Chapter 4
Oscillator
4.1 Introduction
Voltage controlled oscillator (VCO) is another critical block re quired in the 60
GHz transceiver to provide sufficient tuning range to cover the wide spectrum
and large P VT variations with maintained phase noise and compact area,
which is becoming a challenge with conventional tuning methods in CMOS
technology. In this chapter, one inductive tuning metho d is first proposed
based on configuration of current return paths in the secondary coil of a
transformer, which demonstrates a wide frequency tuning range for CMOS
VCO at mm-wave frequencies and shows great potential for integration in
60-GHz transceiver design. The inductive tuning method is further applied to
realize a tunable CRLH T-line for Mobius-ring RTW-VCO design in mm-wave
region. By utilizing the unique features of CRLH T-line to achieve a negative
phase shift with compact area and wide tuning capability, state-of-the-art
performance is demonstrated.
Phase noise and tuning rang e are the two prima ry design tar gets for VCO
designs. During the las t decade, substa ntial knowledge about the wide fre-
quency bands at 60 GHz and be yond has been accumulated to develop the
next generation big-da ta-rate wireles s terminals [39, 130, 74, 131, 132]. The
recent IEEE 802.15.3c standard for wireless local p ersonal network (WPAN)
has defined radio-frequency (RF) allo cation co mposed of 4 RF sub-ba nds at
60 GHz, ea ch with bandwidth of 2.16GHz. Considering the large frequency
range and large process variation in nano-scale CMOS, the utilization of one
single varactor cannot cover such a wide range with good phase noise perfor-
mance, which ha s introduced a grand challenge for 60-GHz VCO des igned in
CMOS technology.
65
66 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
At mm-wave frequencies, the mo st w ide ly used VCO topology is LC VCO
[131, 133, 134, 135, 136]. It consists of an LC tank as resonator, and a cro ss-
coupled transistor pair to generate negative resistance. LC VCO is widely
used at 60 GHz and beyond due to its high oscillation frequency, low phase
noise, simple str uc ture and differential output. The challenge, however, is its
limited tuning ra nge due to the large parasitic capacitance from cross-coupled
transistor pair.
Except for phase noise and tuning range, multi-phase or quadrature out-
put is often required for many big-data communication and imaging appli-
cations a t millimeter-wave frequencies [137, 13 8, 132, 134, 139, 140]. Multi-
phase and quadrature oscillators are normally realized by traveling wave
to generate multi-phase clock outputs with good phase noise performance
[137, 138, 141, 142]. Mobius-ring rotary-traveling-wave (RTW) VCO topology
is commonly adopted due to its advantages such as easy placement of cross-
coupled transistors, good matching of differential blocks and compact area
[142].
Traditionally, RTW-VCO is implemented with conventional right-handed
(RH) transmission line (T-line), with a phase delay directly proportional to the
T-line physical length [141, 142, 143]. Since a total phase delay of 360 -degrees
is req uired for oscillation, a large area is induce d. Recently, left-handed (LH)
T-line has shown to provide a superior performance at high frequency [144],
and also unique features such as the nonlinear dispersion curve [75]. Due to
large parasitic capacitors from cross-coupled transistors that are RH in nature,
the actual implemented T-line is a composite right/left-hand (CRLH) T-line.
By merging the phase shifts from LH and RH c omponents together, CRLH
T-line provides a phase delay independent of its physica l size, and thus can be
designed to be much more c ompact than conventional RH T-line for VCO. On
the other hand, as big-data communication or imaging applications require a
wide-band to e nsure high data rate and also to cover process variation by
CMOS MMIC at a dvanced technology, tuning ability of RTW-VCO has not
been tho roughly studied and achieved as far.
Multi-sub-band operation is normally adopted to enhance the total fre-
quency tuning range (FTR) with reduced VCO gain (K
V CO
) [131]. Conven-
tionally, multi-sub-band opera tion is implemented with capacitive tuning by
switched capacitor banks. When operation frequency sca les up to 60 GHz,
the para sitic capa citance from the ca pacitor bank becomes too large and the
quality facto r of capacitor becomes too low [131, 145], which would severely
limit the achievable FTR. Recently, inductive tuning has become a promising
substitute to replace the capacitive tuning [131, 132, 145, 146], and is normally
implemented by a loaded trans former topology [131, 145, 147]. By tuning in-
ductance instead of capacitance, the limit of parasitic capacitance on FTR is
relaxed. As a result, very wide FTR or multi-band operation can be achieved
[131, 1 32, 145, 146]. The inductive tuning can also be combined with conven-
tional capacitive tuning to realize more sub-bands to reduce K
V CO
[132, 147].
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.