Voltage Controlled Oscillator
(VCO)
1
Presented by:
Hazem Mohamed Fahmy
19-7926
Samuel Aysser Bassaly
16-2577
2
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
3
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
4
What is VCO?
 An oscillator is an electronic circuit that is capable of maintaining
electric oscillations expressed by a periodic function 𝒇 𝒙 = 𝒇(𝒙 + 𝒏𝒌)
where 𝒌 is constant.
 A Voltage Controlled Oscillator is an oscillator whose oscillation
frequency is controlled by a voltage input. The output frequency can
be sinusoidal or Sawtooth.
5
Why VCO?
 An oscillator is a crucial block in any communication
system in order to send and receive a signal over a
specific frequency.
 In low frequencies system (less than 100MHZ) a quartz
crystal is used due to its very accurate fixed frequency,
however in high frequencies (grater than ~300MHZs)
crystal’s quality degrades due to physical limitations.
 Many communication systems require a programmable
carrier frequency and a crystal cost and size on chip is a
drawback and from there arises the importance of using a
Voltage Controlled Oscillator.
6
Why VCO?
7
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
8
VCO Datasheet - 1
 Center Frequency: the output frequency 𝒇𝟎 of the VCO
with its control voltage at its center value.
9
VCO Datasheet - 2
 Tuning Range: the range of output frequencies that the
VCO oscillates at over the full range of the control
voltage. 𝑯𝒛 𝒗𝒔. 𝑽
 Tuning Sensitivity: the change in output frequency per
𝑯𝒛
unit change in the control voltage.
𝑽
10
VCO Datasheet - 3
 Spectral Purity: specified depending on the application. Expressed
as jitter in time-domain, as phase noise in frequency domain.
11
VCO Datasheet - 4
 Center Frequency: the output frequency 𝒇𝟎 of the VCO with its
control voltage at its center value.
 Tuning Range: the range of output frequencies that the VCO
oscillates at over the full range of the control voltage. 𝑯𝒛 𝒗𝒔. 𝑽
 Tuning Sensitivity: the change in output frequency per unit change
𝑯𝒛
in the control voltage.
𝑽
 Spectral Purity: specified depending on the application. Expressed
as jitter in time-domain, as phase noise in frequency domain.
 Load Pulling: determines the sensitivity of output frequency to
changes in its output load.
12
13
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
14
Is there more than one type of a VCO?
 Theoretically speaking, Yes.
 A VCO is implemented using LC oscillator circuit (and
Relaxation?). There are many circuit designs of an LC
oscillator but there is only one technique in order to
design a VCO and that’s by replacing a capacitor in LC
oscillator circuit with a varactor diode (a capacitor that
changes its value according to the applied voltage).
15
What are they?
 There are two types of varactor diode (𝒊. 𝒆.: 𝒕𝒘𝒐 𝒕𝒚𝒑𝒆𝒔 𝒐𝒇 𝑽𝑪𝑶):
 Abrupt Varactor: High Q factor, wide tuning range (0V-60V),
best phase noise performance.
 Hyperabrupt Varactor: Inversely proportional to 𝑽𝒊𝒏 𝟐 (more linear
tuning than abrupt)
16
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
17
How crucial is a VCO? - 1
 Electronic jamming equipment: is an equipment used in transmission of
electronic signal which distorts an original signal. VCO is used in this
equipment in order to prevent an unwanted signal from reaching
the borders of a country by changing the oscillation frequency
against the unwanted signal.
18
How crucial is a VCO? - 2
 Function Generator: is an electronic tool that is used to study and
compare different waveforms. Also used in testing circuits.
 Phase Locked Loop: is a circuit that compares input signal phase
and adjusts its internal oscillator (VCO) frequency such that the
output frequency remains matched with input frequency.
19
How crucial is a VCO? - 3
 Analog-to-Digital Converter: researches are now being done in order to
design an analog to digital converter using a VCO.
 Frequency synthesizer: is a system which is used in generation of any
range of frequencies using oscillator (VCO). Frequency synthesizers
are used in mobile phones, GPS System, satellite receivers and radio
receivers.
20
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
21
How can a VCO fit in my system?
 VCO design has limitations and trade-offs which are based on specifications
previously discussed. In order to find a relation between these specs, good-guyLeeson has presented a model that was named after him Leeson’s Model. The
modelling equation derived is given by
𝑳 ∆𝝎 . 𝑷 𝜶 [
𝒈𝒎
𝝎𝟎 𝟐
𝑰 .𝜞.𝑽𝑫𝑫
.
]
∆𝝎
𝑸𝟐𝑻
where 𝑳 ∆𝝎 is phase-noise, 𝑷 is power consumption,
and 𝑸𝟐𝑻 is Quality Factor
22
How can a VCO fit in my system?
 VCO design has limitations and trade-offs which are based on
specifications previously discussed. In order to find a relation
between these specs, good-guy-Leeson has presented a model that
was named after him Leeson’s Model. The modelling equation
derived is given by
𝑳 ∆𝝎 . 𝑷 𝜶 [
𝒈𝒎
𝝎𝟎 𝟐
𝑰 .𝜞.𝑽𝑫𝑫
.
]
∆𝝎
𝑸𝟐𝑻
where 𝑳 ∆𝝎 is phase-noise
 What happens if we increase Quality factor 𝑸𝑻 ?
Phase-noise and Power consumption will be reduced.
 Why don’t we increase it to infinity?
Because quality factor is limited by the transistor technology.
𝒈𝒎
𝑰
23
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
24
Latest Research
 Wideband LC-VCO design for LTE/LTE-A standards - 2-5
Sept. 2013 - Mediterranean Microwave Symposium
(MMS), 2013 13th - Fahs, B. et. Al.
 0.25-μm BiCMOS process
 20 mW power
 67.3% tuning-range (4856-9779 MHz)
 Phase noise -121 dBc/Hz to -115 dBc/Hz
25
Outline
 Introduction to VCO
 VCO Specifications
 VCO Types
 Applications
 Limitations and Trade-offs
 Latest Research
 Theoretical Questions
26
Theoretical Questions - 1
Why can’t we use a crystal oscillator in high
frequency operations?
Because its quality degrades over high
frequencies due to physical limitations.
Mention two VCO specifications.
Spectral Purity, Tuning Range.
27
Theoretical Questions - 2
 What is the base circuit design of a VCO? (What is the
new equation?)
LC Oscillator Circuit with oscillation frequency 𝒘𝟎 =
𝟏
𝑳𝑪
 What are the types of VCO?
Abrupt Varactor-Based VCO, Hyperabrupt VaractorBased VCO.
 Mention two applications of a VCO.
Electronic Jamming equipment, Frequency
synthesizer.
28
Outline
 Simulator
 General VCO Design
 VCO Analyses
 Generalized Equations
 Paper
 Problems
29
Simulator
 First, I would like to show you a VCO in action.
 You can find the simulator link (with lots of other useful
circuits) in the references.
 Notice that the input is a Saw tooth oscillator for
simulating the output over the full range.
 In real life, it’s usually a constant value / variable
depending on the application.
 Shall we?? 
30
OH BOY !!
 That looks exactly like many circuits we’ve seen before !!
 What makes this circuit an oscillator?
31
Outline
 Simulator
 General VCO Design
 VCO Analyses
 Generalized Equations
 Problems
32
General VCO Design
 VCOs Generally, are eternally looping negative feedback systems.
33
General VCO Design
One more thing…
34
General VCO Design
 Concerning feedback Gain.
 It has to have a specific value of 1.
 Thanks to Heinrich Barkhausen stability criterion.
35
General VCO Design
 Gain should exactly be one.
 Notice that some designs might use something else entirely other
than FB loop. (Not in our scope)
36
General VCO Design
 Could you explain more?
 May the kind asker take a look
at this?
37
Outline
 Simulator
 General VCO Design
 VCO Analyses
 Generalized Equations
 Problems
38
VCO Analysis
 First things first:
There’s no standard rule for VCOs
We apply the feedback concept to get the output.
We apply Leeson’s Equation to get phase noise.
To get math, we need to study specific models:
As this is High Speed circuits course, we will use:
 Negative Resistance LC Oscillator.
39
VCO Analysis
 What is Negative Resistance?
Resistance than when given higher current, generates
lower voltage.
 Advantages of Negative Resistance LC Oscillators:
Simple topology.
Differential implementation
Good for differential circuits.
IC people loves symmetric design 
Good phase noise performance can be achieved.
40
VCO Analysis
 NRLCO.
41
VCO Analysis
 The first step to do is to include all the losses caused by Capacitors
and Inductors.
 Use series resistances.
42
VCO Analysis
 Compile all losses into a parallel network
 (Usually Dominated by R)
43
VCO Analysis
 To simplify analysis, we can split the circuit to half.
 Notice that we do approximate Vs to GND. (This is not 100% true,
but it’s a pretty good estimation).
 Now that we have an always ON transistor (Diode), we can replace
it by a negative resistance (from each come the naming).
44
VCO Analysis
 We need Q to be as large as possible.
 Then we must have Rp as large as possible, WHY??
 The answer can be found in the general equations section.
 Choose bias current for large swing (eqtn. Deduced later).
 Transistor size must achieve relatively large Gm.
 Usually 1 / Rp
45
VCO Analysis
46
VCO Analysis
 If we assume the amplitude is large, I-bias is fully steered
to one side at the peak and the bottom of the sinusoid:
47
VCO Analysis
 If Amplitude is very large, we can assume that I1(t) is square wave:
48
VCO Analysis
 To get Fundamental component:
 Finally the resulting Amplitude will be:
49
Outline
 Simulator
 General VCO Design
 VCO Analyses
 Generalized Equations
 Problems
Generalized Equations
50
fr =1/2π LC
50
51
Outline
 Simulator
 General VCO Design
 VCO Analyses
 Generalized Equations
 Problems
Problems
52
▪
52
53
Problems

54
References
 http://en.wikipedia.org/wiki/List_of_LTE_networks
 http://en.wikipedia.org/wiki/Voltage-controlled_oscillator
 http://en.wikipedia.org/wiki/Varicap
 http://www.radio-electronics.com/info/data/semicond/varactor-varicapdiodes/hyperabrupt.php
 A CMOS Voltage Controlled Ring Oscillator with Improved frequency Stability,
APPL. MATH. INFORM. AND MECH. vol. 2, 1 (2010), 1-9.
 A 1.8GHz CMOS Voltage-Controlled Oscillator, Behzad Razavi, S.23
 A 5.9-GHz Voltage-Controlled Ring Oscillator in 0.18-m CMOS, John P. Uyemura,
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 39, NO. 1, JANUARY 2004
 A 1.4-GHz 3-mW CMOS LC Low Phase Noise VCO Using Tapped Bond Wire
Inductances, Tamara I. Ahrens et al.
 MIT OpenCourseWare
 ‘http://www.falstad.com/circuit/index.html’
55