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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: haoyu@ntu.edu.sg; jyren@fudan.edu.cn

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.

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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