Transcript VCO Specifications - GUC - Faculty of Information Engineering

1
Voltage Controlled
Oscillators
Prepared by :
Yasmin Mohamed
Salma fareed
Maryam Magdy
Supervised by : Dr.Mohamed Abdelghany
Introduction
VCO
Questions
VCO Types
Latest
Research
Specs &
Data sheet
Limitations
and Tradeoffs
Applications
2
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
HyperAbrupt
5
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.
6
[1]
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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[1]
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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[1]
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.
9
[1]
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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[1]
Introduction
VCO
Questions
VCO Types
Latest
Research
Specs &
Data sheet
Limitations
and Tradeoffs
11
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
[2]
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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.

[2]
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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 = ±14 %

[2]
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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
Questions
VCO Types
Latest
Research
Specs &
Data sheet
Limitations
and Tradeoffs
Applications
19
Limitations
Spectral 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 .
[3]
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Limitations

Frequency domain :

Phase noise : frequency stability of a signal .
[3]
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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.
[3]
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Introduction
VCO
Questions
VCO Types
Latest
Research
Specs &
Data sheet
Limitations
and Tradeoffs
Applications
23
VCO Latest Research
The two LC-VCOs are designed in 0.25-μm BiCMOS process
Wideband LCVCO design
for LTE/LTE-A standards 2-5 Sept. 2013 Mediterranean Microwave
Symposium (MMS), 2013
13th –
Fahs, B. et. Al.
The first oscillator provides 67.3% tuning-range (48569779 MHz), below than -116 dBc/Hz phase noise at 1MHz offset and 20 mW power consumption from 1.2 V
supply voltage.
The second oscillator offers 68.7% tuning-range (48219867 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.
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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
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Oscillators Feedback Concept
Conditions of
oscillations:
=1
Phase
(
)=0
[4]
Most Popular VCOs
ƒ
LC Oscillator: low phase
noise, large area
Ring Oscillator: easy to
integrate, higher phase
noise
[5]
LC Oscillators
Basic Idea:
-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
Analysis of Negative Resistance Oscillator
[5]
Analysis of Negative Resistance Oscillator (Step 1)
[5]
Typically, losses are dominated by series resistance in the
inductor.
Analysis of Negative Resistance Oscillator (Step 2)
[5]
-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.
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
[2]
[5]
- By symmetry, assume I1(t) is a square wave
-We are interested in determining fundamental component (DC and
harmonics filtered by tank)
Calculation of Oscillator Swing
[5]
Different Configurations for LC VCOs
[5]
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]
[8]
-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.
[7], [8]
-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.
[7], [8]
-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.
[7]
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.
[7]
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.
[7], [8]
VCOs Comparison
[9]
Square Wave Generator(SQWG):
[10]
[10]
[10]
[10]
[10]
[10]
[10]
[10]
Triangular Wave Generator(TRW):
[10]
[10]
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[10]
[10]
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Linear Voltage Controlled Oscillator:
[10]
Linear Voltage Controlled Oscillator:
[10]
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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-speedcommunication-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&con
text=utk_gradthes
[10]http://eee.guc.edu.eg/Courses/Electronics/ELCT703%20Microe
lectronics/lectures_pdf/ch04-wavefunction_generators.pdf