34 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
Figure 2. 15: Metamaterial-based THz transceiver design including
(a) high power THz signal source by MPW based zero-phase coupled
oscillator network (CON), (b) high gain THz antenna by CRLH T-
line, and (c) high sensitivity THz super-regenerative receiver by TL-
SRR/TL-CSRR based quench-controlled oscillator.
the ε < 0 and µ > 0 condition is satisfied in TL-CSRR in the fr equency range
r
1
C
c
L
c
< ω <
r
C + C
c
CC
c
L
c
(2.21)
where an evanescent wave is formed.
2.4 CMOS Coherent THz Electronics by
Metamat erial
2.4.1 Coherent Source
Coupled oscillator network (CON) [87] is a well-known structure to synchro-
nize output power and reduce phase noise. For a closed-loop CON with N
oscillator s, the phase shift (∆φ) between adjac ent oscillators need to satisfy
the condition of ∆φ = 2kπ/N, (k = 0, ±1, ±2 , ...) as illustrated in Figure 2.16.
The c ombined output admittance (Y
OU T
(ω
0
)) and current (I(t)) of all
CMOS Metamaterial Devices 35
Loop Phase = 2kπ, (k=0, ±1, ±2, ...)
Output
Figure 2.16: Closed-loop coupled oscillator network with center com-
bined o utput.
oscillator s can be calculated as:
Y
OU T
(ω
0
) =
n
P
i=1
Y
i
(ω
0
)
I
OU T
(t) = I
0
·
n
P
i=1
cos(ωt + φ
i
)
(2.22)
where Y
i
, I
0
and φ
i
are the output impedance, the amplitude and phase of
the output current from each oscillator unit-cell, respectively. Clearly, I(t)
is maximized as N · I
0
when all oscillator outputs ar e in-phase (2k/N =
0, ±1, ±2, . . .). Beca use Y
OU T
is also N times larger by parallel connecting N
oscillator outputs, the total available output power is N times increased by the
CON due to P
OU T
= 0.5 · |I
OU T
|
2
/Y
OU T
when compared to that of a single
free-running oscillator. However, if the coupling network is implemented by
the conventional T- line , at least an equivalent length of λ/2 is required with
2k/N = 1, which is not compact with large loss.
The resulting phase noise (L(∆ω)) at frequency offset ∆ω can also be im-
proved under the zero-phase condition satisfied for the CON (in 1/f
2
region)
is [88]:
L(∆ω) = 10log
8πZ
0
ω
2
0
i
2
T
NP
diss
Q
2
L
∆ω
3
!
(2.23)
where
i
2
T
is the squared noise current density; P
diss
is the power dissipated;
and ω
0
is the oscillation frequency. Z
0
and Q
L
are the impedance and the
36 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
quality factor of the co upling T-line, respectively. Ideally, with N oscillator
unit-cells co upled, the phase noise is N times smaller compared to the single
free-running o scillator. Note that similar improvement cannot be achieved by
sp ending the same amount power at one single oscillator. Firstly, increasing
the supply voltage close to the breakdown voltage has serious reliability issues;
secondly, phase nois e cannot be reduced when increasing the supply voltage.
2.4.2 Coherent Transmission
A single-ended dual-fed distributed amplifier (SEDFDA) topology can be used
to realize distributed amplification with extra bandwidth traded for better
power performance. CRLH T-line-based ZP S is implemented in SEDFDA to
optimize all transistors’ power performanc e simultaneously with compact size
and low loss.
A 2D active CRLH T-line network is further proposed as the power-
combining topology with high power-combining efficienc y. The ZPS connec-
tions in the pro posed 2D a ctive CRLH T-line network are adjusted such that
each combining branch resembles a SEDFDA with ZPS connection. In this
way, both high-e fficient power combining and distributed a mplification can be
simultaneously achieved.
Figure 2.17 shows the singe-ended version o f the proposed power-
combining topology. B y using C RLH T-line realized Z PS, a new 2D distributed
power-combining network can be constructed. The CRLH unit-cell can replace
the traditional λ/2 T-line foe in-phase distributed amplification along hori-
zontal direction to achieve the serial power combining. The parallel power
combining for all horizontal branches is then realized by zero-degree power
combiner with sho rt equal-length T-lines along the vertical direction. With
the serial power combining in the 1st level and parallel power combining in
the 2
nd
level, a 2D distributer power combining network is realized for simul-
taneous distributed amplification and power-combining. Such a topology can
be further extended for pha sed-array applications by replacing ZPS with an
array of tunable phase-shifters.
The proposed topology can simultaneously improve power and bandwidth
performance o f PA. For example, PA power performance can bo viewed from
two aspects: output power per area (P
out
/area ) and output power per DC
power consumption (PAE). The 2D power-combining network provides a high
density of transistor, and therefore improves P
out
/area . T he distributed topol-
ogy provides a wide bandwidth, while the SEDFDA implemented with CRLH
T-line-based ZPS trades extra bandwidth with improved efficiency, thus im-
proving PAE. As a result, the power performance can be improved together
with bandwidth performance.
Note that for a fix ed transistor size, the total o utput power depends on the
number of distributed stages N and parallel combining branches M. Therefore,
the power handling ability of the proposed PA partially depends on distr ibuted
stage numbe r N, which is limited by the T-line loss and phase error.
CMOS Metamaterial Devices 37
Figure 2.17: Single-ended version on proposed SEDFDA PA topology
based on 2D distributed power-combining network with the use of
CRLH ZPS s.
2.4.3 Coherent Detection
Millimeter-wave (mm-wave) imaging systems have been demonstrated to de-
tect covered objects for security and phar macy screenings [89, 90, 91, 92, 93].
Compared to other semiconductor implementations of mm-wave imaging cir-
cuits, CMOS is favored for system-on-chip integration of mm-wave circuits
with digital baseband as well as large-arrayed imagers. However, due to the
loss in propagation path as well as substrate and inefficient transmitting power
of MOS transistors, a highly sensitive receiver is much more desirable.
The sensitivity is mainly relevant to bandwidth and noise figure. Super-
regenerative receiver (SRX) is proven to have a superior sensitivity over direct-
conversion one due to its highe r os cillatory amplification [89, 90, 93, 94]. For
example, in [93], the sensitivity was improved by a passive structure with a
high-Q metamaterial r esonator in terms of higher oscillatory amplification.
But the passive approach has limitation to improve the sensitivity further
because of its single oscillato r. As an alternative, active structures, such as
coupled oscillato r network (CON) have been used to reduce the noise and
improve the output p ower at the same time, and improve the sensitivity in
further [95]. But in that struc ture, the coupling of two osc illators is not in-
phase, which results in limited oscillatory amplification.
38 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits
2.4.4 Transceiver Architecture
Figure 2.15 shows the block diagram of the proposed metamaterial-based THz
transceiver de sign. Non-resonant-type metamaterial can be used in the designs
of high power signal sources and high-gain on-chip antennas; reso nant-type
metamaterial can be used in the designs of high sensitivity signal detection.
In the design of high-power signal sources by MPW-based zero-phase coupled
oscillator network (CON), N oscillators can be coupled in-phase to generate a
N times higher co mbined output power as well as N times lower phase noise.
In the design of high-gain CRLH T-line-based on-chip LWA, the zero-phase
propagation in the CRLH T-line can generate in-phase radiation to largely in-
crease the antenna gain in a very sma ll area. In the design o f high-sensitivity
super-regenera tive receivers by TL-SRR/TL-CSRR-ba sed quench-controlled
oscillator s, with the sharp stop-band introduced by the metamaterial res-
onators, high-Q oscillatory amplifications are generated to largely improve
the receiver s ensitivity.
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