TELKOMNIKA Telecommunication Computing Electronics and Control

Vol. 21, No. 4, August 2023, pp. 872~880

ISSN: 1693-6930, DOI: 10.12928/TELKOMNIKA.v21i4.22341  872

Design and performance analysis of low phase noise LC-voltage

controlled oscillator

Ramchandra Gurjar, Deepak Kumar Mishra

Department of Electronics and Instrumentation Engineering, Shri G S Institute of Technology and Science, Indore, India

Article Info ABSTRACT

Article history: Voltage controlled oscillator (VCO) offers the radio frequency (RF) system
designer a freedom to select the required frequency. Today’s wireless
Received Nov 30, 2021 communication system imposes a very stringent requirement in terms of
Revised Aug 08, 2022 phase noise generated in VCO. This study presents an inductive source
Accepted Oct 26, 2022 degeneration technique to improve the phase noise performance of the
inductance-capacitance (LC)-VCO. Double cross-coupled topology has been
chosen for the proposed VCO. The post layout simulations with the parasitic
Keywords: resistance, inductance, capacitance (RLC) extracted view is carried out with
united microelectronics corporations (UMC) 0.18 µm process by spectre
Figure of merit simulator of cadence tools. The proposed VCO provides a phase noise
Low phase noise of -124.3 dBc/Hz @ 1 MHz. The tuning range obtained is 19.87% with a
MOS varactor centre frequency of 2.46 GHz which makes it suitable for industrial,
Tuning range scientific, and medical (ISM) band applications. It consumes a power of
Voltage controlled oscillator 2.10 mW. Also, a good figure of merit of -189 is achieved. The total layout
area occupied is 477×545 µm2.
This is an open access article under the CC BY-SA license.

Corresponding Author:
Ramchandra Gurjar
Department of Electronics and Instrumentation Engineering
Shri G S Institute of Technology and Science, Indore, India
Email: rcgurjar94@gmail.com

1. INTRODUCTION
The global voltage controlled oscillator (VCO) market is predicted to increase substantially due to
rapid technological advancements in the very large scale integration (VLSI) sector and the wide application
of voltage-controlled oscillation in numerous end-user industries. There is a need for wide tunable reference
frequency in almost all wireless or wireline tasks, which supports multi-standard applications. Despite a large
amount of research and development, radio frequency (RF) designers still find the VCO to be a difficult
component. As the need for wireless communications grows, and new applications enter the market at higher
frequencies, VCOs are subjected to more demanding standards. VCO may be realized with many
configurations according to the applications and performance requirements. The available fundamental
approaches are ring oscillator [1]-[3], inductance-capacitance (LC) oscillator [4], [5] tunable active inductor
(TAI) based oscillator [6], [7], and relaxation oscillator [8]. Apart from the phase noise (PN); tuning range,
power consumption, and output waveform are also necessary VCO specifications. Unfortunately, there are
direct tradeoffs between these specifications like ring oscillator, or TAI-based VCOs have larger tuning range
but lower phase noise while resonator-based oscillators have lower phase noise but suffer in terms of tuning
range. The performance of VCO has a very significant impact on the overall performance of RF front end they are
being used in [9]. In high-performance applications where low PN is required, VCO using LC tank is preferred.
The minimum phase noise requirement in the VCO is set by the specific communication standard. Various low PN
techniques like noise filtering [10]-[12], tail current shaping [13], [14], self-switched bias [15], [16] have been

Journal homepage: http://telkomnika.uad.ac.id

TELKOMNIKA Telecommun Comput El Control  873

reported in the literatures. In this paper, we present an inductive source degeneration (ISD) based low PN
technique to improve the PN performance of the LC-VCO. The organization of the paper is as: section 2
discuss the methodology for implementing LC-VCO. Circuit architecture and detailed analysis of the
proposed design are presented in section 3. Section 4 elaborate the layout, post layout simulation and
performance comparison followed by conclusion.

2. METHODOLOGY FOR IMPLEMENTATION OF LC-VCO

VCO designers generally need a methodology to evaluate the performance parameters. It helps to
optimize the various components of the circuit. Double cross-coupled (CC) topology is very much popular
among the LC-VCO designers. It has been widely used by the many researchers [17]-[24]. The CC topology
offers large amplitude, symmetrical waveform and higher transconductance. All these advantages make the
double CC differential topology an optimal choice, and hence the same has been adopted in this work.
The design of an optimized LC tank is of prime importance.

2.1. VCO core design

In order to get the low power consumption, large tuning range, and low PN performances. The LC
resonator and active circuitry of VCO must be optimized. The design of inductor and varactor is challenging to
achieve a low PN VCO. In this design, the size of N-channel metal-oxide semiconductor (NMOS) and P-channel
metal-oxide semiconductor (PMOS) transistors, inductors, varactor value, and parasitics are critical physical
parameters.
− Metal oxide semiconductor (MOS) transistor sizes
The size of transistors is one of the essential design parameters which impacts the various
performance parameters; hence it may be considered for the optimization procedure. The large size of the
transistor gives a better transconductance which helps to overcome parasitic losses offered by the LC tank
and thus helps in start-up oscillations at the cost of tuning range. Hence the aspect ratio of the transistor is
chosen to generate sufficient negative resistance. Also, as the symmetry has to be maintained for better PN,
the sizing of NMOS and PMOS has been chosen keeping in mind the following relations.
µ𝑛𝐶𝑂𝑋 𝑊
𝐼𝑑𝑠𝑎𝑡 = . . (𝑉𝐺𝑆 − 𝑉𝑡ℎ )2 (1)
2 𝐿

𝑑𝐼𝑑𝑠𝑎𝑡 𝑊
𝐺𝑚𝑛 = = µ𝑛 𝐶𝑂𝑋 (𝑉𝐺𝑆 − 𝑉𝑡ℎ ) (2)
𝑑𝑉𝐺𝑆 𝐿

Similarly for PMOS:

𝑊
𝐺𝑚𝑝 = µ𝑝 𝐶𝑂𝑋 (𝑉𝐺𝑆 − 𝑉𝑡ℎ ) (3)
𝐿

The minimum length of 180 µm for the transistors has been used.
− Circular spiral inductor
Lesson’s phase noise model [25] suggests large Q of the tank to achieve the lower PN. The selection
of an inductor in the design of VCO is an important aspect. A three terminal circular spiral inductor model
L_SLCR20K_RF from UMC 0.18 µm RF compemetary metal oxide semiconductor (CMOS) process library
in spectre RF has been employed to design the proposed VCO. The various parameters of the spiral inductor
having 7.14 nH inductance is given in Table 1. The inductance value can be adjusted by the number of turns,
width and diameter. The inductance value affects the tank amplitude and start-up constraints.

Table 1. Spiral inductor’s parameter (UMC process technology)

Parameter Size/value
Diameter (µm) 126
Width (µm) 6
Number of turns 5.5
Inductance (nH) 7.14

− MOS varactors
The tuning of a spiral inductor is not possible by some control voltage, so we need some varactor to
implement the VCO. Because of its wider capacitance range compared to junction varactor, the inversion
metal oxide semiconductor (IMOS) varactor has been employed as a tuning element of LC VCO.

Design and performance analysis of low phase noise LC-voltage controlled oscillator (Ramchandra Gurjar)
874  ISSN: 1693-6930

1
𝑓0 ≅ (4)
2𝜋√𝐿(𝐶𝑣 +𝐶𝑎𝑣𝑔 +𝐶𝑝𝑎𝑟 )

Where 𝐶𝑎𝑣𝑔 is the instantaneous average capacitance due to sinusoidal voltage at the gate terminal of
varactor, 𝐶𝑣 is the varactor capacitance and 𝐶𝑝𝑎𝑟 is capacitance due to presence of parasitic. Other than these
capacitances, load capacitance (𝐶𝐿 ) or external capacitance, whose value depends upon the application (buffer),
is also added. These external capacitances also affect the power dissipation. Therefore the value of tuning
elements i.e., 𝐿 or 𝐶 has to be recalculated. This means the VCO requires redesign for different load [1]. This
makes the oscillator design a challenging task. So as this (4) suggests that only inductance or capacitance are
tuning elements. The large tuning indicates better design in terms of controllability.
The PMOS transistors instead of NMOS transistors have been used in the varactor design because of its
low flicker noise. To reduce the flicker (1/f) noise contributed by MOS switches, the device area can be increased
as shown in (5). A larger area means a lower tuning range, so we need to adjust the area properly. The value of
𝐾 for PMOS is 50 times lower than NMOS, so it gives less drain current thermal noise than NMOS.
̅̅̅̅̅
𝑖2 1 =
𝐾 1 2
. . 𝑔𝑚 (5)
𝑛, 𝑊𝐿𝐶𝑜𝑥 𝑓
𝑓

2.2. Design flow

Various design flow for the LC-VCO have been proposed in the literatures [22], [26]. Design
specifications originate from the application where VCO is being used. Technology file or foundry to be used is
also an important criterion. Next, there is a need to identify the design variable available for the design.
The important design variables are MOS transistor size, the geometrical parameter of the inductor, and a number of
transistor arrays in the varactor. The design flow is shown in Figure 1.

Figure 1. Design flow for the proposed LC-VCO

TELKOMNIKA Telecommun Comput El Control, Vol. 21, No. 4, August 2023: 872-880
TELKOMNIKA Telecommun Comput El Control  875

3. INDUCTIVE SOURCE DEGENERATION BASED LC-VCO

The noise associated with tank and active circuits are the intrinsic noise sources that cannot be
permanently removed. Active devices contribute major noise as compared to other components of the VCO.
The designers have minimal options like device selections, and setting bias operating points. To minimize the
phase noise. The PN models discussed in [25], [27] gives some insights into low PN VCO design. As per the
lesson’s equation [25], tank Q, active circuitry, signal power, and other frequency parameters are the important
consideration for low phase noise. The flicker noise reduction mechanism was discussed in [28], [29]. Various
techniques exist to reduce the phase noise performances in VCO [29], [12], [30], [31], [16]. The VCO design
techniques need to be investigated in several aspects to obtain a better PN performance. The tail current transistor
used in conventional LC-VCO as shown in the Figure 2(a) contributes to phase noise. As reported
in [12], [15], [10] noise from tail transistor can be effectively suppressed by filtering techniques. Here, we present
the inductive source degeneration technique for low phase noise VCO design.

3.1. Circuit architecture and analysis

Figure 2(a) shows the conventional design of LC-VCO while Figure 2(b) shows the proposed double CC
VCO with inductive source degeneration. The advantages of these differential topologies are that they can directly
drive circuits that require a differential input. The complementary cross-coupled configuration has been used to
compensate losses produced by tank circuits and active sources. This complementary structure gives double
transconductance compared to conventional all NMOS structure at the cost of tuning range and thus relaxes the
start-up criteria. The negative resistance which is required for start-up oscillation may be expressed as:
−2
𝑅𝑛𝑒𝑔 = ≤ 𝑅𝑒𝑞 (6)
𝑔𝑚𝑛 +𝑔𝑚𝑝

Where 𝑅𝑛𝑒𝑔 is the equivalent negative resistance offered by double cross-coupled transistor, 𝑅𝑒𝑞 is the
parallel resistance provided by LC tank, 𝑔𝑚𝑛 and 𝑔𝑚𝑝 are the transconductance of NMOS and PMOS
transistor respectively.
In differential CC LC oscillator, tail current generator at source is considered the major source of
flicker noise. The close-in PN is dominated by the up-converted flicker noise of the tail current [30], [32], [33].
Most of the complementary structures either use NMOS or PMOS tail current sources. In any balanced
circuit, even harmonics flow in a common mode path; therefore, there is a need for high impedance to even
harmonics of the oscillation frequency. The high impedance offered by the tail current source also helps to
avoid the degradation of the resonator’s quality factor [12].
Source degeneration scheme is an excellent technique to suppress flicker noise up-conversion into
phase noise [34]. Several circuit structures using source degeneration techniques such as inductive degenerated,
resistive degenerated, capacitive degenerated, and LC filtering technique [12], [31] are presented in various
research papers. By removing the tail current generator in the proposed design, the close in phase noise could be
improved, but this will impair the quality factor of resonator and due to the absence of high impedance,
oscillator will be more sensitive to ground noise. So, instead of tail current generator, an inductor is inserted.

(a) (b)

Figure 2. VCO: (a) traditional implementation of double CC VCO and (b) topology with added ISD

Design and performance analysis of low phase noise LC-voltage controlled oscillator (Ramchandra Gurjar)
876  ISSN: 1693-6930

In perfectly balanced LC-VCO, odd and even harmonics are present. The odd harmonics exist in a
differential path with no current flowing through the tail transistor. Opposite to that, even harmonics flow
from supply to ground path, including the tail transistor. The nonlinearities in the oscillator are responsible
for converting low-frequency noise of the tail transistor into high-frequency noise around the even harmonics
and the down-converting to the PN around the carrier. The effect of higher-order harmonics on the phase
noise is neglected oweing to their low level.
The inductor provides a high impedance common node for differential pairs at the cost of the area.
A spiral inductor (𝐿𝑠 ) is used to resonate in parallel with parasitic capacitance 𝐶𝑝 at source node (𝑆). If the
value of inductor (𝐿𝑠 ) is chosen in such a way that 𝜔𝑠 (resonant frequency at source node) is equal to the
second harmonic frequency (2𝜔𝑜 ) of the oscillator, then impedance (𝑍𝑠 ) at source node seen from
differential transistor is approximately infinite [35]. The Resonant frequency at output node is depicted in
Figure 3(a). The Resonant frequency at source node (𝑆) as shown in Figure 3(b) and Figure 3(c) is:
1
𝜔𝑠 = = 2𝜔𝑜 (7)
√𝐶𝑝 𝐿𝑠

Where 𝜔𝑠 is the resonant frequency at source node, 𝐿𝑠 is the series inductor and 𝐶𝑝 is the total capacitance at
the source terminal (𝑆) of the oscillator including parasitic capacitance and source capacitance of NMOS
transistors. This can be shown:
𝑗2𝜔𝑜 𝐿𝑠 𝑗2𝜔𝑜 𝐿𝑠
𝑍𝑠 = = (2𝜔𝑜 )2
(8)
1−(2𝜔𝑜 )2 𝐿𝑠 𝐶𝑝 1−
(𝜔𝑠 )2

It can been seen from (8), as 𝜔𝑠 (Figure 3(b)) is approximately equal to second harmonic of
oscillator frequency (𝜔𝑜 ) (Figure 3(a)), the impedance 𝑍𝑠 at source node approaches infinity, and the 𝑄 of the
inductance-capacitance tank is maintained. A symmetrical waveform is also helpful to get the lower PN [36].
So in the proposed design, 𝑊/𝐿 ratio of tramsostors of CC is chosen to have equal rise and fall time in the
waveform. To obtain symmetrical waveform, the transconductance of PMOS and NMOS transistors should
be equal. This leads to (9):

𝑊𝑛 𝑊𝑝
√2𝜇𝑛 𝐼𝑑𝑠 𝐿𝑛
𝐶𝑜𝑥 = √2𝜇𝑝 𝐼𝑑𝑠
𝐿𝑝
𝐶𝑜𝑥 (9)

Where 𝜇𝑛 and 𝜇𝑝 are the surface mobilities of NMOS and PMOS channel respectively, 𝐶𝑜𝑥 is the capacitance
𝑊𝑝 𝑊𝑛
per unit area of the gate oxide, and are the effective channel width-length ratio of PMOS and NMOS
𝐿𝑝 𝐿𝑛
device respectively.

(a) (b) (c)

Figure 3. Simulated waveform: (a) single-ended output signal (𝜔𝑜 ), (b) second harmonic of oscillating
frequency (2𝜔𝑜 ) at source node (𝑆), and (c) at the source and output node

The higher sensitivity of the oscillating frequency to voltage supply (frequency pushing) can be lowered
by inserting an inductor (𝐿2 ) having 3.5 nH inductance between a supply voltage and resonator. In addition to that,
it also provides a high impedance path between resonant tank and power supply (𝑉𝐷𝐷 ). Variation in the oscillation
frequency has been obtained by varying the control voltage of IMOS varactor consisting of 5 parallel units of two
series connected back to back PMOS transistor.

TELKOMNIKA Telecommun Comput El Control, Vol. 21, No. 4, August 2023: 872-880
TELKOMNIKA Telecommun Comput El Control  877

4. RESULTS AND DISCUSSION

The performance of the design is greatly affected by the parasitic resistance and capacitance present in
the layout. The good layout design considerably reduces the degrading of the system performance.
The complete layout of the proposed inductive source degeneration-based LC-VCO is drawn using Cadence
Virtuoso layout suite XL with UMC process parameter as shown in Figure 4. It consists of three spiral inductor,
a varactor bank, and CC pairs of the transistor. The spiral inductors occupy the 99% area in the layout. The total
area is 477 µm × 545 µm. After design rule check (DRC) and layout versus schemetic (LVS), quantus parasitic
extraction was performed to extract the parasitics offered by the design. The dimensions and values of the
transistor are given in Table 2.
Post layout simulation from the extracted cell view is required to evaluate the effect of parasitic on
the system’s performance. This also helps a designer to get the results closer to reality. Here in this work,
post layout simulations were carried out with UMC 0.18 µm process by spectre simulator of cadence tools.
The periodic steady state (PSS) simulation is performed to evaluate the tuning range of the proposed VCO.
IMOS varactor consisting of PMOS transistors has been used to regulate the tuning range. The VCO exhibits
a turning range from 2.22 GHz to 2.71 GHz when the control voltage varies from 0.8 V to 1.8 V, as shown in
Figure 5. Phase noise performance represents the spectral purity of the output signal. The plot shown in
Figure 6 shows the single sideband PN with respect to relative frequency from the carrier for the proposed and
conventional design.

Figure 4. Layout of the proposed Figure 5. Tuning range of the Figure 6. PN performance with and
ISD based LC-VCO proposed VCO without ISD technique

Table 2. Transistor aspect ratio of the proposed VCO

Components Device size / values Fingers
𝑀1 , 𝑀2 2.4 µm / 0.18 µm 1
𝑀3 , 𝑀4 6.0 µm / 0.18 µm 1
PMOS transistor of varactor 9 µm / 0.18 µm 10

At offset frequency of 1 MHz, the PN achieved for this technique is -124.3 dBc/Hz, which can meet
the specification of ISM band applications. Using the proposed technique, PN improvement of -11.7 dBc/Hz
at a frequency offset of 1 MHz is achieved compared to the conventional design. A buffer amplifier is not
considered in the design but output power must have a reasonable value. The output spectrum is shown in
Figure 7. The VCO gain (𝐾𝑉𝐶𝑂 ) with respected to tuning voltage is ploted in Figure 8. The Monte Carlo
(MC) simulation for 1000 sample has been performed to examine the effect of process variation on the phase
noise. The MC simulation based histogram is ploted in Figure 9. The VCO draws a current of 1.166 mA from
the supply voltage of 1.8 V. The performance of VCO may be evaluated in terms of various specifications like
PN, power dissipation, output amplitude, and tuning range. There are several figure of merits (FOMs) to
evaluate the performance metrics of a VCO. The following equation is widely used to evaluate the performance
of VCO [37].

𝑓𝑜 𝑃𝐷𝐶
𝐹𝑂𝑀 = 𝐿(𝑓𝑜𝑓𝑓 ) − 20 𝑙𝑜𝑔 ( ) + 10 𝑙𝑜𝑔 ( ) (10)
𝑓𝑜𝑓𝑓 1𝑚𝑤

Where 𝐿(𝑓𝑜𝑓𝑓 ) represents the PN at offset frequency and 𝑃𝐷𝐶 is the power consumption in mW.
The comparison performance of the proposed works with other state of work is given in Table 3.

Design and performance analysis of low phase noise LC-voltage controlled oscillator (Ramchandra Gurjar)
878  ISSN: 1693-6930

Table 3. A performance comparison of the proposed work with others

Specifications This work [18] [38] [11] [39]
Process technology (µm) 0.18 0.13 0.18 0.18 0.18
Supply voltage (V) 1.8 1.0 1.8 0.5 1.8
Centre frequency (GHz) 2.46 2.28 2.45 2.34 2.47
Frequency range (GHz) 2.22 -2.71 2.17-2.40 2.42 -2.48 2.22-2.46 2.38-2.56
Tuning range (%) 19.87 10 2.44 10 7.28
PN (dBc/Hz @ 1 MHz) -124.3 -114.7 -124 -119.4 -97.76
Power dissipation (mW) 2.10 0.262 2.86 3.23 3.78
FOM (dBc/Hz) -189 -188.15 -187.25 -181.3 -161
Layout area (µm × µm) 477×545 365 ×473 521 × 234 390×860 -

Figure 7. Output power spectrum Figure 8. VCO gain with respect to tuning voltage

Figure 9. MC simulation-based histogram of PN

5. CONCLUSION
In this paper, the proposed design of LC-VCO was discussed with a focus on phase noise
performance. The basic step in the design of VCO is the selection of a suitable topology. The design
methodology for VCO was presented. The design, implementation, and layout of CMOS LC-VCOs were
accomplished using Virtuoso analog design environment with UMC process parameter, and post layout
simulations were performed using spectre. The low phase noise technique, namely inductive source
degeneration were investigated. With low phase noise ISD, satisfactory performance of the VCO with phase
noise of -124.3 dBc/Hz at 1 MHz offset frequency was obtained. The post layout simulation results confirm
that these VCOs can meet the specification for applications in the 2.2 GHz to 2.7 GHz unlicensed ISM bands.
The performance outcomes validate the effectiveness of the topologies and methodologies used in the design.

ACKNOWLEDGEMENTS
Authors extend thanks to SMDP-C2SD (A Project of Ministry of Electronics and Information
Technology, Govtorment of India) for providing VLSI-Electronic Design Automation tools in the laborotary.

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TELKOMNIKA Telecommun Comput El Control  879

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BIOGRAPHIES OF AUTHORS

Ramchandra Gurjar received the B.E. in Electronics and Communication

Engineering and M.E. in Digitial Technique Instrumentation from Rajiv Gandhi Technological
University, Bhopal (India) in year 2001 and 2006, respectively. He obtained his Ph.D. in
Electronics Engineering from RGPV, Bhopal. He is currently associate professor in SGSITS,
Indore (India). His current research interests include design of active inductor, VCO, PLLs and
FPGA based design. He can be contacted at email: rcgurjar94@gmail.com.

Deepak Kumar Mishra received the B.E. degree in Electronics and M.E. in
Applied Electronics and Servomechanism from the DAVV (Formerly- University of Indore)
Indore in 1979 and 1984 respectively. He obtained his Ph.D. from DAVV, Indore (India). He
is currently professor in SGSITS Indore. His research area includes the design, development
and testing of microprocessor and microcomputer based electronic instrumentation, RF IC
design, analog and mixed signal VLSI circuit design, dynamic testing of high speed A/D
converters, low power VLSI design, and system on chip design for biomedical signal
processing. He can be contacted at email: mishradrc@gmail.com, dmishra@sgsits.ac.in.

TELKOMNIKA Telecommun Comput El Control, Vol. 21, No. 4, August 2023: 872-880