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8/9/2019 Super Reg 135 Ghz
1/4
A 131.5GHz, -84dBm Sensitivity Super-regenerative Receiver by
Zero-phase-shifter Coupled Oscillator Network in 65nm CMOS
Shunli Ma1, 2
, Hao Yu2, Yang Shang
2, Wei Meng Lim
2, and Junyan Ren
1
1State Key Laboratory of ASIC and System, Fudan University, Shanghai, 200433, China
2School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
Email: [email protected]; [email protected]
Abstract A CMOS high-sensitivity super-regenerativereceiver is proposed for millimeter-wave imaging systems.With quench-control signals, two LC-tank oscillators arecoupled in-phase by zero-phase-shifter network in a positivefeedback loop. This leads to a high oscillatory amplificationand improves the detection sensitivity. The circuit is realizedin 65nm CMOS with a core area of 0.06 mm2. Measurementsshow that the receiver features a sensitivity of 84dBm, anoise-equivalent-power of 0.615fW/Hz
0.5,a noise-figure of
7.26 dB and a power consumption of 8.1mW.
Index Terms zero-phase-shifter; coupled oscillatornetwork; high sensitivity; super-regenerative receiver
I. INTRODUCTION
Millimeter-wave (mm-wave) imaging systems have
been demonstrated to detect covered objects for security
and pharmacy screenings [1]-[6]. Compared to other
semiconductor implementations of mm-wave imaging
circuits, 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 desirable.
The sensitivity is mainly relevant to bandwidth and
noise figure. Super-regenerative receiver (SRR) is proven
to have a superior sensitivity over direct-conversion one
due to its higher oscillatory amplification [1][2][5]. For
example, in [5], the sensitivity was improved by a passive
structure with a high-Q metamaterial resonator in terms of
higher oscillatory amplification.But the passive approach
has limitation to improve the sensitivity in further because
of its single oscillator. As an alternative, active structures,
such as coupled oscillator network (CON) have been usedto reduce the noise and improve the output power at the
same time [7,9], and improve the sensitivity in further [1].
But in that structure, the coupling of two oscillators is not
in-phase, which results in limited oscillatory amplification.
In this paper, an in-phase coupled CON architecture is
proposed to improve the sensitivity of SRR. As shown in
Fig.1, the input power is amplified by two oscillators,
which are coupled in phase in a positive feed-back loop.
Then, the output voltage envelope is detected, indicating
the input power level. The main design challenge is how
to realize in-phase coupling between two oscillators.
The key idea of this paper is using a zero phase shifter
(ZPS) to couple two quench-controlled oscillators in
phase. Compared to the transformer-coupling method [1],
ZPS approach does not introduce the extra phase between
two oscillators, as shown in Fig.1 (a). As a result, the SRR
sensitivity can be improved in terms of both reduced noise
figure (NF) and increased oscillatory amplification.
The proposed SRR with ZPS-coupled CON is designed
in 65nm CMOS at 131.5GHz with a core area of 0.06mm2
.The circuit measurement shows that the receiver features a
sensitivity of 84dBm, a noise equivalent power (NEP) of
0.615fW/Hz0.5, a NF of 7.26dB and a power of 8.1mW.
The rest of this paper is organized as follows. Section II
describes the principle of the SRR. Section III shows the
circuit and layout design. Measurements and conclusions
are given in Section IV and Section V, respectively.
Fig.1 (a) Proposed SRR structure, in-phase output (E,F), andsin-wave quench signal; (b) Envelope shape response (VP) of
oscillator under different input power, and envelope detectoroutput (Vd)
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II. SRR SENSITIVITY ENHANCEMENT
BY ZPS-COUPLED CON
In this section, the fundamentals of the SRR circuit are
described at first and then, the sensitivity enhancement by
the ZPS-coupled CON is discussed.
A.
Fundamentals of Super-regenerative Amplification
In order to understand the sensitivity enhancement from
the coupling of two quench-controlled oscillators, one can
apply the feedback model in a linear time variant (LTV)
analysis of SRR [8]. A simplified circuit model as well as
its feedback model is shown in Fig. 2(a) for conventional
SRRs with single quench-controlled oscillator. Its time-
varying transfer function is
ZVs, t
(1)
where is 1/LC , Z is L/C , and t is dampingfunction:
t 1 GtR (2)where is a constant.Note that the receivers behavior is mainly determined
by ac characteristics of the damping function [6].
When the damping signaltin each quench cycle is aramping signalt , the gain functiontandthe sensitivity function gtof the SRR become
t
(3)
gt e
(4)
where is the slope (GR), and is a constant.
B. Quench-control by In-phase Coupled CON
The simplified circuit and feedback loop model of theSRR with two coupled quench-controlled oscillators are
shown in Fig.2 (b). Its transfer function can be expressed
as follow:
ZNVs, t
(5)
where Zs is the impedance of the parallel resonator(or RLC), and Zs is the impedance of serial resonator(or ZPS).
Note that Gt and Gt are determined by thephase difference of the injected signal between the two
oscillators [1]. At the interested frequency around, theimpedance of serial resonator (or ZPS) is much smaller
than parallel resonator (or RLC). As such, equation (5)
can be further simplified as
ZNVs, t
(6)
where high-order terms are neglected due to small value at
the beginning of the start-up.
Substituting Zs into equation (6), the transferfunction of the proposed SRR can be expressed as
ZNVs, t
(7)
where the new damping functiontbecomest 1 Gm1tR1 e. (8)
Note that the absolute value Gtis equal toGt,and a phase difference is introduced due to the phasedifference from the injected signals. Therefore, when the
damping signal is a ramping signal with slope , thedamping function becomes
t 1 1 e.As a result, the gain functiontand the sensitivity
function gtbecome
t 1
(9)
gt 1
. (10)One can observe that the gain and sensitivity functions
are both influenced by the phase difference of the injectedsignal between two oscillators. When the phase difference
becomes zero, both the gain and the sensitivity functions
can be optimized.
C. Sensitivity Enhancement
We further compare the gain function and sensitivity
function of the conventional SRR with that of the
proposed ZPS-coupled SRR by
U
e
(11)
G
e
. (12)
One can observe that the gain of the SRR enhancement
is exponential with t . When a signal frequencyaround is injected into LC-tank-I, it is amplified andinjected into LC-tank-II in phase. Then, it is further
amplified by LC-tank-II and re-injected into LC-tank-I.
Thus, a positive feedback loop is established when in-
phase coupling is realized by the ZPS, where the oscillator
amplification gain is increased with the improved
detection sensitivity.
o
o
o o
Fig.2 (a) Traditional SRR circuit model and its feedback model;
(b) Proposed SRR circuit model and its feedback model
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III.SRRCIRCUIT DESIGN
The schematic of the proposed SRR is shown in Fig.3.
It consists of two ZPS-coupled LC-tank resonators, one
common source input buffer and one output envelope
detector. Relatively small sized (2m/100nm) NMOSFET
(M3) are connected in both oscillator tanks, working asvaractors for frequency tuning. By tuning control voltages
VTUNE1and VTUNE2, the process mismatch in two LC tanks
is well cancelled to ensure that free running frequencies of
two tanks are the same. As a result, CON synchronization
mainly depends on the ZPS based coupling network. The
quench-controlled transconductances are implemented by
a cross-coupled transistor pairs (M1and M2), of which the
tail current is controlled by M4. Note that M1and M2have
an identical size of 60nm length and 12m width, and M4
has a size of 60nm length and 60m width. The input of
LC-tank-I is connected to a common source buffer (M6),
of which the input is matched to 50 by L1 and L2. A
dummy transistor (Mdummy) is introduced to compensate
parasitic capacitor unbalance. The output of LC-tank-II is
connected to a differential envelope detector (M5).
The design of passive part such as ZPS [10] is shown in
Fig.4. The inductor is implemented with top metal layer.
The radius of the inductor is 26m and the width is 10m.
The simulated inductance is 66pH, and the quality factor
is 20. The ZPS is implemented by a serial connection of
two inductors LZ and one capacitor CZ [10]. An EM
simulation shows a zero-phase-shift at 131.5GHz with a
small insertion loss of 0.4dB.
IV. MEASUREMENT RESULT
The proposed SSR is fabricated in 65nm CMOS process,
and the die micrograph is shown in Fig. 5. The core area is0.06mm2, and total area is 600m500m including input
and output pads. The receiver is measured on probe station
with RF signal provided by a microwave signal generator
through GSG probe. A 12MHz sinusoid quenching signal
is applied by a function generator with 0.6V dc level and
peak-to-dc voltage swing is swept in a range of 0~300mV.
The receiver operates under 1V power supply. The current
consumption of each LC-tank is 3.8mA, while the one of
LNA is 0.5mA.
The operating frequency of the SRR is measured at
131.74GHz, which is also the self-oscillation frequency. A
tuning range of 1GHz is observed when sweeping VTUNE
in a range of 0~1V. Good input power matching is alsoachieved for NF reduction and sensitivity improvement.
As shown in Fig.6, S11 is below 10dB from 122GHz to
140GHz. The bandwidth is around 680MHz. Note that the
maximum gain is 41dB that is almost 13dB higher than
conventional SRR design [2].
In addition, as shown in Fig.7, the sensitivity of the
receiver is measured as 84dBm. NEP is defined as the
signal power in 1Hz bandwidth of unity signal-to-noise
ratio, equivalent to /, and measured as 0.615fW/Hz0.5.
M1 M1 M2
VTUNE1
Vquench
M2
LZ LZCZ
LT LT
ZPS
LNA with
input matching
network
LC Tank-I LC Tank-II
VB
OutputRD
RB
M5
Detector
VB
M5
L1
L2
VG
M3M3
M4 M4
M3 M3
RBSignal input
CA CA
CON
Mdummy
M6
VTUNE2
Fig.3 Circuit diagram of proposed SRR with ZPS-coupled
oscillators
0 50 100 150 200
55
60
65
70
75
80
85
12
14
16
18
20
22
24
0 50 100 150 200
-35
-30
-25
-20
-15
-10
-5
0
-20
0
20
40
60
80
100
26m
Fig.4 Layout of ZPS and simulation results of inductor and ZPS
Fig.5 (a) Chip photo of 131.5GHz SRR in 65nm CMOS; (b)
Measured self-oscillation frequency of 131.74GHz and outputpower of 23.10dBm
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Finally, NF is measured as 7.26dB by S/KTBat a roomtemperature. As shown in TABLE-I, the proposed receiver
are compared to recently published mm-wave imaging
receivers [1]-[5], which achieves better sensitivity, noise
figure and NEP.
V.CONCLUSION
A CMOS super-regenerative receiver is demonstrated
based on ZPS-coupled oscillators for 131GHz. Compared
to traditional SRR designs, the receiver shows an
improved sensitivity by 10dBm due to the additional
positive feedback loop introduced between the two in-
phase-coupled oscillators. The circuit is implemented in
65nm CMOS with a core area of 0.06mm2. Measurements
show that the receiver achieves a sensitivity of 84dBm, a
NEP of 0.615fW/Hz0.5, a NF of 7.26dB and a power
consumption of 8.1mW. The compact size with improved
sensitivity is ideal for the application of large-arrayed
mm-wave imaging applications.
ACKNOWLEDGEMENT
The authors acknowledge Global Foundrys 65nm RF
CMOS tape-out, and funding by MOE Tier-1 RG 26/10
grant from Singapore and NSFC 61176028 grant from
China.
REFERENCES
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1-4.
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Fig.6 Measurement results: i) the maximum gain of 41dB; andii) input S11parameter
Fig.7 Measured sensitivity of 84dBm and the maximum output
voltage of 138mV
TABLE I PERFORMANCE COMPARISON WITH RECENT MM-WAVE RECEIVERS
[3] [4] [1] [2] [5]This
work
Technology65nm
CMOS
65nm
BiCMOS
65nm
CMOS
65nm
CMOS
65nm
CMOS
65nm
CMOS
Supply (V) 1.2 1 1.2 1 1.2 1
Frequency (GHz) 94 94 144 183 95.5 131.5
Sensitivity (dBm) 66 57 74 72.5 78 84
Noise Figure(dB) N/A 12 10.2 9.9 8.5 7.26
Bandwidth(GHz) 23 26 0.94 1.4 0.56 0.68
NEP (fW/Hz0.5) N/A 10.4 1.3 1.51 0.67 0.615
Power (mW) 93 200 2.5 13.5 2.8 8.1
Core Area (mm2) 0.31 1.25 0.021 0.013 0.014 0.06