Study of Voltage controlled oscillator for the applications in K-band and the
proposal of a tunable VCO
GARIMA KAPUR
Department of Electronics & Communication Engineering, Jaypee Institute of Information Technology, Noida-62,
INDIA
Abstract: The advances in wireless technology have made the transfer or sharing of information simple and efficient
thereby maximizing its impact to society around the globe. Due to these advances more memory space is required to
store such a large transfer of information. This can only be done by reducing the device size which means scaling of
MOS transistor to deep submicron levels. The most important part of wireless technology is the wireless trans-receiver.
Its role is to transmit or receive the information to (or from) the wireless device. In the wireless trans-receiver, the
frequency synthesizer is responsible for generating a stable output frequency which is used further to mix the received
signal down to lower frequencies and vice-versa. This stable output frequency is generated by using Phase Locked
Loop (PLL). While working at high frequency at the range of 12GHz to 40 GHz, where the operation is carried out at a
very high speed and the coverage is done with the multiple beams, the circuits used at high frequency should be
compatible with high speed. So, in this paper one very basic component which is the heart of communication system
i.e., VCO is studied and the design parameters has been listed in this paper.
Keywords: RFIC, VCO, Oscillators.
Received: April 15, 2022. Revised: June 6, 2023. Accepted: July 15, 2023. Published: August 2, 2023.
1. Introduction
With the advancement of wireless technology and number of
users with a limited range of bandwidth unexpectedly
increasing demands improved performance like high data rate
with multimedia applications. This also leads the designers to
design and fabricate the wireless components. With the
technological advantages and automation algorithms in the
past three decades, it is now viable to fabricate all the
components of a transceiver in any wireless communication
system on a single IC.
Because of the increased number of applications and demand
in all the field of communication especially in the area of
wireless communication where the high rate of data which is
transmitted on daily basis results in the need of increase in
bandwidth for the communication systems. This all is possible
because of the advancement in VLSI technology and the
automation industry. So, the range of frequencies from 12GHz
to 40 GHz are the major area of interest in which carrier
signals of very high frequency range are used, [1]. Where the
applications are mainly focused for military arm forces,
communication used in aircrafts and satellite, radio and radar
communication. The range from 27GHz to 40 GHz is used in
high throughput satellite applications and is widely available.
2. Voltage Controlled Oscillator Design
In any communication system whether it is a transmitter or a
receiver; low noise amplifier (LNA), voltage-controlled
oscillator (VCO) and phase lock loop (PLL) are the main part of
the system. In those systems, voltage-controlled oscillator plays
a very important part in any communication systems. The high
frequency signals which are used as a carrier signal, these
signals are obtained with the help of voltage-controlled
oscillator circuits. These days with the advancement in CMOS
technology where inductors can be realised using MOSFETs, so
by using active inductors oscillators are designed which can
be used to generate the signals up-to the range of GHz, [2],
[3].The traditional method used to design VCOsare either
by using CMOS ring oscillator or by using Harley and
Collpit’s oscillator which uses LC as a tank circuitt. The
advances in wireless technology have made the transfer
or sharing of information simple and efficient thereby
maximizing its impact to society around the globe. Due to
these advances more memory space is required to store such a
large transfer of information. This can only be done by
reducing the device size which means scaling of MOS
transistor to deep submicron levels. The most important
part of wireless technology is the wireless trans-receiver. Its
role is to transmit or receive the information to (or from)
the wireless device. In the wireless trans-receiver, the
frequency synthesizer is responsible for generating a stable
output frequency which is used further to mix the received
signal down to lower frequencies and vice-versa. This stable
output frequency is generated by using Phase Locked Loop
(PLL). While working at high frequency at the range of
12GHz to 40 GHz, where the operation is carried out at a very
high speed and the coverage is done with the multiple
beams, the circuits used at high frequency should be compatible
with high speed. So, in this paper one very basic
component which is the heart of communication system i.e.,
VCO is studied and the design parameters has been listed in this
paper, [4], [5], [6].
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Garima Kapur
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2.1 VCO based on inductor capacitor pair
A very basic voltage-controlled oscillator with an inductor and
capacitor is shown in Fig.1. The circuit contains inductor L and
capacitor C which are parallel to each other. In the circuits
parasitic components are shown as RL and RC for the inductor
and capacitor respectively. The overcome the energy loss
associated with these parasitic components MOSFETs or
CMOS can be utilized to have the negative resistance. The
energy which is lost in the tank circuit is given by the equation
Eq.1.
Eq.1
Fig. 1 VCO based on inductor capacitor pair
It can be observed from (1) that the power loss in the tank
circuit is inversely proportional to the value of the inductance
and the operating frequency. It can be seen that the power loss
in the tank circuit decreases linearly if there is a decrease in the
series resistance R as it is directly proportional to the loss, and
it also decreases quadratically with an increase of the tank
inductance. This energy loss due to the parasitic components
should also be overcome by adding MOSFET to the circuit. To
compensate the parasitic resistance, a negative resistance -R
can be introduced to the circuit by using active devices so that
both the unwanted stray elements can be cancelled out. This is
done by using the transistors in cross-coupling topology in the
tank circuit, this is shown in Fig.2.
Fig. 2 Cross coupled oscillator
The idea behind using the cross coupled transistors is to have
the same value of conductance (gm) as provided by the
negative resistance from the oscillator, [7].
2.2 Ring VCO
A ring oscillator can be designed by using a number of buffer
stages, in the basic approach the output of the last stage is
connected back to the input of the initial stage. The criteria for
the oscillation is that the circuit must give a phase shift of 2π or
0 and voltage gain should be greater that equal to 1, [8].
Fig.3Ring oscillator
In the circuit every stage is used to generate the delay and the
phase shift of π=N, where N is used to define the number of
delay stage. The another required phase shift is provided by a
dc inversion, [9].
2.3 Difference Between LC voltage-Controlled
Oscillator and Ring VCO
The Voltage controlled oscillators as we know that it plays
very important role in the design of Phase Lock Loop
(PLL) circuit. In this study paper the most commonly used
VCOs CMOS ring oscillator and LC tank oscillators are
shown in the previous section. The LC oscillator circuits
can be used as they have the advantage that they have
better noise characteristics, but with the drawback that this
approach may have the large dimensions, so cannot be
used where the phase shift operation is required. Whereas,
the ring voltage-controlled oscillator has the better
performance parameters as compared to LC oscillator but
they also have the demerit that these circuits have low
power requirements and less area on the chip, so they can
be prone to noise. So, because of these clear advantages of
ring oscillators, ring oscillators are preferred over the LC
oscillators, [10], [11].
3.Design Topologies
The VCO is the only RF block in a PLL. Its basic function is to
generate a constant RF frequency in wireless transceivers.
Besides having a simple design architecture, it is the most
challenging block to design because of its operation at high RF
frequencies in which the phase noise becomes significant and
its parameters gets deviated from desired values. When voltage
at the input of VCO changes then frequency at the output is
varied. The VCO can be realized either as a ring oscillator or as
resonant oscillators.
As shown in Fig.4(a) an inductor L and capacitor C are parallel
to each other. The resonance frequency of this LC circuit is
given by Eq.2.
ωres=1/√LC Eq.2
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At the given resonant frequency, the impedance of the inductor
and capacitor which are written as jLωres, and 1/(jCωres)
respectively , are opposite and equal to each other, thus
resulting an infinite impedance. But in actual practical circuits
these, passive components have stray impedance indicated as
resistive components, as shown in Fig.4(b) and the quality
factor for inductor is given as , Q as given in Eq.3
Q= Lω/Rs Eq.3
Because the value of quality factor(Q) of the capacitor is much
larger than the value of quality factor Q of the inductor,
therefore the losses due to Rc are considered as negligible. The
series model of the circuit is presented in the form of Fig.4(c)
Fig.4: a) LC parallel circuit b) Oscillator Resonator with Series
Resistance of the Inductor and Capacitor, c) series model of the
circuit.
Now, converting the resistor (Rs) which in series with inductor
(L) in Fig. 5(a) into the parallel form in Fig.5(b)
we get
L≈ L p Eq.4
R ≈ Q 2Rs Eq.5
Fig.5: a) Inductor with Parasitic Series Resistance, b)
Series Resistance Conversion into Parallel Resistance
It can be concluded from Eq.5; the quality factor of the
inductor plays an important role in determining the amount of
energy lost in the tank. Fig.6 shows a simple gain stage based
on an LC tank.
Fig.6: RLC Gain Stage used in the circuit
4. Design Parameters
Almost every trans-receiver which is designed for wireless
applications requires a tunable reference frequency. Thus, an
ideal VCO is required that will generate an output which is
linearity proportional to the applied input voltage. And except
from this linearity parameter some another parameteris also
there which plays an important role in the designing of
oscillators. Some of the parameters are discussed below:
4.1 Linearity
Linearity and the tuning of that linearity is the most important
parameter of a VCO. Ideally, linear tuning is required, but in
actual implementation the nonlinear behavior of VCO is
observed as components used are also non-linear. Linearity is
the required to have the VCO gain (KVCO) constant as given
in Fig.7.
Fig.7: Ideal VCO Tuning Linearity
4.2 Range of Tuning
The range of tuning of voltage-controlled oscillator is based on
the following two specifications:
(i) The center frequency of the tuning range must be remained
constant with the frequency of oscillation.
(ii) Frequency deviation due to even slight variations in
temperature results nonlinearities in VCO characteristics. To
minimize the effect of these variations, so, a wide tuning range
is selected. But these nonlinearities can be minimized by
narrowing down the tuning range. Therefore, a tradeoff exists
between nonlinearities and tuning range.
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4.3 Power Consumption
In a PLL, most power is dissipated by the VCO as compared to
other components. In this paper, VCO is studied for RF trans-
receiver therefore; priority is given to the tuning range and
phase noise as compared to power.
4.4 Phase Noise
The sidebands present around the central frequency in
frequency domain system is called the phase noise and the same
sidebands in time-domain system is called jitter. Linear time-
invariant model of phase noise given by Leeson is represented
in Eq.6.
Eq.6
QL = loaded quality factor
fo = oscillation frequency
Ps = signal power of oscillation
fm = offset frequency
F = noise factor of active devices
k = Boltzman’s constant
T = temperature (Kelvin)
fk = flicker noise corner frequency in the phase noise
,
and
are the sensitivity of the VCO to the control voltage, supply,
and bias current respectively.
We observe that amongst various factors that can reduce phase
noise, the circuit designer can control only three factors
namely, loaded quality factor, noise factor of active device and
signal power of oscillations.
(i) The loaded quality factor of the device (QL) is
extracted by the amount of series resistance in the
LC tank.
(ii) Therefore, high quality factor resonators
inherently have lower phase noise.
(iii) The output power (Ps) is inversely proportional to
phase noise. But output power must be
minimized. So, we have a tradeoff between power
and phase noise.
(iv) The noise factor (F) is directly proportional to
phase noise. Lowering the noise factor is directly
related to the active components used in the VCO.
Therefore, devices with lower flicker noise are
better for this application.
5. Performance Comparison of VCO
Designs
Various VCO structures has been studied and their performance
parameters has been compared. The comparison of performance
parameters has been done for the designs which are designed
for the frequency range between 24GHz to 40GHz i.e., suitable
for the application of 5G circuits.
The circuit presented, [12], is used for the generation of a
24GHz oscillatory signal. This signal has been generated
using a 12GHz voltage-controlled oscillator indirectly
cascaded with passive mixer. The circuit has been
implemented in 0.18um CMOS technology, the advantage of
using the passive mixer reduces the power consumption and
also increases the tuning range of the device.
6. Varrious Vco Designs Used in the
Frequency Range of 24Ghz to 40Ghz
As it has been studied that the VCO is one of the important
blocks in the communication system. For the application of
VCOs in Radar, it is used to find the highest frequency range of
the system. In VCOs varactor diodes are used to tune the
frequency of the device. A complementary Cross – coupled LC
VCO presented in [13], that uses the MOSFET in accumulation
mode for the tuning of the device. The LC tank resonator is
used in the circuit contains on-chip differential inductor and a
pair of MOSFET that is used in accumulation mode for a good
linearity. As another application of VCO an integrated PLL has
been shown, [14], with low phase noise. The circuit can be used
for wireless high quality video streaming. The circuit also uses
the cross-coupled VCO used with varactor capacitance. The
circuit provides the wide tuning range, less power consumption
as well as low phase noise. Next approach that can be used and
indicated, [15], that uses the tunable active inductor
approach. This circuit presented in [15], is also designed
for the radar application and for the K and Ku band of
applications. The circuit indicates the high value of Q-factor
and low phase noise. VCO indicted in [16],
[DTMOS] provides high transconductance gm. In the
given circuit two N-type MOS are connected in parallel
with the conventional VCO circuit.Capacitive division
technique is used to increase the voltage swing and to lower
the phase noise value.
The design of cross coupled VCO is presented in Fig.8 and has
been simulated at 90nm technology using BSIM4 MOSFETs
and the operation is carried out by the charging and discharging
of inductor and capacitor. There will be the die down wave in
the frequency of oscillations because of the lose of energy. This
lose of energy can by represented by adding the Rp resistance
in parallel to L and C.
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Fig.8: Cross-Coupled Voltage Controlled Oscillator
The circuit has been designed on ADS design tool and the
simulation is done at 90nm technology. The output has been
plotted differentially between Vop and Vom nodes. To avoid
the degradation of VCO tuning range the capacitively loading
on the output nodes should be avoided. The device size is
decided by considering the loss in the circuit. When the net
currents are balanced, net voltage across the LC tank circuit is
zero and at that point the noise will affect the circuit
performance. The tuning of the VCO can be done by unis the
tunable active inductor. The oscillation produced are shown in
the Fig.9. and the frequency of oscillations can be calculated by
taking the fourier transform of the (Vop-Vom). The tuning of
the circuit is proposed by replacing the inductor L by gyrator-C
based inductor in which the direct tuning can be done by the
biasing applied at the feedback MOSFET as shown in Fig.10.
Fig.8:Frequency of oscillations of Cross-Coupled VCO
Fig.10:Tunable Active inductor design
7. Conclusion
The role of voltage-controlled oscillator in the field of wireless
communication and other area of application has been studied.
The design parameters which should be kept in mind to while
designing the VCO has been presented in the paper. And as an
application in the field of radar and another wireless
communication system has been seen. Some of the designs used
the tunable active inductor approach, which can be further
improved by changing the design technology. A simple
approach where a inductor L is replaced by single ended
inductor is proposed in this study paper. The further
modification can be done by using the varactor diodes for the
charge storage in the tank circuit.
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Policy)
The author contributed in the present research, at all
stages from the formulation of the problem to the
final findings and solution.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The author has no conflict of interest to declare that
is relevant to the content of this article.
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