A Novel Design of Voltage Controlled Oscillator by Using the Method of Negative Resistance

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 8, No. 6, December 2018, pp. 4496~4504
ISSN: 2088-8708, DOI: 10.11591/ijece.v8i6.pp4496-4504  4496
Journal homepage: http://iaescore.com/journals/index.php/IJECE
A Novel Design of Voltage Controlled Oscillator by Using the
Method of Negative Resistance
Ayoub Malki1
, Larbi El Abdellaoui2
, Jamal Zbitou3
, A.Errkik4
, A .Tajmouati5
, Mohamed Latrach6
1,2,3,4,5
LMEET Laboratory FST of Settat Hassan 1st University, Morocco
6
Microwave group ESEO Angers France
Article Info ABSTRACT
Article history:
Received Mar 3, 2018
Revised Jul 17, 2018
Accepted Jul 30, 2018
The objective of this paper is to develop a new design of a voltage controlled
microwave oscillator by using the method of negative resistance in order to
fabricate VCO with very good performance in terms of tuning rang, phase
noise, output power and stability. The use of hybrid microwave integrated
circuit technology’s (HMIC) offers a lot of advantage for our structure
concerning size, cost, productivity, and Q factor. This VCO is designed at
[480MHz; 1.4GHz] frequency for applications in the phase locked loop
(PLL) for signal tracking, FM demodulation, frequency modulation, mobile
communication, etc. The different steps of studied voltage controlled
oscillator’s design are thoroughly described. Initially designed at a fixed
frequency meanwhile the use of a varactor allow us to tune the frequency of
the second design. It has been optimized especially regarding tuning
bandwidth, power, phase noise, consumption and size of the whole circuit.
The achieved results and proposed amendment are the product of theoretical
study and predictive simulations with advanced design system microwave
design software. A micro-strip VCO with low phase noise based on high gain
ultra low noise RF transistor BFP 740 has been designed, fabricated, and
characterized. The VCO delivers a sinusoidal signal at the frequency 480
MHz with tuning bandwidth 920 MHz, spectrum power of 12.62 dBm into
50 Ω load and phase noise of -108 dBc/Hz at 100 Hz offset. Measurement
results and simulation are in good agreement. Circuit is designed on FR4
substrate which includes integrated resonators and passive components.
Keyword:
Active device
Microstrip
Oscillator
Phase noise
Resonator
Voltage controlled oscillator
Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Ayoub Malki,
Faculty of Sciences and Techniques,
University Hassan 1st,
University Complex Casablanca road, Km 3.5, B.P: 577 Settat, Morocco.
Email: ayoub.malki88@gmail.com
1. INTRODUCTION
The spectrum congestion of frequencies in the microwave field, the research for new applications in
the telecommunication domain, and constraints of integration lead currently to the development of
microwave systems. In regard to these systems, oscillators (simple or voltage controlled oscillator) are
essential devices and their design presents many difficulties and challenges for laboratories and research
centers. So such systems can’t benefit a noticeable development unless their production price become
reasonable; the difficulty of designing such circuits is therefore still spreading due to the seek for low costs.
The solution is to design a monolithic or hybrid microwave integrated circuit realizing full integration of
components in the same structure. This technology would allow the mass production of these structures in the
same series of operations [1]-[2].

Int J Elec & Comp Eng ISSN: 2088-8708 
A Novel Design of Voltage Controlled Oscillator By using the Method of Negative Resistance (Ayoub Malki)
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Voltage controlled oscillator are actually very important and indispensable module in the design of
communication system employing in microwave technology. The objective’s design is to get very stable
output signal controlled in frequency with very low phase noise, power management (consumption, output
power), pulling, matching impedance and the rejection of unwanted frequency bands. The role of the VCO is
to provide approximately very perfect signal to the antenna to avoid the use of very high gain amplifier and
the use of filters to bloc undesirable frequencies. Also the noise generates into the output signal of the VCO
is susceptible strongly to degrade the sensitivity of the communication system. This noise called phase noise
is distributed around the main frequency. In the other hand, in addition to the difficulty of VCO designs , the
technological tendency, the requirements of the regulations and the speed of development of the
telecommunications have increasingly forced to fabricate voltage controlled oscillator with very precise
performances in term of size, consumption, phase noise, stability and output power. For that we present in
this work, the study and complete design of a low phase noise voltage controlled oscillator, operating at
[480MHz; 1.4GHz] on epoxy FR4 with performances more better than founded in the literature.
We begin by presenting the oscillator theory including the fundamental oscillations conditions, the
negative resistance structures and the hybrid integrated voltage controlled oscillator specifications. The
choice of the topology is then argued. The last part of this work relates to the design process up to the
fabrication and testing of our VCO. The importance of the layout realization step is widely developed. It
allowed to proose an optimized and original architecture. The presentation and analysis of the obtained
results with the published results in the literature positively end this work [3].
2. FIXED FREQUENCY OSCILLATOR DESIGN
In this paper, before to design voltage controlled oscillator with known tuning bandwidth we
propose firstly to design an oscillator at fixed frequency to acquire the high performance of our oscillator.
Secondly we replace the resonator of the fixed frequency oscillator by model of varactor based on diode to
allow us to control the frequency of the voltage controlled oscillator.
An oscillator consist generally of four independent parts: matching network, active element
(amplifier), resonator and bias network feedback element. During the procedure design, to improve the
characteristic of the proposed model each component must be analyzed separately (choice of transistor,
choice of positioning of tuning element, bias design and the architecture of the resonator). Finally the study
of the interactive behaviors between the different components of oscillator is very important during the
procedure design because this step allow us to get very precise performance of our structure [4].
The theory of oscillators can be firstly approached by the modeling of a counter-reacted system
which is none other than the transmission approach shown in Figure 1(a). A non-linear amplifier with
transfer function G is responsible for supplying the power to the load, it is then associated in the oscillator
loop with a selective transfer function filter β (f). It is therefore a positive feedback that reinjects part of the
output signal at the input of the amplifier to build the oscillation [5]-[6].
(a) (b)
Figure 1. Oscillator as (a) feedback loop (b) negative resistance.
A sustained oscillation in steady state, requires that two conditions, known as the Barkhausen
criterion, be simultaneously satisfied. Such a looped system reaches its stable oscillation state when the
closed loop transfer function is exactly equal to 1 [7]. This implies that the total phase shift on a loop turn is
zero. Mathematically the Barkhausen criterion can be written as follows:

 ISSN: 2088-8708
Int J Elec & Comp Eng, Vol. 8, No. 6, December 2018 : 4496 - 4504
4498
║β(f0)*G║=1 (1)
Φ(β(f0)*G)=0 (2)
Although the model shown in Figure 1 (a) can be used to analyze and determine the necessary and
sufficient conditions for oscillation, it is more judicious in the case of the design of VCO, to use the model
shown in Figure 1 (b) whose analysis is performed using the concept of negative resistance. This concept
explains that the tuned circuit, once it is supplied with voltage, will oscillate continuously if there is active
device to compensate the resistance that absorb energy. The function of the amplifier therefore has the role of
producing the negative resistance necessary to maintain the oscillation, ensuring a quantity of energy equal to
that absorbed [8].
The energy in an oscillator is inevitably dissipated due to the non-ideality of the circuit elements.
Therefore, energy must be used to maintain the oscillation. This can be achieved with the resulting negative
resistance of transistor created by the transconductance g of transistor. For this study, an ultra-low noise SiGe
transistor from Infineon, the BFP740, was chosen. Infineon BFP740 is a general purpose transistor that offers
excellent performance at high frequency. The BFP740 is housed in a low cost low parasitic 3 lead F59. This
transistor is recommended by Infineon to fabricate low-noise microwave oscillators or amplifiers and it
delivers outstanding performance for a wide range of wireless application up to frequencies of 12 GHz. Its
transition frequency is given at 150 GHz and it has a ultra-low noise figure.
At the start of an oscillator the noise existing inside each electrical component must be excited to
produce output signal. This excitation is the result of the negative energy created by the transistor. For the
transistor deliver a sufficient amount of negative energy, it must be sufficiently unstable when the power
source is connected to the system. This instability is translated by the module of return loss greater than the
unit in the input port (|S11|˃) and the stability factor less than the unit (|K|˂1). It’s necessary that both ports of
the amplifier are unstable because both sides of the amplifier serve a particular purpose in the built up of
oscillation.
𝑲 =
𝟏−|𝑺 𝟏𝟏| 𝟐−|𝑺 𝟐𝟐| 𝟐+|𝑺 𝟏𝟏 𝑺 𝟐𝟐−𝑺 𝟏𝟐 𝑺 𝟐𝟏| 𝟐
𝟐|𝑺 𝟐𝟏 𝑺 𝟏𝟐|
 
Based on the scattering parameters of BFP740 at common base given by the manufacturer Infineon
at 1 GHz, we can conclude that the transistor operates at the stable area. In order to make it unstable we have
added a feedback element. So by optimization of these feedback elements parameters and the adjustments of
polarization parameters the transistor is becoming unstable.
The study of the active device stability in the common base configuration can be discussed using the
ADS simulation specially the tool Stabfact to get the stability factor k. This method consist to change the
frequency around the frequency of oscillation sitting in the S-parameters component (start=0.4 GHz and
stop= 1.4 GHz). The resulting k factor versus frequency is shown in Table.1. The active device in a common
base configuration with the feedback element are potentially unstable at [480MHz; 1.4GHz] [9].
Table 1. K Factor for Tte Active Device
Frequency Sabfact:K
400MHz -0.81
700 MHz -0.82
900MHz -0.84
1.1GHz -0.82
1.4GHz -0.79
The bias of the transistor is provided by a resistor bridge R1, R2 and a collector supply voltage of 5v.
A very detailed study is brought to see the different bias parameters because it plays a very important role to
achieve the performances of our oscillator in term of power, efficient and phase noise. In this study we tried
to have a stable transistor operation point and to minimize the power consummation because the
characteristics of our transistor depend directly on its polarization [10].
An important issue in the design of a high performance oscillator is the design of a high quality
resonator. As we have indicated, the quality factor has a significant impact on the performance of phase
noise. It is therefore important to choose the high quality resonator in order to meet the best specifications.
The chosen resonator is a micro strip resonator of length λ/2. The circuit being in hybrid technology, the
surface stress is much less than with an integrated circuit. The use of this type of resonator improves

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oscillator specifications better than LC resonator in terms of size and noise. The aim is to have inductance
and capacitance where one can store energy without loss and at the same time the ability to integrate with a
varactor. The quality factor of this micro strip resonator can be defined as follows:
Q =
w0
∆w
(4)
Q = 2π
Energie stokée
Energie liberée par cycle
(5)
At frequencies near of resonance frequency, the microstrip line can be replaced by an RLC series circuit
whose values can be estimated using the following relationship:
Ls =
Q Rs
w0
(6)
Cs =
1
Q Rsw0
(7)
RS the series resistance determined by the actual part of the input impedance of the resonator at
resonance, W0 is the pulsation at resonance, Ls inductance component of the resonator and CS capacitance
component of the resonator. To reduce the series resistance of the resonator we tried the maximum possible
to reduce the width of micro strip lines. So a minimum characteristic impedance chosen is 25.
The last step in this work is to design matching impedance. Impedance matching is a technique used
in electricity to optimize the transfer of electrical power between a transmitter (source) and an electrical
receiver (load) 1 and optimize the transmission of telecommunications signals.the theory of maximum power
determines that the impedance of the load must be the conjugate complex of the impedance of the generator;
Now the whole circuit including the active device with its polarization, resonator and matching impedance is
as in Figure 2.
Figure 2. The schematic of the negative resistance microwave oscillator.
For the simulating of the obtained oscillator we can use the harmonic balance used for nonlinear
structure which is frequency domain analysis tool. The frequency spectrum, the steady state and the phase
noise results are shown successively in Figure 3 (a), Figure 3 (b) and Figure 3(c).

 ISSN: 2088-8708
4500
(a) (b)
(c)
Figure 3. Simulated result of fixed oscillator in (a) time domaine (b) frequency domaine (c) phase noise.
It is seen from Figure 3 (a) that the waveform of the output signal is purely sinusoidal at 1 GHz.
The output power at the fundamental frequency is 14.909 dBm as shown in Figure 3 (b). We can also observe
that the power of the harmonics is very negligible compared to the fundamental we can also observe that the
power of the harmonics is very negligible compared to the fundamental one. Which further improves the
distortion of the oscillator. In the author hand the phase noise prediction of the microwave oscillator is 117.6
dBc/Hz.
The proposed voltage controlled oscillator performances were compared to some existing works in
the literature in terms of phase noise, output power and distortion, the comparison results conducts to the best
results. Generally, in the literature, the best results are about -110dBc/hz from 100 Hz of the carrier in term of
phase noise and 7dBm in term of output power. If we compare theses values with reached values in our work
(phase noise: -117.6 dBc at 100 Hz from carrier; output power: 14.909 dBm and the harmonic about -8.42
dBm), we can conclude that the proposed VCO has excellent performances compared to the reported works.
3. VOLTAGE CONTROLLED OSCILLATOR DESIGN
In the following section the design of voltage controlled oscillator is given. This design example
will use the same structure used in the fixed frequency version but we try to replace the resonator by varactor.
The objective of this step is the frequency variation of the fixed frequency oscillator. This is allowed when
the resonator parameters can be modified, namely its inductance and capacitance. There are many methods,
each has their advantages and disadvantages. An integration of varicap based on diode is very adequate to our
structure. The varactor diode, also called varicap diode, reverse polarized will have a variable capacitance
between these two terminals.
In order to choose a suitable varactor for the design, we have to decide on the tuning bandwidth.
This particular requirement is for a tuning bandwidth 45 MHz/V over the tuning range of 1 to 20V. To give

4501
us some margin to cover the external parasite effects we choose a bandwidth of 50MHz/V. The varactor
should have a minimum of parasitism, so we opt for a SMT device. The simulation of the complete circuit
with the varactor was run and re-run for each value of the voltage from 1V to 20V. The results of variation
voltage versus frequency and output power are given in Table 2.
Table 2. Tuning Frequency of VCO in Function of Voltage
Voltage Frequency
0.5V 500MHz
5V 700 MHz
10V 900MHz
15V 1.1GHz
20V 1.4GHz
4. EXPERIMENTAL RESULTS AND DISCUSSION
Good isolation between components as well as satisfactory decoupling between the DC ports and
the RF lines are critical considerations for achieving an RF or microwave design. To avoid introducing
asymmetry into the oscillation waveforms and to induce additional phase noise on the output signal,
particular attention must be given to the symmetry of the complete layout. The voltage controlled oscillator
was fabricated on the epoxy substrate with the dielectric constant of 4.4, the metal thickness of 0.035 mm,
and height is 1.60 mm; as hybrid microwave integrated circuit structure. All the circuit components are in
SMD (Surface Mount Device) package. Figure 4 illustrates the photograph of the completed fabricated
voltage controlled oscillator. It occupies a surface of 28*30mm2
integrating the pads of power supplies.
Before the realization of the circuit, the electrical diagram extracted from the final layout, was simulated for
the last time taking into account the given minimum and maximum values of the critical components
(inductance, varactor, capacitances).
Figure 4. Photograph of the fabricated voltage controlled oscillator
The fabriquated voltage controlled oscillator is characterized succevelly in time domaine, frequency
domain and phase noise using Tektronix DPO 7245 digital phosphor oscilloscope, Agilent E4440A PSA
spectrum analyzer and Agilent 5505A. Figure 5 (b) shows that The VCO delivers 12.62dbm at 1 GHz,
Figure5(c) shows that phase noise levels of output signal is -108 dBc/Hz at 100 Hz at an oscillation
frequency of 1 GHz and from Figure 5 (a) The output voltage has ideal sinusoidal shape with peak to peak
voltage swing of 2.6V.
The measured and simulated parameters of our voltage controlled oscillator present approximately
the same results in term of tuning bandwidth, frequency and output power and some offset in phase noise
(9dBc). This offset between measured and simulated phase noise is normal due to the fabrication,
approximations taken by ADS especially phase noise, the quality of components and the integration of the
varicap.
If we compared the obtained results of this work (phase noise -108 dBc/hz , tuning bandwith 920
MHz) with the published results of some scientific reports on microwave oscillator designs [11]-[17], we can
conclude that this voltage controlled oscillator has good results in term of tuning bandwith, phase noise, good
output power and frequency accuracy. These results were the product of deep study of each parameter of the
voltage controlled oscillator and especially the interaction between these parametres.

 ISSN: 2088-8708
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(a) (b)
(c)
Figure 5. Measurement result of VCO in (a) time domaine (b) frequency domaine (c) phase noise.
5. CONCLUSION
As a conclusion, we can deduce that we have developed a novel structure negative resistance
voltage controlled oscillator at 480 MHz with tuning bandwith 920 MHz. The fabricated voltage controlled
oscillator produces sinusoidal signal in frequency range [480MHz; 1.4GHz] with start frequency of 480MHz,
output power at 1GHz equal to 12.62dBm, phase noise of the output signal around -108dBc/Hz at 100 Hz.
This work has been designed and adjusted by using a theoretical step and optimization tools applied on active
device, Bias networks, matching impedance and resonator with its varicap,. The comparative study of this
voltage controlled oscillator design with some summary of some scientific reports on voltage controlled
microwave oscillator designs shows that we got better results.
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BIOGRAPHIES OF AUTHORS
Ayoub Malki was born in Settat, Morocco, In january 1988. He received the master degree in
aeronautic and telecommunications from Air royal scool, University of kadi Aayad Marrakech,
Morocco. He is currently working toward the Ph.D. degree in physics and engineering sciences
at Faculty of Sciences and Technologies, Settat, Morocco. His research interests include the
analysis and design of hybrid, monolithic active and passive microwave electronic circuits.
Larbi El Abdellaoui was born in Tiflet, Morocco, in 1961. He received the Ph.D. degree in
electronics from the University of Metz, in 1994, France. He is currently an associate Professor
of Electronics in Faculty of sciences and techniques, University Hassan 1st, Settat, Morocco. He
is involved in the design of hybrid, monolithic active and passive microwave electronic circuits.
Jamal Zbitou was born in Fes, Morocco, in June 1976. He received the Ph.D. degree in
electronics from Polytech of Nantes, the University of Nantes, Nantes, France, in 2005. He is
currently an associate Professor of Electronics in FST, University Hassan 1st, Settat, Morocco
and the head of Computing Networks and telecommunication in LMEET Laboratory in FSTS.
He is involved in the design of hybrid, monolithic active and passive microwave electronic
circuits.
Ahmed Errkik was born in July 1960 in Morocco. He received the Ph.D. degree in physics from
the University of Technology Compie`gne (UTC), Compie`gne, France. He is currently an
associate Professor of physics in FST University Hassan 1st, Settat, Morocco. He is involved in
the design of hybrid, monolithic active and passive microwave electronic circuits.

 ISSN: 2088-8708
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Abdelali Tajmouati was born in, Morocco, in 1962. He received the Ph.D. degree in science
engineering from the University of Perpignan, France, in 1992. He is currently an associate
Professor of Electronics, thermal transfer and thermodynamic in Faculty of sciences and
techniques University Hassan 1st, Settat, Morocco. He is involved in the design of hybrid,
monolithic active and passive microwave electronic circuits.
Mohamed Latrach (IEEE member) received the Ph.D. degree in electronics from the University
of Limoges, Limoges, France, in 1990. He is currently Professor of microwave engineering with
the Ecole Supérieure d’Electronique de l’Ouest (ESEO), Angers, France, where he is head of the
Radio-Frequency & Microwave research group. His research interests include: design of hybrid
& monolithic active and passive microwave circuits, metamaterials, LH materials, antennas,
rectennas and their applications in wireless communications, and wireless power transmission
(WPT).

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