Mbius Microsystems
MEMS and CMOS Approaches to Monolithic Timing and Frequency Synthesis
University of Utah March 28, 2005
Michael S. McCorquodale, Ph.D. Chief Executive and Technology Officer Mobius Microsystems, Inc. Detroit, MI
M. S. McCorquodale Mbius Microsystems Overview
• An Overview of Timing and Frequency Synthesis • Critical Metrics • Entrenched Technologies • Emerging MEMS Approaches • CMOS Approaches • RF Clock Synthesis for the UMICH-WIMS µsystem • Mobius’ Clock Synthesis Technology • Future Work and Summary of Results
2of 79 M. S. McCorquodale Mbius Microsystems
An Overview of Timing and Frequency Synthesis
3of 79 M. S. McCorquodale Mbius An Overview of Microsystems Timing and Frequency Synthesis
Timing Every synchronous semiconductor component requires a clock to operate Frequency synthesis RF systems require precision frequency references for carrier frequency synthesis
Bluetooth/LAN USB Print Server • USB XTAL clock reference • Ethernet XTAL clock reference • Processor XTAL clock reference • Bluetooth radio XTAL reference (on flip side) 4of 79 M. S. McCorquodale Mbius Frequency Synthesis Microsystems Approaches
• The phase, delay, or injection locked “bottom-up” approach – Resonator (of some type) serves as a frequency reference – Sustaining oscillator provides a low frequency reference signal – PLL/DLL/ILL multiplies frequency by 2-4096x
• Drawbacks with this approach – External components (1 resonator + 2 capacitors) • Expensive, large, pin interface – Reference oscillator required • Either included in PLL or design required – PLL dissipates substantial power to multiply frequency • Particularly true for large multiplication factors – Performance degrades as frequency increases • For multiplication factor N, noise increases by N2 (to be shown) – Lock and start-up time can be long (e.g. >10,000 cycles) 5of 79 M. S. McCorquodale Mbius Frequency Synthesis Microsystems Approaches
• The free-running “direct” approach – RC (phase shift), ring, relaxation oscillators – Designed on-chip for the desired frequency – No external components required (monolithic); No reference
• Drawbacks with this approach – Very inaccurate: frequency ±20% untrimmed, ±2% trimmed – Very unstable over power supply & temperature variation: ±2% – High jitter – Typically found in 4-bit microcontrollers
6of 79 M. S. McCorquodale Mbius Frequency Synthesis Microsystems Approaches and Implementations
Phase-locked Free-running
fref
PFD CP LPF Nfref fo
÷N
Discrete Hybrid Monolithic crystal
µC clock
7of 79 M. S. McCorquodale Mbius Microsystems
Critical Metrics
8of 79 M. S. McCorquodale Mbius Summary of Microsystems Critical Metrics
• Frequency and time domain metrics – Short-term frequency stability: Jitter and phase noise – Total frequency accuracy: Drift over process, voltage, temperature (PVT), and aging – Rise/fall times – Duty cycle – Start-up time • Environmental conditions – Sensitivity to microphonics, moisture, etc. • Cost – Fabrication process technology – Production trimming requirements – Packaging requirements 9of 79 M. S. McCorquodale Mbius Short-Term Microsystems Frequency Stability
= ω Ideal Oscillator Output vo (t) Vo cos ot = + ε ω +φ vn (t) (Vo (t))cos( ot (t)) Noisy Oscillator Output
Timing Jitter Phase Noise Time domain uncertainty in period Power at frequency offset from fundamental P v Ideal Period
t
f P fo
vn t t1 t2 t3 4
t
T T T f 1 2 3 fo 10 of 79 M. S. McCorquodale Mbius Short-Term Microsystems Frequency Stability
v Ideal Period Short-Term Timing Jitter Expressions t • n-cycle = − J n (k) var(tk +n tk ) • Period (1-cycle) v n t t t t = − = = 1 2 3 4 J 1 (k ) var( t k +1 t k ) var( Tk ) J • Cycle-to-cycle t = − J cc (k ) var( Tk +1 Tk )
T1 T2 T3
P Phase Noise Power Spectral Density (PSD) Power relative to fundamental at offset fm from fo fo+fm ⎛ ⎞ + No = Sv ( fo fm ) ⎜ ⎟ Po Po f ⎝ ⎠ fm fo 11 of 79 M. S. McCorquodale Mbius Frequency Accuracy Microsystems and Precision
Nominal frequency acc./prec.
• Accuracy is how close the actual f − f A = actual ref frequency is to the desired (fref) f fref • Precision is how much the maximum frequency deviates from the mean − = fmax f ( f ), an issue that must be addressed Pf in production f
• Will frequency trimming be required? If so, what will it cost (test time)
12 of 79 M. S. McCorquodale Mbius Frequency Microsystems Sensitivities or Drift
V ∂f • Power supply S f = DD V DD ∂ f V DD 1 ∂f TC = • Temperature f f ∂T G ∂f S f = • Microphonic G f ∂G
All expressions can be determined by analysis
13 of 79 M. S. McCorquodale Mbius Long-Term Microsystems Frequency Stability
Long-term frequency stability • A measure of frequency variation over a long period of time • Commonly called aging ∆f
f Long-term instability
Short-term instability
t 14 of 79 M. S. McCorquodale Mbius Microsystems
Entrenched Technologies
15 of 79 M. S. McCorquodale Mbius Entrenched Microsystems Technologies
• Quartz – Piezoelectric bulk acoustic wave (BAW) resonators – ±50 to ±250ppm total accuracy – kHz to 100MHz – Primary applications: frequency and clock synthesis • ZnO – Piezoelectric surface acoustic wave (SAW) resonators – ±100 to ±250ppm total accuracy – 100 to 900MHz – Primary application: IF filters • Ceramic – Ceramic material which is induced to be piezoelectric – ±0.25 to ±5% total accuracy – kHz to 50MHz – Primary application: clock synthesis 16 of 79 M. S. McCorquodale Mbius Microsystems
Emerging MEMS Approaches
17 of 79 M. S. McCorquodale Mbius Two General Microsystems Approaches
• Resonator replacements – Utilize micromachining to develop integrated mechanical resonators which can replace discrete resonators – Intended to enable the realization of an integrated time/frequency reference • Improve VCO performance with enhanced passive components – Develop high-Q varactors and inductors in order to realize low phase noise VCOs – Not intended to replace the reference, but related to improving the performance of frequency synthesis blocks (allows Q-factor of reference oscillator to be relaxed)
18 of 79 M. S. McCorquodale Mbius Emerging MEMS Microsystems Approaches
• Capacitively-coupled microresonators – Surface micromachined poly-Si structures with capacitive actuation
• Benefits Clamped-clamped beam poly-Si microresonator – Very high-Q (>10,000) demonstrated [Nguyen, McCorquodale, et al.]
• Challenges – High motional resistance (>kΩ) – Nonlinear transduction causes flicker noise upconversion in oscillator circuits – Specialized packaging required – Process not CMOS-compatible – Frequency trimming required – Moderate temperature coefficient – Microphonic sensitivity may be high Disk poly-Si microresonator [Nguyen, et al.]
19 of 79 M. S. McCorquodale Mbius Emerging MEMS Microsystems Approaches
Sense Electrode
• Piezoelectrically-coupled ZnO Film microresonators – ZnO film couples actuation to surface Drive Electrode Tuning Capacitor Device Layer micromachined poly-Si beam Oxide – Remainder of device identical to Handle Layer previous microresonator Piezoelectric microresonator [Ayazi, et al.] • Benefits – Much lower motional resistance than previous microresonator (~100Ω)
• Challenges – Same as remaining challenges for previous microresonators
20 of 79 M. S. McCorquodale Mbius Emerging MEMS Microsystems Approaches
• Piezoelectric film bulk acoustic wave resonators (FBAR) – Similar to an integrated XTAL, but a film Drive • Benefits Electrode – High-Q – Low motional resistance – No specialized packaging required FBAR [Ruby, et al.] Thin • Challenges Piezoelectric – Not CMOS-compatible Film – Accuracy difficult to control
• Actually in products
Substrate
Electrodes Sense • Best application: multiple Piezoelectric references/filters within one Reflectors Electrode
package Substrate
21 of 79 M. S. McCorquodale Mbius Emerging MEMS Microsystems Approaches
• Passive RF MEMS – Micromachined varactors and inductors of various topologies • Benefits – Higher Q than planar passive components and thus lower phase noise in VCOs – Often tunable via mechanical actuation Micromachined parallel plate varactor – Some devices CMOS-compatible [Young, Boser] • Challenges – Most devices not CMOS-compatible – Microphonic sensitivity high for large aspect ratio devices – Some devices not practical for high volume Micromachined suspended inductor production [Yoon] 22 of 79 M. S. McCorquodale Mbius Microsystems
CMOS Approaches
23 of 79 M. S. McCorquodale Mbius General Microsystems Approach
• Construct a reference oscillator at desired frequency with available CMOS components: resistors, capacitors, transistors, diodes, etc. • Use standard and well-known oscillator topologies • Design compensation circuitry for voltage and temperature drift • Trim frequency out of fabrication • Best accuracy achieved to date ~±1.5% over PVT
24 of 79 M. S. McCorquodale Mbius Phase Shift Microsystems Oscillator
Basic operation C CC • Minimum of three RC pairs to -A form 180º phase shift R RR • Inverting amplifier creates final 180º phase shift
• fo dependent on RC component values ω = 6 o RC Challenges • Poor temperature stability Notes • Poor short-term stability • Very common in MCUs • Moderate accuracy (trimmed) • All discrete Si clock components • Moderate Si area utilize phase shift topology
25 of 79 M. S. McCorquodale Mbius Relaxation Microsystems and Ring
Relaxation: Basic operation Ring: Basic operation Characterized by one equivalent Odd number of inverters in a ring or an storage element as reference even number with wire inversion π ω = 2 ω = gm o o 2ntd 2RCCD
RR 12… n
M M 1 2 Challenges for both • Very inaccurate C • Poor short-term stability • Poor temperature stability
26 of 79 M. S. McCorquodale Mbius Microsystems
RF Clock Synthesis for the UMICH-WIMS µsystem
27 of 79 M. S. McCorquodale Mbius µsystem Microsystems Applications
Environmental Sensors Biomedical Implants
Heavy Cochlear Metal Implant Sensing
Deep Brain Implants
µGas Chromatograph 28 of 79 M. S. McCorquodale µ Mbius system Microsystems Development Requirements
System Requirements Purpose Solution
System battery operated Low power digital core, Operation voltage from and battery EOL voltage 900mV AFE, low power 1.8V to 900mV ~900mV clock reference Support processing Sensor data requires Low-latency clock “bursts” from processing only frequency switching and 2kHz to 66MHz periodically fast start-up time Minimize form factor and Fully encapsulated Monolithic clock packaging cost, some µsystem reference applications are aqueous AFE captures analog sensor data, ISA supports Sufficient local data Minimize transmitted data sufficient instructions for processing and storage and associated power data processing, 64KB on-chip SRAM 29 of 79 M. S. McCorquodale Mbius WIMS µsystem Microsystems By Rob Senger, Block Diagram Eric Marsman, Dan Burke, and Matt Guthaus Monolithic Clock Synthesizer Register Files
Fetch Decode Execute
Memory Management Unit
By Fadi Gebara
and External Memory Keith Kraver Boot 64KB Timer USART SPI Test Loop ADC ROM RAM X2 X3 X2 Int. Cache Int.
Vin+ Buffer Σ∆ PGA ADC
Vin- Buffer Analog Front End
30 of 79 M. S. McCorquodale Mbius Research Microsystems Goals
• Develop a clock synthesizer that is: Monolithic (no external components) Difficult CMOS-compatible (in Si) Difficult Small (Si footprint) Difficult Low-Power Difficult Fast (start-up and frequency switching) Difficult High-accuracy Very difficult Low-jitter Very difficult Low-drift over PVT Very difficult • Demonstrate clock synthesizer: – As an autonomous block – As the sole clock reference for the µsystem • Develop any supporting technology 31 of 79 M. S. McCorquodale Mbius General Microsystems Approach
• Initial Research Thoughts – Achieving low jitter and acceptable PVT drift requires a harmonic reference (Q >> 1) – Mechanical references are too expensive/complicated to integrate with CMOS – Look at the problem from the RF world: The LC oscillator is a workhorse, but LC sizes at ~66MHz are too large to integrate & are low-Q
• Thoughts Toward the Developed Approach – Must free-run and not phase-lock for fast start-up and low-latency switching – Turn the ubiquitous frequency synthesis approach upside-down
• Start at high fo, square, & translate to more stable low frequency
• Starting at a high fo limits the LC size and keeps Si area down • Investigate use of micromachining to improve Q for LC components – Develop some electrical frequency trimming approach – PVT compensation • First determine overall performance of LC oscillator for clocking • Next determine compensation techniques
32 of 79 M. S. McCorquodale Mbius The Importance of Microsystems Frequency Translation Phase and frequency are related by a linear operator φ ω = d dt
Frequency mult./div. results in phase noise mult./div. = ω +φ = ω + φ vn (t) Vo cos( ot n (t)) vn,mult (t) Vo cos(N ot N n (t))
Using narrowband FM approximation
⎛ ⎞ ⎛ ⎞ N o = N o ± 2 ⎜ ⎟ ⎜ ⎟ log( N ) Po Po ⎝ ⎠ f m ,mult . / div . ⎝ ⎠ f m Linear freq. trans. results in quadratic change in noise power
33 of 79 M. S. McCorquodale Mbius Relationship Microsystems with Q-Factor
Leeson Phase Noise Model 2 N FkT 1 ⎛ f ⎞ ⎛ o ⎞ = ⎜ o ⎟ ⎜ ⎟ 2 ⎜ ⎟ ⎝ C ⎠ fm C 8Q ⎝ fm ⎠ Q-factor quadratically related to phase noise PSD Frequency Translation ⎛ ⎞ ⎛ ⎞ N o = N o ± 2 ⎜ ⎟ ⎜ ⎟ log( N ) Po Po ⎝ ⎠ f m ,mult . / div . ⎝ ⎠ f m Frequency translation also quadratically related to phase noise PSD
Nmult for XTAL+ PLL up to 4096 High-Q, but large degradation: Can consider “effective” Q
34 of 79 M. S. McCorquodale Mbius Converting Phase Microsystems Noise to Period Jitter
Two common expressions for converting phase noise to jitter
f 2 ⎛ N ⎞ [Kundert] J = m ⎜ o ⎟ 3 ⎜ ⎟ Uses Lorentzian assumption by [Demir] f o Po ⎝ ⎠ f m
ω
8 ∞ ⎛ N ⎞ π [Drakhlis] J = ⎜ o ⎟ sin 2 fτ df τ Τ 2 ⎜ ⎟ = = 1/fo o ∫0 Po ⎝ ⎠ f m
• Kundert’s expression is actually just a specialized case of the expression by Drakhlis (i.e. an estimate) • Phase noise is related to jitter by a square root
35 of 79 M. S. McCorquodale Mbius Effect of Frequency Microsystems Translation
Using phase noise conversion expression, determine jitter:
⎛ N ⎞ f 2 1 ⎛ N ⎞ f 2 J = N 2 ⎜ o ⎟ m = J/ N J = ⎜ o ⎟ m = N J mult ⎜ ⎟ 3 div 2 ⎜ ⎟ 3 Po (Nfo ) N Po ( fo / N) ⎝ ⎠ fm ⎝ ⎠ fm
Considering fractional, or ppm, jitter: J J = ppm T / 10 6
J / N N J J = = N J J = = J / N ppm ,mult (T / N ) / 10 6 ppm ppm ,div NT / 10 6 ppm
Frequency translation can enhance or degrade jitter 36 of 79 M. S. McCorquodale Mbius Frequency Microsystems Translation Summary
Reference Frequency Frequency Variable/Metric Oscillator Multiplication Division
Output
Frequency fref Nfref fref /N (Hz)
SSB ⎛ No ⎞ ⎛ N ⎞ ⎛ N ⎞ ⎜ ⎟ ⎜ o ⎟ + 20log(N) ⎜ o ⎟ − 20log(N) Phase Noise PSD ⎜ ⎟ ⎜ P ⎟ ⎜ ⎟ Po ⎝ o ⎠ f Po (dBc/Hz) ⎝ ⎠ fm m ⎝ ⎠ fm
Period Jitter J (s) J / N N J Relative Period Jitter J J ppm / N ppm N J ppm (ppm)
37 of 79 M. S. McCorquodale Mbius Typical Ring-PLL Microsystems Phase Noise and Jitter (N = 64)
0 Bottom-up Reference Oscillator Bottom-up VCO Bottom-up Synthesizer Output -25
Consider Intel SA-1110 -50 • Strong-ARM mobile PLL Loop BW µP for PDAs -75 • 3.6864MHz XTAL • 64X Ring-PLL Significant • Max f = 236MHz -100 clk noise at large Phase Noise Spectral Density (dBc/Hz) ~20log(64) -125 offsets
-150 1 2 3 4 5 6 7 10 10 10 10 10 10 10 Offset Frequency (Hz)
38 of 79 M. S. McCorquodale Mbius Typical LC Oscillator Microsystems Phase Noise and Jitter (N = 16)
Consider typical LC-VCO 0 Top-down Reference Oscillator • Free-running Top-down Synthesizer Output • f = 3.78GHz o -25 • ~-110dBc/Hz @ 100kHz • 16X divider
• fclk = 236MHz -50
Can show that jitter is -75 Little noise the same for the ring at large ~20log(16) PLL and the LC offsets oscillator -100
Can jitter be improved Phase Noise Spectral Density (dBc/Hz) with micromachining? -125
Can accuracy be -150 1 2 3 4 5 6 7 addressed with tunable 10 10 10 10 10 10 10 component? Offset Frequency (Hz) 39 of 79 M. S. McCorquodale Mbius CMOS Micromached Microsystems Varactors and Inductors Prototype Development • TSMC 0.18µm MM/RF process • Post process in SSEL with PAD Etch IV • Realize suspended inductors from interconnect and varactors from MiM and M5- M6 interconnect to achieve high-Q variable tank for low jitter and frequency trimming support
Suspended Inductor • Some minor etching of inductor metal observed • No field etching observed • Inductor appears suspended • Also investigated use of PGS 40 of 79 M. S. McCorquodale Mbius Inductor Test Microsystems Results Test approach • Probe using an Cascade RF-1 probe station
• Test using an Agilent 8753ES network analyzer Standard • Short-open-load (SOL) cal. of ACP-40-GSG-100 Suspended • Measure S and convert to Y 11 11 Suspended with PGS • De-embed parasitic pad capacitance
j1 j1 j1
j0.5 j2 j0.5 j2 j0.5 j2
j0.2 j0.2 j0.2
0 0 0 0.2 0.5 1 2 0.2 0.5 1 2 0.2 0.5 1 2
-j0.2 -j0.2 -j0.2
-j0.5 -j2 -j0.5 -j2 -j0.5 -j2
-j1 -j1 -j1 6nH designs 8nH designs 10nH designs
41 of 79 M. S. McCorquodale Mbius Suspended Microsystems Inductor Test Results Im(Y ) • Calculate Q from de-embedded Y parameters using: Q = 11 11 Re(Y ) • Observed up to 13% increase in Q 11 • PGS did not increase Q Standard Suspended
Suspended with PGS
6nH Inductors 8nH Inductors 10nH Inductors 9 Standard 8 Standard 8 Released Released Released with PGS Standard 8 7 Released with PGS 7 Released Released with PGS 7 6 6
6 5 5 5
4 4 4 Quality Factor Quality Factor Quality
3 Factor Quality 3 3
2 2 2
1 1 1
0 0 0 01234501234501234 Frequency (GHz) Frequency (GHz) Frequency (GHz)
42 of 79 M. S. McCorquodale Mbius MEMS Varactor Test Microsystems Results
• MiM 4-by-4 varactor array • Trench cleared very well • No etch in field • M5-M6 varactor also developed (aspect ratio too large to be practical)
M5-M6 dielectric etched, but MiM dielectric does not etch at all! 43 of 79 M. S. McCorquodale Mbius Results from Microsystems CMOS Passive RF MEMS
• Inductor suspension technique successful • PGS technique not appropriate for target inductor sizes • Need new varactor approach – M5-M6 varactors are too large due to large gap • Why use a varactor? – To increase frequency accuracy by introducing trimming mechanism • Why use MEMS? – Junction and MOS varactors are lower Q – MiM structure is accurate and bias/temperature stable • Is there another way? – Use MiM and tune with a different technique – shown next
44 of 79 M. S. McCorquodale Mbius Frequency Modulation Microsystems via Harmonic Work Imbalance
Transconductance amplifier + bias Resonant tank, LC
_ RL RC + + -g v m _ _ + L Cf
Ibias generation
45 of 79 M. S. McCorquodale Mbius Frequency Modulation Microsystems via Harmonic Work Imbalance
• Consider current into and voltage across net tank g -amp injects current Waveform m capacitance onto net capacitance is distorted 15 600 • Transconductance amplifier
10 400 sustains oscillation by injecting current into the resonant tank 5 200
0 0 (t) (mV)
(t) (mA) • Causes work imbalance; C C i v
-5 -200 modulates the oscillation frequency -10 -400
-15 -600 1 1.5 2 2.5 3 3.5 4 • Work imbalance is a strong t (ns) function of bias current (gm), thus bias current can be utilized to trim absolute frequency
46 of 79 M. S. McCorquodale Mbius Characteristic Microsystems of fo(gm)
• fo(gm) is always of the shape shown below
• gmo corresponds to minimum the gm required for start-up (no distortion)
• gmsat corresponds to the point at which gm is so large that current injection onto the tank cap. approaches an impulse (maximum waveform distortion)
• fo ← gm ← I, where I is the current in the transconductance amplifier • Bias current, I, can be made temperature dependent
fo 1 NOTE: g = mo R fmax P Where RP is the total No equivalent parallel oscillation loss across the tank
fmin
gm gmo gmsat 47 of 79 M. S. McCorquodale Mbius Radio Frequency Microsystems Oscillator Core
• Complementary cross- coupled configuration • PMOS tail for low flicker 100 100 noise upconversion 0.18 0.18
• Devices in red are isolated 6nH with deep NWELL option
• MiM capacitors configured 8.3pF 8.3pF for release (if chemistry is 40 40 ever determined) 0.18 0.18 • 900MHz target frequency
48 of 79 M. S. McCorquodale Mbius Discrete Clock Microsystems Synthesizer Architecture
• RF core drives a high bandwidth amplifier which squares the signal and decreases the rise/fall times • D flip-flops serve as frequency dividers • 50Ω buffers required for off-chip instrumentation fo fo fo … fo 2 4 8 2n
fo D Q D Q D Q + AMP DFF DFF DFF … - Q Q Q
50Ω 50Ω 50Ω
49 of 79 M. S. McCorquodale Mbius Fabricated Discrete Microsystems Clock Prototypes
Prototype clock synthesizer in TSMC 0.18µm MM/RF Preliminary wafer testing with Cascade RF-1
Device packaged in 16-pin dual-inline package (DIP) Test printed circuit board (PCB) for environmental testing 50 of 79 M. S. McCorquodale Mbius Results: Short-Term Microsystems Frequency Stability
• Jitter measured using stats mode of a Tektronix CSA11801A DSO NOTE: the p-p 1x10-9 jitter for XTAL • 50,000 samples acquired at each edge oscillators around = 2 − 2 = = • DUT RMS jitter (x14.1 for p-p): J DUT J 2 J 1 1.97 ps 55 ppm 30MHz is ~50ps This work: ~27.8ps
28MHz 28MHz clock clock edge 1 edge 2
51 of 79 M. S. McCorquodale Mbius Results: Accuracy, Microsystems Precision, and
Trimming Range 900
898 Frequency accuracy/precision 896 894
• Design target = 900MHz 892
• Measured mean = 891.4MHz Frequency (MHz) 890 888 • Inaccuracy = 0.96% 886 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 • Precision = ±0.75% around mean Die Number
906
904
902 Trimming Range 900 898
• Available range is 2.2% for trimming 896
894 Frequency (MHz) Frequency • Captures 900MHz target 892
890
888
886 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Current Mirror Bias (mA) 52 of 79 M. S. McCorquodale Mbius Temperature Microsystems Drift
Frequency vs. Temperature • Trimmed oscillator core to 900MHz at room temperature 908.00 • Total TC +0.67% to -0.89% with no 906.00
compensation (very good and linear) 904.00 • f dominated by TC of real loss in TC 902.00 inductance 900.00 • Complete relationship has been derived 898.00 Frequency (MHz)
• TC can be compensated via a variety 896.00 of mechanisms 894.00 • Conclusion: free-running LC reference 892.00 oscillators exhibit low drift over PVT
890.00 • Next step: develop into µsystem with -40-200 20406080100 Temperature (°C) additional support 53 of 79 M. S. McCorquodale Mbius Low-Latency Microsystems Glitch-Free Frequency Switching for µsystem
S0 S1 S2 S3
fmax CLK1 fmax/2 2 fmax/2 3 fmax/2 CLK2 . D Q … D Q fout . DFF DFF .
15 SELECT fmax/2
CLKout glitch
2fmax • The clock ref. for the µsystem synthesizes signals from 1.1GHz to 2kHz • With top-down free-running architecture, frequency can be switched with very low latency • Must consider glitches • Use synchronizers to prevent meta-stable state 54 of 79 M. S. McCorquodale Mbius µsystem Test and Microsystems Measurement Set-Up
AFE E4405B Memory CSA11801A Loop Cache
Pipeline Peripherals Memory
CLK Memory Packaged HP82000 test chip
Micrograph of fabricated µsystem Test and measurement Set-up • TSMC 0.18µm MM/RF • HP82000 • ~3.5 million transistors • Tektronix CSA11801A • 12.8mm2 (clock = 0.3mm2 or < 3%) • Agilent E4405B • Packaged test chip 55 of 79 M. S. McCorquodale Mbius Results: Low-Latency Microsystems Glitch-Free Frequency Switching
• Case 1: Switching from 1MHz to 33MHz • Case 2: Switch rapidly from 4 to 8 to 16 to 33MHz
56 of 79 M. S. McCorquodale Mbius Results: Short-Term Microsystems Frequency Stability
Concern • How does processor switching affect short-term stability? Case 1: 33MHz • Only clock tree switches Case 2: 33MHz • Processor runs loop code Results • Less than 1dB degradation observed at 10kHz • Increased spurious power observed (3.5ps to 4.5ps jitter)
57 of 79 M. S. McCorquodale Mbius Summary of Microsystems Results
• Demonstrated a CMOS-compatible passive RF MEMS fabrication process (varactor not used) • Demonstrated a high accuracy and ultra low jitter monolithic clock reference • Demonstrated that free-running LC oscillators exhibit low drift over PVT (and can be improved) • Demonstrated top-down clock in µsystem and low-latency frequency switching for power management • Demonstrated that substrate noise injection due to switching in µsystem does not affect clock performance substantially
58 of 79 M. S. McCorquodale Mbius Microsystems
Mobius’ Clock Synthesis Technology
59 of 79 M. S. McCorquodale Mbius General Microsystems Approach
• Cost – Abandon use of all MEMS technology – Migrate to vanilla logic processes (i.e. 1P4M, 2P4M, etc.) • Technology – Focus on achieved performance in CMOS and techniques by which it can be improved – Utilize RF LC reference and frequency divide to achieve low jitter – Calibrate frequency with capacitor bank and develop automatic calibration hardware – Use circuit techniques to compensate frequency drift over PVT
60 of 79 M. S. McCorquodale Mbius RF-TCHO™: Mobius’ Microsystems Monolithic Clock Synthesis Technology
Transconductance fo(T) compensation Process variation comp. amplifier + I(T) bias Resonant tank, LC module, Cv+f(T) module, Cf(bp-1,…,b0)
Cv+f(vctrl) _ RL RC + + Resonant v (T) Frequency -g v ~1-2GHz ctrl _ m _ generation correction, + C (b ,…,b ) L f p-1 0 Cf Cv+f(vctrl)
f I(T) Simplified schematic of cal generation b ,…,b Mobius’ Radio Frequency p-1 0 generation Temperature Compensated Harmonic Oscillator Automatic frequency (RF-TCHO™) technology calibration macro
61 of 79 M. S. McCorquodale Mbius RF-TCHO™ Microsystems Transconductance Amplifier + I(T) Bias
Transconductance amplifier + I(T) bias Resonant tank, LC
_ RL RC + + -g v ~1-2GHz m _ _ + L Cf
I(T) generation
62 of 79 M. S. McCorquodale Mbius First Order Model for Microsystems
Predicting fo
1 1 f ≈ o π + 2 L(C f Camp )
• Expression above is the classic LC oscillator frequency expression where the reactance (phase) is zero • Typically L and C have negligible temperature coefficients (TCs)
• Ignoring RL and RC, fo does not depend on temperature
• Camp is the capacitive load presented by the transistors in the
gm-amplifier • Expression above is only ≈ 5% accurate
63 of 79 M. S. McCorquodale Mbius Second Order Model Microsystems
for Predicting fo(T)
1 1 (C + C )R2 (T ) − L f (T ) ≈ * f amp L o 2π L(C + C ) (C + C )R2 (T ) − L f amp f amp C
• RL and RC introduce a phase shift across the LC network; thus, the frequency expression is modified (solved by finding zero phase of the new network)
• RL and RC represent real losses which have temperature coefficients; thus, fo is a function of T
• Note, in this model, if RL and RC are exactly equal and have the same TC, then fo(T) is constant for all T • Expression above is ≈ 1-2% accurate 64 of 79 M. S. McCorquodale Mbius Frequency Modulation Microsystems due to Harmonic Work Imbalance
• Amplifier sustains oscillation by injecting energy (current) into the resonant tank • Capacitor absorbs nearly all energy (current in L can’t change instantaneously)
• Causes harmonic work imbalance between L and C; fo lowered to reconcile imbalance • Induced frequency modulation varies over temperature and bias conditions
• Temperature dependent reference, I(T), stabilizes fo(T) due to harmonic work imbalance
gm-amp injects current Waveform Distortion is temperature/bias dependent onto net capacitance is distorted and can be utilized for fo(T) compensation
15 600 20 0.8 Increasing T T = -40C T = 30C 15 0.6 10 400 T = 100C
10 0.4 5 200 5 0.2
0 0 0 0 (t) (mV) (t) (mA) C C i v Voltage (V) Current (mA) -5 -0.2 -5 -200
-10 -0.4
-10 -400 -15 -0.6
-15 -600 -20 -0.8 1 1.5 2 2.5 3 3.5 4 33 33.5 34 34.5 35 35.5 36 t (ns) Time (ns)
65 of 79 M. S. McCorquodale Mbius Microsystems Frequency (fo) Generation
1 Y • I(T) is a composite signal I(T) of PTAT, PTAT2, and/or CTAT M1 M2 current
generators which L(T) RL (T)
stabilize fo(T)
C (T) R (T) C (T) • 1:Y mirror v C f reduces power dissipated in M3 M4 reference leg
66 of 79 M. S. McCorquodale Mbius RF-TCHO™ I(T) Bias Microsystems Generation
• I(T) is generated by combining temperature-dependent reference currents
2 • Temperature-dependent ICTAT IPTAT IPTAT I(T) reference currents can be generated with many different topologies 1 S • The shape of I(T) is determined Current Mirror by the relative magnitude of each temperature-dependent current source I I I • I(T) often does not permit the
complete removal of fo(T), but it T T T can always be used to help 2 ICTAT IPTAT IPTAT stabilize fo(T) 67 of 79 M. S. McCorquodale Mbius RF-TCHO™ Microsystems Temperature
Compensation, Cv+f(T)
Transconductance fo(T) compensation amplifier + I(T) bias Resonant tank, LC module, Cv+f(T)
Cv+f(vctrl) _ RL RC + + v (T) -g v ~1-2GHz ctrl m _ generation _ + L Cf Cv+f(vctrl)
I(T) generation
68 of 79 M. S. McCorquodale Mbius RF-TCHO™ Microsystems Temperature
Compensation, Cv+f(vctrl)
• fo(T) is difficult to predict To one side of without measured Si due to x-1 x-1 2 C 2 Cf limited modeling the resonant v tank • Make fo(T) correction
programmable and select the bx-1 correct compensation factor V post-fabrication DD • Use an x-bit bank of AMOS/IMOS varactors in parallel with the fixed tank vctrl(T) capacitance and switch the 1Cv 1Cf ratio between the two • Control varactors with a b0 temperature dependant
control voltage, vctrl(T),and VDD create a temperature dependant capacitance,
Cv+f(T) 69 of 79 M. S. McCorquodale Mbius Third Order Model for Microsystems
Predicting fo(T,VDD)
Correction factor accounting for Parasitic resistive losses in temperature dependent harmonic series with the tank inductance work imbalance and capacitance
δ 1 1 [C (T ) + C (T ) + C (v (T ))]R2 (T ) − L(T ) f (T ) ≈ (1− (T,V ))* * f amp v ctrl L o DD 2π L(T )(C (T ) + C (T ) + C (v (T ))) [C (T ) + C (T ) + C (v (T ))]R2 (T ) − L(T ) f amp v ctrl f amp v ctrl C
Variable tank capacitance Fixed tank inductance used maintain frequency (minimal temperature accuracy over temperature dependence) Fixed tank capacitance and capacitance from MOS devices in the transconductance Expression above can be amplifier (minimal temperature better than 0.1% accurate dependence) with appropriate models
70 of 79 M. S. McCorquodale Mbius What is Microsystems δ (T,VDD)?
• A method of harmonic balance can be used to determine δ (T,VDD)
⎛ 1 ∞ n2 −1 ⎞ δ (T,V ) = ⎜ (h (T,V ))2 ⎟ DD ⎜ 2 ∑ 2 i(n) DD ⎟ ⎝ 2Q n−2 n ⎠
• hi(n) is the nth Fourier coefficient of the current waveform, i(t), injected into the tank normalized to the Fourier coefficient of the fundamental
• Note, if hi(n) = 0 for all n ≥ 2, then fo is temperature and bias independent in relationship to harmonic work imbalance
71 of 79 M. S. McCorquodale Mbius RF-TCHO™ Microsystems Process Variation
Comp., Cf(bp-1,…,b0)
Transconductance fo(T) compensation Process variation comp. amplifier + I(T) bias Resonant tank, LC module, Cv+f(T) module, Cf(bp-1,…,b0)
Cv+f(vctrl) _ RL RC + + Resonant v (T) frequency -g v ~1-2GHz ctrl _ m _ generation correction, + C (b ,…,b ) L f p-1 0 Cf Cv+f(vctrl)
I(T) generation
72 of 79 M. S. McCorquodale Mbius RF-TCHO™ Microsystems Process Variation
Comp., Cf(bp-1,…,b0)
• Large W/L required to keep
Ron small • W/L large → large C → db 2p-1C increase finger count To resonant tank trim b • Still often problems with p-1 p-1 large Ctrim → split Ctrim 2 Ctrim 1C bp-1 trim Ctrim b0
C W/L 1Ctrim db bp b0
• Constant R & TC load Parallel binary-weighted fixed capacitor banks problems (use dummies) • Binary-weighted capacitor array adds/subtracts switched C C trim dummy capacitance and modulates frequency • Simple concept, but many complicated details bp bp • Recently developed proprietary “switchless” trimming 73 of 79 M. S. McCorquodale Mbius Automatic Frequency Microsystems Calibration Macro (AFC™)
Transconductance fo(T) compensation Process variation comp. amplifier + I(T) bias Resonant tank, LC module, Cv+f(T) module, Cf(bp-1,…,b0)
Cv+f(vctrl) _ RL RC + + Resonant v (T) frequency -g v ~1-2GHz ctrl _ m _ generation correction, + C (b ,…,b ) L f p-1 0 Cf Cv+f(vctrl)
f I(T) AFC™ generates cal generation bp-1,…,b0 generation bp-1, …, b0 based on
calibration frequency Automatic frequency calibration macro
74 of 79 M. S. McCorquodale Mbius Sample Customer Microsystems Application
Before: FS-USB to RS-232 Bridge Controller (chip shown • FS-USB to RS-232 Bridge in USB cell phone cable) Controller – Application: Cables to bridge PC USB ports to mobile devices – Specification: 48MHz ± 0.25% over PVT and <20ps p-p jitter – Existing implementation: 12MHz XTAL and 4X PLL After: FS-USB to RS-232 Bridge Controller (chip shown on test board) • With Mobius’ RF-TCHO Macro – Eliminate PLL + XTAL – 1.536GHz reference oscillator – PG in November: Chartered 0.35µm 2P4M – Within specification over PVT
75 of 79 M. S. McCorquodale Mbius Microsystems Test Results
Frequency vs. Temperature for Calibrated Mobius FS- • ±0.25% accuracy achieved USB RF-TCHO Clock over PVT 48.15
(for T = -40 to 80C) 48.1
• Record total accuracy in Si 48.05
• Essentially, developed an 48
all-CMOS time reference (MHz) Frequency 47.95
• Can be utilized to 47.9 synchronize other less 47.85 accurate Si references -40-200 20406080 Te mpe rature (C) VDD=LO VDD=HI Low Specification High Specification
76 of 79 M. S. McCorquodale Mbius Microsystems
Future Work and Summary of Results
77 of 79 M. S. McCorquodale Mbius Microsystems Future Work
• Continue to demonstrate fast, RF-referenced, ultra-low jitter, and high-accuracy integrated clock generators and timing references in CMOS with “hard core” RF/analog techniques • Move from a record-setting total PVT accuracy of ±0.25% toward better than ±500ppm in CMOS
• fTC still very difficult to predict (now programmable) • Continue to investigate emerging timing and frequency synthesis technologies, but MEMS technologies currently do not appear to be economically viable and suffer from performance challenges 78 of 79 M. S. McCorquodale Mbius Summary of Microsystems Results
• Exciting research and commercial activity in monolithic timing and frequency synthesis • MEMS technologies require better economic analysis (i.e. application, cost, market, and business model) to determine market viability • With creative concepts in circuit design, CMOS continues to address integration demand at low cost • Is MEMS really solving a problem in frequency synthesis? Must answer this question before developing new technologies
79 of 79 M. S. McCorquodale