1Oscillator Design

Behzad RazaviElectrical Engineering Department

University of California, Los Angeles

2Outline

z Introductionz Basic Ringsz Frequency Tuningz LC Oscillators

3Small-Signal View

4Ring Oscillators

5Linear Model

6Amplitude Limiting

7Basic Rings

8Other Rings

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

10

Two Schools of Thought

Use differential rings lower supply sensitivity

But Use inverters with supply acting as control line. wider tuning range

But

11

Differential Ring VCOs (I)

z But large swing variation across tuning range

z Ring with Replica Biasing

[Young, JSSC, Nov. 92]

12

Differential Ring VCOs (II)

13

Tuning by Interpolation

z Interpolation does not work well at low speeds.

14

Example of Wide-Range Tuning

[Maneatis, JSSC, Nov. 03]

15

Single-Ended VCOs

[van Kaenel, JSSC, Nov. 98]

[Mansuri, JSSC, Nov. 98]

16

CCO with Regulation

[Yan, ISSCC05]

17

But how to generate complementary outputs?

[Grozing, ESSCIRC 03]

z Synchronize two rings:

18

Other Examples

[Searles, ISSCC07] (AMD)

[Desai, ISSCC07] [Straayer, JSSC, April 09]

19

18-GHz Ring in 65 nm

[Gebara, ISSCC07]

20

Measured Tuning Range

[Gebara, ISSCC07]

21

Another Example

[Kossel, ISSCC05]

22

Delay Stage

[Kossel, ISSCC05]

23

Simulated Behavior

[Kossel, ISSCC05]

24

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

25

Basics

26

MOS Varactors

Simpler to use than pn junctions. C/V characteristic scales with technology.

27

Q-Range Trade-Off

28

Symmetric Inductors

Inductors driven differentially have a higher Q.

29

Output Swing

Peak differential output voltage swing is given by:

30

One-Port View

Example of negative resistance:

31

3-Point Oscillator

32

Oscillation Condition

Convert series resistance to parallel:

33

Differential Topology

R1 appears in series with the parallel combination of L1 and L2, lowering their Q and avoiding CM oscillation.

34

Cross-Coupled Oscillator

Looks like a diff pair with positive feedback.

Oscillation freq is given by:

35

Problem of Swings

Peak Vds must not stress the transistors.

36

Supply Sensitivity

Voltage-dependent Cdb results in a finite Kvcofrom Vdd to output frequency:

37

One-Port View

Oscillation condition easier to meet than in 3-point topologies:

38

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

39

Use of Symmetric Inductor

Requires accurate model of inductor. cant begin design without a useful

inductor library.

40

Tuning Range Limitations

41

Effect of Varactor Q

Now include the varactor:

42

VCO Type II

Select device dimension to set the output CM level to about Vdd/2.

43

Varactor Modulation by IDD

Noise of current mirror becomes the dominant source.

Does this effect exist in Type I VCO?

44

VCO Type III

Tuning range:

With 5% bottom-plate parasitic cap:

45

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.

46

Oscillation Amplitude vs. Frequency

Suppose the tank inductor has only a series resistance:

Oscillation amplitude falls as freq is lowered.

47

Discrete Tuning

But on-resistance of switches lowers tank Q:

48

Use of Floating Switch

49

LC VCO Design Procedure

50

Application as Reference

[McCorquodale, ISSCC08]

51

Results

[McCorquodale, ISSCC08]

52

Mathematical Model of VCOs