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IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

DESIGN AND PERFORMANCE ANALYSIS OF VCO FOR
STANDARD GSM USING MEMS

KHALID A. S. AL-KHATEEB* AND WAJDI F. AL-KHATEEB

Department of Electrical and Computer Engineering,
Kulliyah of Engineering, International Islamic University Malaysia,

P.O. Box 10, Kuala Lumpur, 50728, MALAYSIA

*E-mail: khalid@iiu.edu.my

ABSTRACT: The design of a prototype monolithic Micro Electro-Mechanical Systems
(MEMS) electronic circuits, namely the Voltage Controlled Oscillators (VCOs) is
presented. The components can achieve the stringent requirements of wireless
communication applications such as GSM cellular telephony. The VCO meets the low
phase noise specifications of -136 dBc/Hz at large offset frequency of 3MHz, over the
appropriate frequency range. The model of the monolithic VCO is based on the topology
of the Colpitts Oscillator. It is relatively less complicated, which facilitates the practical
integration of the MEMS components into the configuration. The variable capacitor and
the monolithic 3-D coil inductor are suitable for low phase-noise and low power
consumption at the application frequencies. A PSpice simulation model was developed
with MEMS switching devices that can be integrated into the system. The model helps in
determining the design parameters, which affect the performance and operation
reliability of the RF transceiver system, for which a prototype has been tested and proved
successful.

KEYWORDS : MEMS; Electronic Circuits; VCO; GSM; Phase Noise.

1. INTRODUCTION

Mobility and portability are features, which have a strong driving force in the

miniaturization process of wireless communication interfaces. They have raised much
interest in single-chip applications. The integration of passives devices such as variable
capacitors and inductors, typically involves some trade-offs between noise, power
consumption, linearity, frequency range, gain and supply voltage. Higher Q-factors and
lower insertion losses however, may mitigate some of the trade-offs. A low phase noise
VCO operating at radio frequency (RF) can be designed by integrating a variety of
suitable MEMS components. Other applications that may involve MEMS capacitive
switches are output-stage audio amplifiers, which should satisfy the GSM specifications.
The important design parameters however, have to be identified before the implementation
of such circuits. MEMS can be used to replace standard variable capacitors (varicaps) and
inductors. Both of these passive components can be fabricated on silicon substrates and
thus are amenable to monolithic integration by standard IC processes. The MEMS
technology is attractive for the implementation of on-chip high-quality RF variable
capacitors and inductors, especially in low noise power amplifiers, matching networks,

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

and monolithic low-noise VCOs. In fact, the Q-factor of these components may ultimately
determine the phase noise performance, which is critical in high performance
communication systems [1]. The introduction of MEMS in RF systems increases the
functionality and improves the performance. The challenge however remains in how to
minimize the phase noise while maintaining lowest possible power consumption..

2. THE PHASE NOISE

The requirements of a versatile VCO are low phase noise, low power consumption,

wide tuning range and a high power output. However, to achieve these characteristics all
at once is a challenging task, because of trade-offs. Much of the research is aimed at
reducing the phase noise, which is a rapid short term random fluctuation. It can be
expressed in terms of signal to noise power ratio in unit bandwidth. The expression, which
can be used to calculate the noise power at the offset frequency fm, is based on the classical
Leeson’s model [2]:

…… ………… ( 1 )

Where; F is the excess noise factor, k Boltzmann constant, T the absolute temperature, Ao
the amplitude of oscillation, Q the resonator loaded quality factor, Rp is the parallel
resistance to model losses in the resonator, and Δf1 / f

3 is the corner frequency in the phase
noise spectrum.

For an ideal oscillator, the shape of the spectrum is an impulse at a resonant frequency,

ωo. However, in actual oscillators, a skirt shape is formed around ωo. Figure 1(a) shows
the single spike of energy at the center carrier frequency, ωo and Figure 1(b) shows the
skirts around ωo due to phase noise. Actually, the phase noise is expressed as the power at
particular offset, ωm from ωo. The signal power is measured in a 1 Hz bandwidth at ωm
[3]. The unit of phase noise is dBc/Hz represented as decibels below the carrier per hertz.

(a) (b)

Fig. 1: (a) Output spectra of ideal oscillator, (b) Output spectra of practical
noisy oscillator.

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

In the prototype MEMS oscillator, there is an additional noise source due to the

mechanical thermal vibration, which is included instead of thermal and flicker noise. In
practice, this kind of additional noise is due to the vibrations of the suspended plate which
cause variation in the capacitance and affect the overall output of the phase noise [1].

3. MEMS PASSIVE DEVICES IN VCO

There are two approaches to design and fabricate a VCO using MEMS passive devices.

The first is to replace the MOS capacitors with MEMS, which is good for a wide tuning
range. The second is to use a monolithic 3-D coil inductor. The high Q varicap is a key
element in a low phase noise VCO. The MEMS varicap also enables a complete
monolithic fabrication of RF VCO for on-chip IC compatible devices, with lower loss,
larger tuning range and higher linearity [4]. The high-Q variable capacitor can be realized
by a surface micro-machined all aluminum microstructure. The top and cross-section are
shown in Fig. 2. It consists of 1 µm-thick aluminum plate suspended in air 1.5 µm above
the bottom layer and anchored with four folded beam suspensions acting as springs.
Aluminum sheet resistance is low, which is critical to minimize the ohmic losses, and the
fabrication temperature of 150ºC [1] is also low. The plate size of 200 µm by 200 µm and
1.5 µm nominal air gap results in a nominal capacitance value of approximately 200 pF.
Thus, large capacitance can be obtained by parallel connection [5]. In RF transceivers, the
DC tuning voltage is typically limited to 3.3 V or less. Thus with a 1.5 μm air gap and a
200 μm square plate it requires 3.8 N/m for a 3.3 V operation. This corresponds to a
mechanical resonant frequency of 30 kHz [1]. Each of the suspension four folded beams is
100 μm long and 20 μm wide as in Fig. 2.

Fig. 2: Micro-machined variable capacitor.

A high Q inductor is another key element in low phase noise VCO, since the inductors

produced by silicon processes cannot provide high Q factors, due to ohmic loss in the thin
metal layers and eddy-currents [6]. Several methods have been proposed. However, the
bond-wires types are chosen as an attractive alternative because the Q values are at least
an order of magnitude higher than the on-chip spiral inductors. They have been

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

implemented in RF VCOs and high efficiency power amplifiers. The 3-D microstructure
also minimizes the capacitive coupling and eddy current loss, leading to larger Q-factor at
higher frequencies. Bigger inductors and smaller capacitors allow operation with less
power [6].

In a high Q inductor every mm of bond-wire contributes about 1 nH. Therefore, a few

nH requirement for wireless applications can be provided readily, with a lead frame of a
standard IC package [1].

Fig. 3: 3-D coil inductor.

The 3-D coil inductor, shown in Fig. 3, consists of two 5 μm thick turns, 50μm wide
copper traces electroplated around an insulating core with a 650µm by 500µm cross-
section. The core is alumina, which has negligible loss tangent, which is another key
parameter to ensure high Q. A core width of 500µm is found experimentally to be the
minimum that avoids tilting during attachment. The fabrication process is described in
details elsewhere [8].

4. THE DESIGN PROCEDURE OF THE VCO

There are many types of oscillator topologies which can be used to construct an RF

VCO for frequencies below 1 MHz. For frequencies above 1 MHz however, LC feedback
oscillators are normally used. Due to the frequency limitations of most op-amps, transistor
amplifiers are used as the gain elements. Thus, a popular topology, which proved
successful in many commercial modules, is the Colpitts oscillator (Fig. 4), due to its
simplicity, robustness and wide range of operating frequencies, from IF to RF [8].

The frequency is determined by the resonant frequency of the feedback network, which

is a parallel resonant tank circuit, in which any change in the inductor or capacitor will
change the oscillation frequency, fo given by:

………………….. ( 2 )

2 T
f

L C





IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

where, CT = Cv + C12, Cv = (Cvar x Co)/(Cvar + Co), and C12 = (C1 x C2) / (C1 + C2)

(Note: The above equation is accurate only if the LC circuit has a high Q).

Fig. 4: Colpitts circuit topology for VCO.

Fig. 5: The actual circuit design of the VCO.

Considering the Colpitts VCO (Fig. 5), which uses parallel-mode LC tank

configuration, the analysis is straightforward. The FET is a medium power amplifier as
compared to a bipolar transistor. It may provide double the power for a given frequency or
double frequency for a given power. In fact, FET can be used up to 30 GHz. At 10 GHz,
the FET itself may actually provide several watts of output power [9]. In the simulation
model however, the VCO operates at a frequency fo of approximately 1 GHz.

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

The task for phase noise performance analysis is to reconfigure the Colpitts oscillator as
an amplifier with positive LC feedback, as in Fig. 5. Hence a readily available set of
design equations can be used, which are clear and convenient. Instead of only analyzing
the phase noise, this model can also be useful in calculating the loop gain and the
amplitude of the oscillation [9].

The factors that affect the accuracy of phase noise simulation and measurement are

many. It may be possible addressed these factors accurately with a prudent selection of the
passive components and proper resonator modeling. For an acceptable level of simulation
accuracy of a VCO that operates at RF and above, all components in the linear network
such as transmission lines and discontinuities must be accurately characterized over
several harmonics of the fundamental oscillation frequency. This is important because the
accuracy of the oscillation signal affects the noise analysis, and the noise analysis itself
depends on the linear network. This means that any inaccuracy in the network
characterization will affect the quality of the phase noise simulation. The simulation
accuracy of the VCO will ultimately determine the parameters of all the fabricated parts,
including the dimensions of the circuit board, material properties, and component models
of any parasitic behavior [10].

5. ANALYSIS OF THE VCO DESIGN

Microwave Office is a software, which can be useful in simulating many circuit designs.

Therefore it has been for the simulation of the VCO. It also enables the prediction of the
performance characteristics. The analysis can either be performed in the frequency domain
using the harmonic balance technique, or in the time domain using transient simulators
such as SPICE and Spectre. The simulation method involves three main steps. First, it
attempts to locate the start-up frequency with respect to the well-known loop gain
criterion. When the loop gain saturates to a magnitude of 1 in the large-signal steady state,
the simulator will step the probe voltage in an attempt to detect loop gain saturation. Then,
the neighborhood of gain saturation is used as the starting point for the analysis. Then the
voltage and frequency of the probe are adjusted in a way that results in zero probe-current.
After setting all the requirements, as in Fig. 6, the simulation begins to perform the
analysis automatically.

The analysis uses a special device called oscillator probe that allows fast and robust

oscillator simulations, even for the case of extremely high resonator Q. The oscillator
probe refers to the combination of the source and the ideal impedance element. In the
Microwave Office simulator, the oscillator probe is denoted as OSCAPROBE, as shown in
Fig. 7. The most significant probe parameters are Fstart and Fend. These parameters
indicate the range of search for start-up frequency. Another parameter is the Fsteps, which
refers to the number of steps used in the search for start-up frequency and rarely needs to
be changed from default. However, in extremely high-Q cases, Fsteps may be increased or
the frequency range is narrowed. The recommended location of the oscillation probe is at a
node connecting the resonator and the active device.

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

OSCNOISE
ID=NS1

OFstart=1e-8 GHz
OFend=0.1 GHz

OFsteps=5
SwpType=LOG

Harm={1,2}

Fig. 6: Flow chart of the analysis. Fig. 7: Schematic symbol of OSCAPROBE.

The main objective of simulating the VCO is to measure the phase noise performance,

then to compare it with the values of a real prototype VCO at the offset frequency of
3MHz. The OSCNOISE simulator shown in Fig 8 was employed to determine the phase
noise performance of the VCO. The results are shown in Fig. 9. The phase noise is
measured in dB/Hz offset from the carrier and is plotted on a log frequency scale. In this
simulation, the OFstart and OFend parameters were set to 1e-008 GHz and 0.1 GHz
respectively.

Fig. 8: Schematic symbol of OSCNOISE.

It is shown that at an offset frequency of approximately 3MHz, the value of the phase

noise is -138.05 dBc/Hz, which is in good agreement with the actual value for the
prototype -136 dBc/Hz. The noise components in conventional LC tuned oscillators are;
electrical thermal noise, flicker noise (l/f noise), supply voltage noise, and the noise
contribution from the substrate. In practical MEMS tuned LC oscillators however,
additional phase noise is introduced by the mechanical thermal vibration of the variable
capacitors. The vibration of the suspended plates causes variation in the capacitance value,
which results in phase noise or jitter in the output frequency [1]. The noise power spectral
density due to plate displacement can be expressed as follows:

OSCAPROBE

ID=X1
Fstart=1 GHz
Fend=2 GHz
Fsteps=200
Vsteps=40

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

Fig. 9: Plot of the simulated phase noise.

…. …. …. …. ( 3 )

Where k is Boltzmann constant, T is the absolute temperature, b is damping coefficient
due to the surrounding gas ambient and internal dissipation of the system, ωn is the
mechanical resonant frequency of the capacitor, km is the structure compliance and QM is
the mechanical quality factor. Additional phase noise can be further expressed as given
below:

… … … … … ( 4 )

Where xo is the nominal air gap of the capacitor, N is the number of parallel-connected

devices and α is the ratio between the nominal tank tunable capacitance and its parasitics,
fo and fm are the oscillation and the offset frequencies, respectively.

These two equations enable the phase noise induced by the Brownian motion to be

determined at various offset frequencies. For a typical design condition in which xo =

1.5μm, QM ≅ 1 at 1 atm, ωn = 2π (30 kHz), N = 4, α ≅ 0.5, and fo = 1GHz, the phase
noise at offset frequencies fm of 10KHz, 100KHz, and 3 MHz are respectively [1]
64dBc/Hz, -105dBc/Hz, and -136dBc/Hz, Typical wireless communication applications
specify low phase noise requirement at relatively large offset frequency, for example –136

IIUM Engineering Journal, Vol. 11, No. 1, 2010 Al-Khateeb and Al-Khateeb

dBc/Hz at 3 MHz offset for GSM. In order to fulfill these norms, the VCO phase noise
must be maintained better than –135 dBc/Hz at 3 MHz offset over the appropriate
frequency range of 855 MHz to 863 MHz.

6. CONCLUSION

MEMS tunable VCO is a viable proposition for GSM applications. It can provide

superior performance characteristics over other types of VCO. It is expected to make large
impact in many applications, especially in wireless mobile telephony. The phase noise
analysis performed by simulation and described explicitly in this work is one of the most
important aspects of VCO design. The Microwave Office software tool can be very useful
in designing such circuits and predicting their performance. It is a powerful tool for
simulation and design analysis of linear and nonlinear circuits and for measurements
carried out under specifically set conditions. The practical application of the VCO
simulation model was the fabrication using real MEMS passive devices i.e. the high-Q
variable capacitors and 3-D coil inductors. The prototype in which MEMS constitute the
frequency determining components of low phase-noise with low power consumption may
prove to be commercially attractive, as it meets the stringent performance requirements of
GSM at the application frequencies.

REFERENCES

[1] D. J. Young, V. Malba, A. F. Bemhardt, and B. E. Boser, “A Micromachined RF
Low Phase Noise Voltage-Controlled Oscillator for Wireless Communications”, IEDM
Dig. Tech. Papers, pp. 285-300, 24 April 2001.

[2] D.B. Lesson, “A Simple Model of Feedback Oscillator Noise Spectrum”, Proc. IEEE,
54 (2), pp. 329-330, 1996.

[3] V. K. Saraf, “Low Power Wide Tuning Range LC-VCO using RF MEMS Passives”,
Master thesis, Univ. of Carnegie Mellon, Pittsburgh, PA, August 2004.

[4] B. S. Darade and T. A. Parmar, “Low Phase Noise Fully Integrated VCO”, 18th Intl.
Conference on VLSI Design, Jan 2005.

[5] D. J. Young and B. E. Boser, “A Micromachined Variable Capacitor for Monolithic
Low-Noise VCOS”, Solid-State Sensor and Actuator Workshop, Dig. Tech. Papers,
pp. 86-89, June 1996.

[6] E. C. Park, Y. S. Choi, J. Yoon, S. Hong and E. Yoon, “Fully Integrated Low Phase-
Noise VCOs With On-Chip MEMS Inductors”, IEEE Transactions on Microwave
Theory and Techniques, vol. 51, no. 1, January 2003.

[7] D. J. Young, V. Malba, J. J. Ou, A. F. Bemhardt and B. E. Baser, “Monolithic High-
Performance Three-Dimensional Coil Inductors for Wireless Communication
Applications”, IEDM Dig. Tech. Papers, pp. 67-70, December 1997.

[8] R. H. Berube, “Computer Simulated Experiments for Electronic Devices Using
Electronic Workbench”, Prentice-Hall, Inc., 2000.

[9] C.O’Connor, “Develop A Trimless Voltage-Controlled Oscillator”, Microwaves & RF
Magazine, pp. 94-105, 2000.

[10] D.Vye, “Improving VCO Phase Noise Performance Through Enhanced
Characterization”, High Frequency Electronics Magazine, pp. 56-60, 2004.