Voltage Controlled Oscillators

Prepared by :

Yasmin Mohamed

Salma fareed

Maryam Magdy

Supervised by : Dr.Mohamed Abdelghany

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Introduction

VCO

VCO Types

Specs & Data sheet

Applications

Limitations and Trade-

offs

Latest Research

Questions

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VCO People want to be connected all the time .

High bandwidth needed.

Quartz crystal used for frequencies less than 100 MHZ

Higher than 300 MHZ , physical limitation occur .

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VCO A Voltage Controlled Oscillator is an oscillator whose oscillation

frequency is controlled by a voltage input. The output frequency can be sinusoidal or Sawtooth.

Frequency synthesizers, navigation systems, instrumentation systems, and telecommunication devices

Oscillators are electronic circuits designed to produce a repetitive electronic signal .

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Varactor

Diode

Abrupt Hyper-Abrupt

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Varactor Diodes A diode that has a variable capacitance which is a function of the voltage

that is impressed on its terminals.

Tuning / varactor diodes are operated reverse-biased, and therefore no current flows.

The width of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary.

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Varactor Diodes

Relations : capacitance is inversely proportional to the depletion region thickness depletion region thickness is proportional to the square root of the

applied voltage the capacitance is inversely proportional to the square root of the

voltage applied to the diode.

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Varactor Diodes

Different varactor diodes ,have different values and parameters of PN junction .

Abrupt varactors and hyperabrupt varactors have different properties as detailed below.

Different doping profiles could be applied to the pn junction of the varactor diode , to achieve certain c-v relations.

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Abrupt Diodes

For an abrupt varactor diode the doping concentration is held constant, i.e constant doping level as far as reasonably possible.

Disadvantage : In applications where a linear dependence is required, a lineariser is needed. This

takes additional circuitry that may be an additional burden for some applications, not only in terms of circuitry, but also the slower response speed caused by the lineariser.

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Hyper-abrupt varactor diodes

This provides a narrow band linear frequency variation. much greater capacitance change for the given voltage change

Disadvantage : Low Q factor, only used for microwave appilcations . Up to a few GHZ at most .

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Introduction

VCO

VCO Types

Specs & Data sheet

Limitations and

Trade-offs

Latest Research

Questions

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VCO Specifications

1)Control Voltage :: This is the voltage applied at the input terminal of the oscillator . This varying voltage cause a change in frequency .

2) Deviation : This refers to the amount of change in frequency due to change in voltage. A 5 volt control voltage might result in deviation of 100 ppm .

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VCO Specifications 3)Transfer Function : Denotes the direction of frequency change vs control voltage Positive transfer function : increase in frequency with increase

in voltage negative transfer function : decrease in frequency with decrease

in voltage

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VCO Specifications 4) Linearity The ratio between frequency error and total deviation,

expressed in percent. Frequency error : maximum line away from best straight line

plot through output frequency and control voltage.

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VCO Specifications 4) Linearity Solution ? The maximum deviation from the best straight line is -14 pm and total

deviation is 100 ppm , Linearity = 14 ppm /100 ppm =

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VCO Specifications

Center Frequency: is the output frequency f0 of the VCO with its control voltage at its center value and is expressed in [Hz].

Tuning Range: is the range of output frequencies that the VCO oscillates at over the full range of the control voltage.

Tuning Sensitivity: is the change in output frequency per unit change in the control voltage, typically expressed in [Hz/V].

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VCO Specifications

Load Pulling: quantifies the sensitivity of the output frequency to changes in its output load

Supply Pulling: quantifies the sensitivity of the output frequency to changes in the power supply voltage and is expressed in [Hz/V].

Power Consumption: specifies the DC power drain by the oscillator and its output buffer circuits.

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VCO Specifications

Output Power: is the power the oscillator can deliver to a specified load.

Harmonic suppression: specifies how much smaller the harmonics of the output signal are compared to the fundamental component and is typically expressed in [dBc].

Spectral Purity: can be specified depending on the application, in the time domain in terms of jitter or in the frequency domain in terms of phase noise or carrier/noise ratio.

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Introduction

VCO

VCO Types

Specs & Data sheet

Applications

Limitations and Trade-

offs

Latest Research

Questions

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LimitationsSpectral purity

Time domain :

Amplitude variation and that the zero-crossings of the output waveform are not perfectly spaced in time ,but exhibit random variation around a nominal value called jitter .

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Limitations

Frequency domain : Phase noise : frequency stability of a signal .

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Limitations

Q Factor :

High Q factor implies that there is a small damping factor and the signal is better able to maintain oscillation.

Higher Q more resistant to noise.

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Introduction

VCO

VCO Types

Specs & Data sheet

Applications

Limitations and Trade-

offs

Latest Research

Questions

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VCO Latest Research

The two LC-VCOs are designed in 0.25-μm BiCMOS process

The first oscillator provides 67.3% tuning-range (4856-9779 MHz), below than -116 dBc/Hz phase noise at 1-MHz offset and 20 mW power consumption from 1.2 V supply voltage.

The second oscillator offers 68.7% tuning-range (4821-9867 MHz), a phase noise -121 dBc/Hz to -115 dBc/Hz.

The maximum power consumption is 18.6 mW from a 1.2 V supply voltage.

Wideband LCVCO design for LTE/LTE-A standards - 2-5 Sept. 2013 - Mediterranean Microwave Symposium (MMS), 2013 13th – Fahs, B. et. Al.  

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Theoretical Questions Why can’t we use a crystal oscillator in high frequency operations? Because its quality degrades over high frequencies due to physical limitations.

What are the types of VCO? Abrupt Varactor-Based VCO, Hyper abrupt Varactor Based VCO.

What are relations of Capacitance and voltage ? capacitance is inversely proportional to the depletion region thickness depletion region thickness is proportional to the square root of the applied voltage the capacitance is inversely proportional to the square root of the voltage applied to

the diode.

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Theoretical Questions

Mention two VCO specifications. Spectral Purity -Tuning Range. 

How are Varactor diodes used in c-v relations ? Different doping profiles could be applied to the pn junction of the varactor diode.

Mention two applications of a VCO. Electronic Jamming equipment, Frequency synthesizer.

What happens if we increase Quality factor ? 𝑸𝑻 phase-noise and Power consumption will be reduced.

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Theoretical Questions

 3)What is the base circuit design of a VCO? Oscillator Circuit with oscillation frequency = √ 𝒘𝟎 𝟏 𝑳𝑪 What are two components of spectral purity ? Jitter and phase noise

Oscillators Feedback Concept

= 1Phase (

) = 0

Conditions of oscillations:

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Most Popular VCOs

�LC Oscillator: low phase noise, large area

Ring Oscillator: easy to integrate, higher phase noise

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LC Oscillators

-LC Oscillators are prone to losses which causes oscillations to decay exponentially.

-losses are caused by the series resistances in the inductors and capacitors.

-The challenge is to compensate these losses using a negative resistance , that’s why they are called “Negative Resistance” Oscillators.

-Transistors are used for the purpose of creating this negative resistance.

Basic Idea:

Analysis of Negative Resistance Oscillator

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Analysis of Negative Resistance Oscillator (Step 1)

Typically, losses are dominated by series resistance in the inductor.

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Analysis of Negative Resistance Oscillator (Step 2)

-Split oscillator circuit into half circuits to simplify analysis -We can approximate Vs as being incremental ground -Transistor can be represented with a negative resistor Note: Gm is large signal transconductance value.

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Ways of Improving Design

-Design tank components (inductors and capacitors) to achieve high Q Resulting Rp value is as large as possible

-Choose bias current (Ibias) for large swing (without going far into saturation).

-Choose transistor size to achieve adequately large gm1 (Usually twice as large as 1/Rp1 to guarantee startup)

Calculation of Oscillator Swing

- By symmetry, assume I1(t) is a square wave -We are interested in determining fundamental component (DC and harmonics filtered by tank)

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Calculation of Oscillator Swing

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Different Configurations for LC VCOs

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Problems[3]

Applications of VCO

Function Generators (low frequency oscillators).  High-frequency VCOs are usually used in phase-locked loops for radio

receivers. Voltage-to-frequency converters, with a highly linear relation between

voltage and frequency. They are used to convert a slow analog signal into a digital signal

over a long distance.[7]

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-Harmonic (Tuned) Oscillators: Resonator + amplifier The amplifier replaces the resonator losses and isolates the

resonator from the output. A varactor is used to change the capacitance and hence the

resonant frequency.

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-Relaxation (Untuned) Oscillators: They can provide wide range of optional frequencies with a

minimal number of external components. They are used in monolithic ICs. Three topologies of relaxation oscillators:1. Ground-Capacitor VCOs.2. Emitter-Coupled VCOs.3. Delay-Based Ring VCOs.

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-Relaxation Oscillators Types:

Both ground-capacitor and emitter-coupled VCOs operate similarly. The Time spent in each state depends on the rate of charge or

discharge of a capacitor. For the ring oscillator, the output frequency is a function of each

delay stage.

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Advantages of Harmonic Oscillators over Relaxation Oscillators:

Frequency stability with respect to temperature, noise, and power supply.

They have good accuracy for frequency control, since the frequency is controlled by a crystal or tank circuit.

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Disadvantages of Harmonic Oscillators:

They cannot be easily implemented in monolithic ICs, relaxation oscillator VCOs are better suit for this technology.

Unlike harmonic VCOs, relaxation VCOs are tunable over wider range of frequencies.

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VCOs Comparison

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Square Wave Generator(SQWG):

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Triangular Wave Generator(TRW):

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Linear Voltage Controlled Oscillator:

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Linear Voltage Controlled Oscillator:

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References:1] http://www.cbtricks.com/handyandy/PC-122/Clarifier.htm (varactor)

[2]Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 • http://www.vectron.com

[3] Analog Circuit Design ,Sansen, WillyHuijsing, Johan van de Plassche, Rudy 10.1007/978-1-4757-3047-0_17 Integrated GHz Voltage Controlled OscillatorsU

[4] http://eee.guc.edu.eg/CorsMain/Electronics/ELCT1003%20High%20Speed%20Electronic%20Circuits/schedule.html

[5] http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-976-high-speed-communication-circuits-and-systems-spring-2003/lecture-notes/lec11.pdf

[6] http://users.ece.gatech.edu/pallen/Academic

[7] http://en.wikipedia.org/wiki/Voltage-controlled_oscillator

[8]http://users.ece.gatech.edu/pallen/Academic/ECE_6440/Summer_2003/L130-VCO-I(2UP).pdf

[9]http://trace.tennessee.edu/cgi/viewcontent.cgi?article=3582&context=utk_gradthes

[10]http://eee.guc.edu.eg/Courses/Electronics/ELCT703%20Microelectronics/lectures_pdf/ch04-wavefunction_generators.pdf