Chapter 5
Coupled Osci l l at or
Network
5.1 Introduction
CMOS-based THz signal sources have be en recently demonstrated for co mpact
system-on-chip implementation [153, 154, 155, 156, 157, 158, 159, 160, 16 1]
with applications in high-data-rate communication and non-invasive imag-
ing. As THz signal suffers from larg e pr opagation loss, the generated signal
strength must be strong enough to have sufficient signal- to-noise ratio (SNR)
for detection, which imposes grand challenges for high-power s ignal source
designs in CMOS.
It is challenging to design a hig h-output power THz signal source by single
CMOS oscilla tor source with high efficie nc y a s well as a wide freq ue nc y tuning
range (FTR). In the CMOS process, one single oscillato r usua lly has small out-
put power that is limited by single CMOS transistor with commonly observed
low output power level [162, 154, 77], which is incapable of delivering a strong
THz signal for transmission and processing. Similar to the approach utilized
in a power amplifier [163, 164], output power combining of multiple CMOS os-
cillators can be considered to achieve a large output power at THz frequency.
Several coupled-oscillator-network (CON) structures have been prop osed for
high-output power [161, 87]. A phas e/delay tuning design is utilized in [161]
but cannot e nsure the in-phase coupling condition. In-phase synchroniza tion
by half λ transmission line (T-line) is reported in [87], which is bulky and lossy
when deployed for a large-scale phase-arrayed design. There is great interest
to explore a high-output power signal source by combining the o utputs of an
array of CMOS oscillators such as CON [165].
105
106 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
The combined o utput power of CON is maximized when all the oscillator
unit-cells are in-phase coupled with a pha se difference of 2nπ (n = 0, 1, 2, . . .).
In the conventional CO N design [165], adjacent oscillators are coupled by a
T-line or an equivalent delay network with at least an electrical length of half
λ (n = l), which is bulky and lossy and hence larg ely reduces the output
power and efficiency. The recent work in [163] has shown that the CMOS
metamaterial devices can be utilized for phase-arrayed de sign such as zero-
phase-shifters (n = 0) for power combing in CMOS mm-wave PAs with very
high-output power. Co mpared to the coupling by half λ T-line, the z ero-phase-
shifter can result in z ero-phase coupling with low loss and compact are a.
Recently, plasmon polariton-based waveguide structures have been ex-
plored for low loss energy transfer [166, 167, 168, 59, 60, 169]. As one type of
plasmon polariton-based waveguide structures, magnetic plasmon waveguide
(MPW) has been already applied to the middle distance wireless energy trans-
mission systems [59, 60] as well as one-dimensional sub-wavelength power
transfer in the mm-wave range [166, 167, 168]. Similar to CRLH T-line, MPW
has a zero-phase propagation mode. But it also has several advantages over
CRLH T-line in the CON de sign. Firstly, MPW has lower propagation loss
in the practical CMOS layout. As introduced in Sec. 2.2.1, there will be a
band-gap in CRLH T-line if the series and para llel r esonance fr equencies of
unit-cell are not same, which will c ontribute the propagation loss. On the other
hand, MPW only has one zero-phase mode at the boundary of the stop-band,
where its attenuation factor is zero. Secondly, the plasmon resonators inside
MPW can be easily transferred into oscillator unit-cells by replacing their
equivalent capacitance with transistor-based negative resistance . As such, a
distributed z ero-phase CON is proposed with magnetic-plasmon-waveguide
(MPW)-based oscillator unit-cells. When a number of z ero-phase oscillator
unit-cells are serially connected in a ring with a centralized place ment o f ac-
tive devices, in-phase coupling can be achieved with low loss. The resulting
CON output can be s ignificantly improved with a reduced phase noise because
outputs of all zero-phase coupled (ZPC) oscillator unit-cells a re synchronized
for an in-phase combination. With the further use of inductive tuning [170],
a wide FTR can be achieved for each ZPC oscillator unit-c ell.
In this chapter, firstly, a zero-phase CON with 4 unit-cells is implemented
at 60 GHz in 65nm CMOS process. The measured results show a peak output
power of 2 mW with 2.2% power efficiency, phase noise of -116.7 dBc/Hz at 10-
MHz offset and FTR of 16% from 58 to 69.1 GHz. Secondly, an injectio n-locked
THz signal source with zero -phase coupled oscilla tor network is proposed to
provide high output power and low phase noise within compact chip area. A
zero-phase oscillator unit-cell is developed by inter-digital coupler with 0.4-dB
loss at 70 GHz. With four in-phase coupled unit-cells and push-push frequency
doublers, high output power signal is generated at the center of the proposed
CON at 140 GHz. The measured results show a peak output power o f 3.5
mW with 2.4% power efficiency, power density (or output power/area) of 26.9
mW/mm
2
and 9.7% FTR centered at 133.5GHz.
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.