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Basic LC Vcos: Behzad Razavi Electrical Engineering Department University of California, Los Angeles

This document discusses different types of basic LC voltage-controlled oscillators (VCOs). It covers: 1) The operation of oscillators and key VCO parameters like tuning range, gain, supply rejection, and output amplitude. 2) A one-port view of negative resistance feedback oscillators and the use of resonance in oscillator design. 3) Cross-coupled oscillator topology where oscillation frequency is a function of the inductor and varactor values in the tank.

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272 views19 pages

Basic LC Vcos: Behzad Razavi Electrical Engineering Department University of California, Los Angeles

This document discusses different types of basic LC voltage-controlled oscillators (VCOs). It covers: 1) The operation of oscillators and key VCO parameters like tuning range, gain, supply rejection, and output amplitude. 2) A one-port view of negative resistance feedback oscillators and the use of resonance in oscillator design. 3) Cross-coupled oscillator topology where oscillation frequency is a function of the inductor and varactor values in the tank.

Uploaded by

SiddharthJain
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Basic LC VCOs

Behzad Razavi
Electrical Engineering Department
University of California, Los Angeles

Outline

Operation of Oscillators
One-Port View
Cross-Coupled Oscillator
VCO Techniques
Discrete Tuning

Voltage-Controlled Oscillators
Center Frequency
z Tuning Range:
- Band of Interest
- PVT Variations
z

Gain (Sensitivity)

Supply Rejection

Tuning Linearity
z Intrinsic Jitter
z Output Amplitude
z

Feedback Oscillator

One-Port View

Example of negative resistance:

Use of Resonance in Oscillator Design

Cross-Coupled Oscillator

Looks like a diff pair with positive


feedback.
Oscillation freq is given by:

Use of Symmetric Inductor

Requires accurate model of inductor.


cant begin design without a useful
inductor library.
8

Output Swing

Peak differential output voltage swing is given


by:

How much is the output CM level?

Supply Sensitivity

Voltage-dependent CDB results in a finite Kvco


from VDD to output frequency:

10

One-Port View

11

How Do We Vary the Frequeny?

Use a MOS varactor.

12

VCO 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
Tuning range may be limited.
13

VCO Type II

Select device dimensions to set the


output CM level to about Vdd/2.

14

Varactor Modulation by IDD

Noise of current mirror becomes the


dominant source.
Does this effect exist in Type I VCO?
15

VCO Type III

Tuning range:

With 5% bottom-plate parasitic cap:

16

VCO Type IV

Select device dimensions to set the


output CM level to about Vdd/2.
Output swing twice that of previous
topologies.
But tail noise modulates varactors.

17

Discrete Tuning

But on-resistance of switches lowers tank Q:

18

Use of Floating Switch

19

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