2πN nom G(f) T 2 St q (ej2πft ) - G(f) 2 S Φ n (f) 2πT G(f) -e -j2πft 2 S q (e j2πft ) Σ Δ quantization noise now impacts the overall PLL phase noise - High PLL bandwidth will increase its impact Digital PLL implementation simplifies quantization noise cancellation 97

98 CppSim Behavioral Model of TI Digital Synthesizer Implements basic version of TI all-digital synthesizer with parameters we calculated in this tutorial 98

99 Comparing Behavioral Simulation to Calculations Simulated Versus Calculated Phase Noise Detector Noise VCO Noise Total Noise L(f) (dbc/hz) Overall Calculated PLL Phase Noise TDC Noise DCO Noise GSM Mask (Referenced to 3.6 GHz carrier) Overall Simulated PLL Phase Noise Frequency Offset (Hz) Calculations validated by simulation results! 99

100 Behavioral Simulation of a Digital Fractional-N PLL Check out the CppSim tutorial: - Design of a Low-Noise Wide-BW 3.6GHz Digital Σ Δ Fractional-N Frequency Synthesizer Using the PLL Design Assistant and CppSim 00

101 Summary of Digital Frequency Synthesizers Digital Phase-Locked Loops look extremely promising for future applications - Very amenable to future CMOS processes - Excellent performance can be achieved TDC structures are an exciting research area - Ideas from A-to-D conversion can be applied Analysis of digital PLLs is similar to analog PLLs - PLL bandwidth is often chosen for best noise performance TDC (or Ref) noise dominates at low frequency offsets DCO noise dominates at high frequency offsets Behavioral simulation tools such as CppSim allow architectural investigation and validation of calculations Innovation of future digital PLLs will involve joint circuit/algorithm development 0