Download - CMOS Oscillator Analysis
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1Oscillator Design
Behzad RazaviElectrical Engineering Department
University of California, Los Angeles
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2Outline
z Introductionz Basic Ringsz Frequency Tuningz LC Oscillators
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3Small-Signal View
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4Ring Oscillators
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5Linear Model
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6Amplitude Limiting
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7Basic Rings
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8Other Rings
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9Voltage-Controlled Oscillators
z Center Frequencyz Tuning Range:- Band of Interest- PVT Variations
z Gain (Sensitivity)
z Supply Rejectionz Tuning Linearityz Intrinsic Jitterz Output Amplitude
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Two Schools of Thought
Use differential rings lower supply sensitivity
But Use inverters with supply acting as control line. wider tuning range
But
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Differential Ring VCOs (I)
z But large swing variation across tuning range
z Ring with Replica Biasing
[Young, JSSC, Nov. 92]
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Differential Ring VCOs (II)
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Tuning by Interpolation
z Interpolation does not work well at low speeds.
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Example of Wide-Range Tuning
[Maneatis, JSSC, Nov. 03]
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Single-Ended VCOs
[van Kaenel, JSSC, Nov. 98]
[Mansuri, JSSC, Nov. 98]
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CCO with Regulation
[Yan, ISSCC05]
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But how to generate complementary outputs?
[Grozing, ESSCIRC 03]
z Synchronize two rings:
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Other Examples
[Searles, ISSCC07] (AMD)
[Desai, ISSCC07] [Straayer, JSSC, April 09]
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18-GHz Ring in 65 nm
[Gebara, ISSCC07]
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Measured Tuning Range
[Gebara, ISSCC07]
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Another Example
[Kossel, ISSCC05]
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Delay Stage
[Kossel, ISSCC05]
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Simulated Behavior
[Kossel, ISSCC05]
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LC Oscillators
z Much lower phase noise than rings (for a given power budget and frequency)
z Much faster than ringsz Much narrower tuning range z Main entry barrier: accurate inductor and varactor models
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Basics
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MOS Varactors
Simpler to use than pn junctions. C/V characteristic scales with technology.
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Q-Range Trade-Off
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Symmetric Inductors
Inductors driven differentially have a higher Q.
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Output Swing
Peak differential output voltage swing is given by:
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One-Port View
Example of negative resistance:
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3-Point Oscillator
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Oscillation Condition
Convert series resistance to parallel:
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Differential Topology
R1 appears in series with the parallel combination of L1 and L2, lowering their Q and avoiding CM oscillation.
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Cross-Coupled Oscillator
Looks like a diff pair with positive feedback.
Oscillation freq is given by:
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Problem of Swings
Peak Vds must not stress the transistors.
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Supply Sensitivity
Voltage-dependent Cdb results in a finite Kvcofrom Vdd to output frequency:
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One-Port View
Oscillation condition easier to meet than in 3-point topologies:
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Frequency Tuning (Type I)
To maximize tuning range, we wish to minimize C1.
But C1 is given by:- Caps of M1 and M2 (including 4Cgd)- Cap of L1- Input cap of next stage
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Use of Symmetric Inductor
Requires accurate model of inductor. cant begin design without a useful
inductor library.
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Tuning Range Limitations
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Effect of Varactor Q
Now include the varactor:
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VCO Type II
Select device dimension to set the output CM level to about Vdd/2.
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Varactor Modulation by IDD
Noise of current mirror becomes the dominant source.
Does this effect exist in Type I VCO?
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VCO Type III
Tuning range:
With 5% bottom-plate parasitic cap:
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VCO Type IV
Select device dimension to set the output CM level to about Vdd/2.
Output swing twice that of previous topologies.
But tail noise modulates varactors.
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Oscillation Amplitude vs. Frequency
Suppose the tank inductor has only a series resistance:
Oscillation amplitude falls as freq is lowered.
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Discrete Tuning
But on-resistance of switches lowers tank Q:
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Use of Floating Switch
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LC VCO Design Procedure
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Application as Reference
[McCorquodale, ISSCC08]
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Results
[McCorquodale, ISSCC08]
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Mathematical Model of VCOs