Passive and Semi-Passive Wireless Temperature and Humidity Sensors Based on EPC Generation-2 UHF Protocol

https://www.researchgate.net/publication/273394748_Passive_and_Semi-Passive_Wireless_Temperature_and_Humidity_Sensors_Based_on_EPC_Generation-2_UHF_Protocol

Passive and Semi-Passive Wireless Temperature and Humidity Sensors Based on EPC Generation-2 UHF Protocol

ABSTRACT

This paper proposes passive and semi-passive wireless temperature and humidity sensors based on electronic product code (EPC) global Class-1 Generation-2 UHF communication protocol. The wireless sensors consist of a sensor key chip and off-chip temperature and humidity sensors. The sensor key chip integrates RF/analog front-end circuit, digital baseband processor, nonvolatile memory, on-chip temperature sensor, and sensor interface. The sensor interface connects the off-chip sensors and the sensor key chip. The sensor key chip with the on-chip temperature sensor can operate without battery power (passive mode), and also can co-operate with the off-chip temperature and humidity sensors powered by battery (semi-passive mode). The RF/analog front-end circuit provides the dc power to the sensor key chip and communicates with the interrogator passively. Advanced low-power techniques are adopted to reduce the power consumption of the sensor key chip. The sensor key chip is fabricated in 0.18-μm CMOS process. In passive mode, the maximum wireless sensitivity of on-chip sensor is -15.1/-11.2 dBm for reading and sensing operation, respectively, and the temperature sensing error is -1 °C/0.8 °C over operating range from -20 °C to 50 °C. It achieves a reading/sensing distance of over 9.5/6 m with 4-W effective isotropic radiated power (EIRP) by the commercial interrogator. In semi-passive mode, the temperature and humidity sensing distance of off-chip sensors is 2.7 m.

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Passive and Semi-Passive Wireless Temperature

and Humidity Sensors Based on EPC Generation-

2 UHF Protocol

ARTICLE in IEEE SENSORS JOURNAL · APRIL 2015

Impact Factor: 1.76 · DOI: 10.1109/JSEN.2014.2375180

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Chinese Academy of Sciences

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Nan-Jian Wu

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IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015 2403

Passive and Semi-Passive Wireless Temperature

and Humidity Sensors Based on EPC

Generation-2 UHF Protocol

Shuang-Ming Yu, Peng Feng, and Nan-Jian Wu, Member, IEEE

Abstract—This paper proposes passive and semi-passive

wireless temperature and humidity sensors based on electronic

product code (EPC) global Class-1 Generation-2 UHF communication

protocol. The wireless sensors consist of a sensor

key chip and off-chip temperature and humidity sensors. The

sensor key chip integrates RF/analog front-end circuit, digital

baseband processor, nonvolatile memory, on-chip temperature

sensor, and sensor interface. The sensor interface connects

the off-chip sensors and the sensor key chip. The sensor key

chip with the on-chip temperature sensor can operate without

battery power (passive mode), and also can co-operate with the

off-chip temperature and humidity sensors powered by battery

(semi-passive mode). The RF/analog front-end circuit provides

the dc power to the sensor key chip and communicates with

the interrogator passively. Advanced low-power techniques are

adopted to reduce the power consumption of the sensor key

chip. The sensor key chip is fabricated in 0.18-μm CMOS

process. In passive mode, the maximum wireless sensitivity of

on-chip sensor is −15.1/−11.2 dBm for reading and sensing

operation, respectively, and the temperature sensing error is

−1 °C/0.8 °C over operating range from −20 °C to 50 °C.

It achieves a reading/sensing distance of over 9.5/6 m with

4-W effective isotropic radiated power (EIRP) by the commercial

interrogator. In semi-passive mode, the temperature and humidity

sensing distance of off-chip sensors is 2.7 m.

Index Terms—Wireless sensors, temperature sensors, humidity

sensors, sensor interface, baseband processor, EPC global Class-1

Generation-2 protocol, passive tags, low power design.

I. INTRODUCTION

NOWADAYS, various kinds of wireless sensors have been

widely used in many applications because they can be

Manuscript received October 20, 2014; revised November 17, 2014;

accepted November 19, 2014. Date of publication November 26, 2014; date

of current version February 10, 2015. This work was supported in part by the

National Key Technology Research and Development Program through the

Ministry of Science and Technology of China under Grant 2012BAH20B02,

in part by the National High Technology Research and Development Program

of China under Grant 2012AA012301, in part by the National Science and

Technology Major Projects through the Ministry of Science and Technology

of China under Grant 2012ZX03004007-002, in part by the National Natural

Science Foundation of China under Grant 61306027, and in part by the

Academy-Locality Cooperation Program, Chinese Academy of Sciences,

Beijing, China. The associate editor coordinating the review of this paper

and approving it for publication was Prof. Sang-Seok Lee.

The authors are with the State Key Laboratory for Superlattices and

Microstructures, Institute of Semiconductors, Chinese Academy of Sciences,

Beijing 100089, China (e-mail: yushuangming@semi.ac.cn; fengpeng06@

semi.ac.cn; nanjian@red.semi.ac.cn).

Color versions of one or more of the figures in this paper are available

online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2014.2375180

easily arranged in the fields [1]–[5]. For example, wireless sensors

are increasingly applied to extreme condition detecting,

high-risk environment surveillance, emergency rescue, frozen

food transportation, and tracking humidity and pressure of

scientific equipment [6].

The wireless sensors usually include wireless communication

technologies, such as ZigBee, Bluetooth and wireless

local area networks, to receive commands and transmit

data [7]–[14]. A ZigBee-based intelligent self-adjusting

sensor for home energy management service is implemented

by using ZigBee wireless technology for networking and

communication [7]. Another ZigBee-based non-invasive wearable

monitoring device for physiological parameters, such as

temperature and heart rate of a human subject, has been

developed and reported [8]. A Bluetooth module is selected

to act as a virtual serial data port for digital temperature sensors

in rotor temperature measurement [12]. However, those

wireless technologies are usually used to form sensor networks

for environment monitoring and diagnostics of electric

drives [13]. They have drawbacks of high power consumption,

large device size, high complexity of operation, and low

flexibility.

The passive RFID tag technology exhibits benefits of very

simple structure, low cost and low power. The tags are able

to passively communicate with the interrogator in a zeropowered

backscatter mechanism. Therefore, wireless sensors

based on the passive tag technology have raised increasing

interest among academic and industrial research [15]–[19].

An energy-efficient wireless sensing within a passive multistandard

RFID transponder is enabled by using a successive

approximation analog-digital converter [15]. Another wireless

temperature sensor tag chip is presented with the extra

circuit modules, such as external temperature sensor and

ADC [16]. A passive RFID tag embedded temperature sensor

with time-domain readout scheme for a −30 °C to 60 °C

sensing range was reported [17]. TELID412 RFID sensor by

MicroSENSYS [18] and Fenix RFID Tag by Farsens [19] are

designed as passive RFID sensor devices without sensor interface

for off-chip sensor devices. However, in many applications,

some non-CMOS-compatible sensors, such as humidity

and pressure sensors are also required to be integrated in the

system.

Recently, wireless sensors which include a RFID tag

chip with a sensor interface and off-chip sensors has been

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2404 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015

proposed [20]–[26]. An ultra-low-power four-channel sensor

interface for wireless semi-active RFID transponders based

on a switched-capacitor sigma-delta modulator was described

in [20]. However the analog signal interface is not compatible

with most general off-chip sensors with standard digital

signal interface. RFID sensor prototypes fabricated on a

printed circuit board using discrete components were designed

for full-passive, semi-passive and passive/semi-passive compatible

operation mode [21]–[26]. A solution including a

compact low power microcontroller and a RFID core extended

by a memory unit as data interface was presented in [21].

PE3001 UHF Tag IC by Productivity Engineering (PE) GmbH

was designed with a Serial Peripheral Interface (SPI) which

costs higher pin count than I2C and limits the number of

accessible off-chip sensors [26]. In [23]–[25], the sensors need

external MCU component and extra antenna with off-chip

RF-dc circuit to harvest the RF energy and supply the MCU

and off-chip sensors in passive operation mode. This will

greatly increase the cost and size of the sensor tag.

Comparing with the wireless sensors, where a microcontroller

and an off-the-shelf RFID chip are externally connected,

an ASIC single-chip solution and easier data exchange strategy

between the RFID chip and the microcontroller is more

compact and has a lower power consumption. Therefore, it is

very important to develop a sensor single key chip with wireless

low power communication circuits, on-chip low power

sensor circuits and sensor interface for operating the off-chip

sensors.

This paper proposes passive and semi-passive wireless temperature

and humidity sensors based on EPC global Class-1

Generation-2 (EPC Generation-2) UHF communication

protocol. The wireless sensors consist of a sensor key chip

and off-chip temperature and humidity sensors. The sensor

key chip integrates an on-chip temperature sensor, a sensor

interface and a digital baseband processor. The sensor interface

uses I2C serial bus to connect the off-chip sensors and the

sensor key chip. Some advanced low power techniques are

adopted to effectively reduce the power consumption of the

key chip and to increase the sensing distance. Comparing with

the above-mentioned related works, the proposed wireless sensors

have the following features. First, the sensor key chip with

the on-chip temperature sensor can operate without battery

power (passive mode) and also co-operate with the off-chip

sensors which are powered by a battery (semi-passive mode),

which can be operated by a commercial interrogator. Second,

the sensor key chip implements single-chip solution instead

of using discrete MCU to reduce the power consumption and

cost. Third, the integrated I2C sensor interface can connect

the off-chip sensors and the sensor key chip directly. Fourth,

the sensors are equipped with only one UHF antenna which is

designed and patterned directly on FR4 substrate to perform

both energy-harvesting and communication. This will reduce

the size and cost of the wireless sensors.

The paper is organized as follows. Firstly, the wireless

sensor architecture is presented in Section II. Next, the critical

circuit blocks are designed in Section III. The measurement

results are shown in Section IV. Finally, conclusions are drawn

in Section V.

Fig. 1. Architecture of the wireless sensors.

II. WIRELESS SENSOR ARCHITECTURE

Fig. 1 shows the architecture of the wireless sensors. The

wireless sensors consist of a sensor key chip and off-chip

temperature and humidity sensors. The sensor key chip integrates

RF/analog frontend circuit, digital baseband processor,

non-volatile memory (NVM), on-chip temperature sensor,

and off-chip sensor interface. The communication between

the wireless sensors and the interrogator is based on the

EPC Generation-2 UHF protocol. The wireless sensors can

operate in two modes: passive mode and semi-passive mode.

In passive mode, the wireless sensors can measure temperature

by the on-chip sensor passively. In semi-passive mode, the

wireless sensors can semi-passively measure temperature and

humidity by the off-chip sensors, which are powered by a

battery. The performance of the off-chip temperature sensor is

better than on-chip temperature sensor.

The proposed wireless sensors have several advanced merits.

The interrogator can identify the wireless sensors when there

are a mass of wireless sensors in the application fields.

Secondly the wireless sensors can also execute temperature

sensing operation in passive mode under some battery-less

conditions. Furthermore, when the humidity or high-accuracy

temperature measurements are required, the sensor key chip

can activate the off-chip sensors and perform the measurements

in the semi-passive mode. Therefore, the wireless sensors

can adapt to the various application scenarios flexibly and

improve the battery lifetime.

The communication process between the interrogator and

wireless sensors is described as follows. The interrogator sends

energy and information to the sensor key chip by a modulated

UHF RF carrier. The sensor key chip harvests electrical energy

from the RF carrier, and activates itself. Then the sensor key

chip receives the commands and data from the interrogator

and sends information back by a backscattering scheme. The

communication link between the interrogator and sensor key

chip is half-duplex.

The RF/analog frontend circuit converts the energy of

RF signal into DC power to energize the sensor key chip, and

demodulates (modulates) the received (transmitted) signal. The

digital baseband processor decodes the demodulated signal

and generates the encoded data for modulation. Besides,

the baseband processor executes the received commands and

controls the on-chip and off-chip sensors. The sensor interface,

compatible with the I2C serial bus standard, supports the

communication between the sensor key chip and off-chip

YU et al.: PASSIVE AND SEMI-PASSIVE WIRELESS TEMPERATURE AND HUMIDITY SENSORS 2405

Fig. 2. Block diagram of the digital baseband processor.

sensors. The CMOS-compatible NVM is designed to store the

information such as EPC code, security information or sensing

data after power off.

III. DESIGN AND IMPLEMENTATION OF CIRCUIT BLOCKS

A. Low-Power Digital Baseband Processor

Fig. 2 shows the structure of the proposed digital baseband

processor which is fully compatible with EPC Generation-2

UHF protocol. The system consists of power management

(PM) module, PIE decoder, encoder, command decoder,

state controller, NVM controller, sensor controller, off-chip

sensor interface, cyclic redundancy check (CRC) check

module and slot counter.

In each communication round, the baseband processor

receives the encoded commands and data from the RF/analog

frontend circuit at a rate of 26.7kbps to 128kbps. Then, the

PIE decoder decodes the data symbol and starts the data

processing. The CRC check module calculates the CRC-16

over the received commands or transmitted data to detect error

or protect the certain information. If the CRC check result is

error, all data are discarded and frame is terminated.

The command decoder analyzes and identifies all commands

received from the interrogator. According to the decoded

commands, the state controller performs the corresponding

operations with the input data, such as enabling or disabling

the NVM controller, sensor controller and off-chip sensor

interface respectively.

These commands are responsible for reading data from or

writing data to the NVM, activating the on-chip temperature

sensor, and controlling the off-chip temperature and humidity

sensors through the sensor interface module. Finally the

encoder collects the output frames from the State Controller,

CRC code from CRC check module, and random number

generated in slot counter. The slot counter is designed

based on the Slotted ALOHA algorithm as anti-collision

mechanism [34]. Then, the encoder encodes the output data

according to FM0/Miller encoding method, and sends them

to the modulator for backscattering. The PM module adaptively

and dynamically controls the operations of the circuit

modules of baseband processor to effectively reduce its power

consumption.

We design a set of internal customized commands to

operate the on-chip temperature sensor and off-chip sensors.

Fig. 3. Functional illustration of Power Management Module.

The baseband processor firstly sends the certain command

to start up the on-chip temperature sensor. After finishing

the measurement operation, the baseband processor receives

the sensing result data and saves them into the NVM. The

customized commands also can start up the off-chip sensors,

receive the sensing result data and save them into the NVM

through the I2C interface. The interrogator can read out

the sensing result data in the NVM and finish the sensing

operation.

The designed baseband processor has some advanced

merits. Firstly, it can implement the internal customized

commands and manage the on(off)-chip sensors. Secondly,

it integrates the general I2C serial bus, which can connect any

sensors with I2C serial bus. Thirdly, it is compatible with EPC

Generation-2 protocol. Finally, the wireless sensors operate

in the half-duplex fashion so that a novel PM method can

be designed to activate/shut down the certain circuit modules

adaptively and to reduce the instantaneous power consumption

of the baseband processor.

Because the sensor key chip integrates the on-chip sensor

and I2C serial bus, the power consumption of the chip will

be increased. To improve the operating distance, the power

consumption of the sensor key chip must be minimized. Fig. 3

illustrates the function of the power management module,

which reduce the baseband power consumption.

The operation of the baseband processor is divided into

three kinds of states: RECEIVE state, PROCESS state and

TRANSMIT state. When the baseband processor operates in

RECEIVE state, only PIE decoder and CRC check modules

are activated. After receiving the whole message, processor

turns in to PROCESS state. PM module disables the PIE

decoder and CRC check module, and enables command

decoder and state controller. Then according to the received

commands, state controller performs the corresponding operations,

and prepares the output data. Finally, in TRANSMIT

state, encoder and slot counter module are activated to encode

the message and send the data to the modulator. Thus, the

PM module enables the operating modules and disables the

idle modules dynamically to reduce the power consumption

of the baseband processor.

2406 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015

Fig. 4. Clock gating and operand isolation techniques.

Fig. 5. Block diagram of the on-chip temperature sensor.

Besides of PM method, other low-power circuit design

techniques are also adopted to reduce the power consumption

of baseband processor. As shown in Fig. 4, clock gating

and operand isolation are used to reduce power consumption

of the modules. These power optimization techniques are

adopted at RTL and logic synthesis level. Clock gating reduces

the switching activity of the sequential cells by turning off

the clock signal. Furthermore, operand isolation is applied

to arithmetic modules to reduce the internal and dynamic

power consumption of the combinational cells. The power

consumption of these cells is reduced by eliminating unwanted

transitions of their inputs when their outputs do not affect any

other cells.

Supply voltage reduction is also an effective method to

reduce the dynamic power consumption. In order to minimize

the power consumption, the processor circuit is designed

to operate at 0.8V. On the other hand, the operating clock

frequency is minimized to further reduce the dynamic power.

The clock frequency is optimized by considering the trade-off

between the sensor performance and the power consumption.

B. Low-Power On-Chip Temperature Sensor and

I2C Serial Bus Interface for Off-Chip Sensors

The low power on-chip temperature sensor is powered by

the energy provided by RF rectifier. The block diagram of

the on-chip temperature sensor is shown in Fig. 5. It consists

of a bipolar sensing core, a low power 2nd-order sigma delta

( ) ADC, a dynamic element match (DEM) control module

and a bias circuit. The ADC includes a modulator,

a decimation filter and a clock generator. The bipolar sensing

core which is biased by a p: 1 ratio currents, produces two

voltage signals VBE1 and VBE2, which are complementary to

absolute temperature (CTAT), and can be expressed as follows:

VBEn = (

kT

q

) · ln(

IC

IS

), n = 1,2 (1)

where k is the Boltzmann constant, q is the electron charge,

T is the temperature in Kelvin, IC is the collector current

and IS is the PNP’s saturation current. The voltage difference

VBE = VBE1 − VBE2 is proportional to absolute temperature

(PTAT), which can be expressed as:

VBE = kT

q

· ln(p) (2)

The relationship among the absolute temperature T ,

VBE2 and VBE can be expressed as [35]:

T = A

α

α + X

(3)

where A is a constant, X = VBE2/ VBE and α is the gain

factor which is chosen to make a band-gap reference voltage

VREF = VBE2 + α VBE. For the temperature range from

−20 °C to 50 °C, in consideration of process corners and

design margins, X will not exceed the range from 5 VBE

to 24 VBE. Then the charge balance scheme can be used to

digitize X in the ADC, and α can be simply trimmed in

the digital backend. In the actual implementation, a 2nd-order

ADC is designed to realize noise shaping and improve

the resolution and conversion speed, and reduce the power

consumption. The modulator implemented with differential

switched-capacitor circuits, adopts DEM method to reduce

the mismatch effect of current sources in the bipolar core

and sampling capacitors in the modulator. As a result of the

2nd-order noise shaping, 13 bits temperature data can be

obtained with only 128 bit streams. The measured temperature

data are stored in the NVM, which can be read out through

commercial interrogator.

The off-chip sensor interface is integrated in this wireless

sensor key chip to manage off-chip temperature and humidity

sensors. It is compatible with I2C serial bus interface standard.

The off-chip sensors and sensor key chip are connected with

two bus lines: a serial data line and a serial clock line.

The EPC operation commands control the sensing commands

and data transmissions through the sensor interface. The

off-chip sensors are recognized as slave (master) devices when

receiving (transmitting) the operation commands (the sensing

data) from (to) the sensor key chip. The interrogator first starts

up the off-chip sensors by a measurement command. After

finishing the sensing operation, the off-chip sensors send back

the sensing result digital data to the sensor key chip. Then,

the result data is stored in NVM. Finally the interrogator can

read out the result data in NVM by an EPC read operation

command and shows the measured temperature and humidity.

C. CMOS UHF Rectifier

The rectifier converts input electromagnetic waves into

DC power for the wireless sensor key chip. The UHF rectifier

in this paper is compatible with a standard CMOS process.

YU et al.: PASSIVE AND SEMI-PASSIVE WIRELESS TEMPERATURE AND HUMIDITY SENSORS 2407

Fig. 6. Circuit architecture of the CMOS rectifier.

Fig. 7. Voltage control methods of the rectifier.

As shown in Fig. 6, the CMOS UHF rectifier is based on a

modified charge pump structure. It consists of six cascaded

rectifier cells and an oscillator. A switched capacitor circuit

technique is used to provide active bias to solve the

threshold voltage problem in MOS diodes. The MOS diodes

(NM1 and PM1) are biased by two circuits (Bias1 and Bias2)

respectively to make them work in sub-threshold region. As a

result, the voltage loss during charge transferring caused by

the threshold voltage of the switch transistors is reduced. This

technique improves the voltage gain of the rectifier cell and

the power efficiency of the rectifier.

Because the incoming electromagnetic wave intensity into

the wireless sensors varies over a 30 dB range, to provide a

stable DC voltage and power, some voltage control methods

must be designed. Fig. 7 shows the schematic diagram of the

voltage control circuits, which include regulator, limiter and

protector. For low level of electromagnetic wave intensity, the

voltage is stabilized by the regulator only. For middle level

of electromagnetic wave intensity, the voltage is stabilized by

the regulator and limiter. The limiter can clamp the voltage

below a certain value. For high level of electromagnetic wave

intensity, the voltage is stabilized by the regulator, limiter and

protector using feedback mechanism. The protector consists

of voltage comparator and a switch connecting to the input

ports of rectifier. The comparator is used to compare output

voltage of rectifier and reference voltage, and the comparison

result controls the switch to change the input impedance of

rectifier. As a result, the input signal intensity can be reduced

and the sensor key chip can be protected.

D. Low Power Single-Poly NVM

NVM embedded in the sensor key chip is used to store

information such as EPC code, security codes and sensing data

Fig. 8. (a) Block diagram of the non-volatile memory. (b) Schematic of the

memory bit cell.

Fig. 9. Microphotograph of the sensor key chip.

after power off [36], [37]. The architecture of the proposed

NVM is shown in Fig. 8(a). It consists of a non-volatile cell

array, a volatile register array, a bit line controller, a column

decoder, a sense amplifier column, a charge pump and a

control logic module. The non-volatile cell array and register

array have the equal numbers of corresponding bits. The data

in the non-volatile array can be first loaded into the register

array for the subsequently read operation with a faster speed

and less power. The charge pump generates a high voltage

and a medium voltage for programming. The column decoder

selects the active column. The sense amplifier column detects

the output data of active-column cells during read operation.

Fig. 8(b) shows the schematic of the memory bit cell.

To minimize the process cost and power consumption, the

NVM is realized with single-poly differential floating gate cell

compatible with standard CMOS process and bi-directional

FN-tunneling to achieve reliable low power programming. The

memory bit cell consists of FN tunneling junctions M1-M4

and coupling capacitors C1 and C2. The electrons can tunnel

through M1 and M2 (M3 and M4) to inject into (depart from)

the floating gates.

IV. MEASUREMENT RESULTS

The sensor key chip has been fabricated in 0.18μm

standard CMOS process with the chip area of 1.44 mm2.

Fig. 9 shows the microphotograph of the sensor key chip.

2408 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015

Fig. 10. The test platform for the wireless sensors.

Fig. 11. Measurement communication signal waves between wireless sensors

and interrogator.

Fig. 12. Measured backscattered data of FM0-encoded symbols.

Fig. 13. Power consumption for optimization techniques.

Prototype passive and semi-passive wireless temperature and

humidity sensors based on the sensor key chip have been

fabricated on FR4 substrate. As shown in Fig. 10, the wireless

sensors include the sensor key chip, off-chip temperature and

humidity sensors, dipole copper antenna and lithium battery.

Fig. 10 shows the test platform of the wireless sensors.

The commercial interrogator is used to send energy and a

set of commands and data through the antenna. We use a

RFID protocol analyzer to monitor the communication information

between wireless sensors and interrogator. Fig. 11

shows measurement communication signals between wireless

sensors and interrogator. The interrogator uses Select Command

to selects a particular sensor population, and uses Query

command to start the communication process. The selected

sensor will backscatter a 16 bits random number (RN16)

to the interrogator. Then the interrogator will send ACK

command which echoes the RN16 to acknowledge the sensor.

Finally, the sensor will backscatter the data. Fig. 12 shows

the measured backscattered data of FM0-encoded symbols.

Fig. 14. Measurement result of operating off-chip sensors with I2C serial

bus interface.

Fig. 15. Measured temperature error of the on-chip temperature sensor.

The wireless sensors have been further measured according

to EPC Generation-2 UHF protocol. The results show that

the wireless sensors are compatible with the communication

protocol and it can correctly execute reading, writing and

sensing operation.

Fig. 13 shows a comparison of the measured power

consumption for two different baseband processor designs.

The first version of the design without power optimization

techniques consumes 18.5μW at minimum operating voltage

of 0.9V. By using power optimization techniques described

above, the baseband processor consumes only 7.9μW at

minimum operating voltage of 0.8V. The comparison results

show that a 57% power reduction is achieved by using power

optimization techniques.

Fig. 14 shows the measured communication process of the

I2C serial bus interface between the sensor key chip and

the off-chip temperature and humidity sensors. SCL signal

provides the off-chip sensors a continuous stable clock signal,

and SDA signal is used to transmit data and commands

between the sensor key chip and off-chip sensors. The communication

process is divided into three steps. In the first

step, the off-chip sensors receive the address and commands

from the sensor key chip. Then the off-chip sensors begin

to perform the measurement operation. After finishing the

measurement, they outputs the sensing data back to the sensor

key chip through SDA signal line. Finally, the sensing data

is stored in the NVM which can be read out by commercial

interrogator. The measurement results show that the sensor

interface can make the sensor key chip operate the off-chip

sensors.

The measured performance of the rectifier indicates that it

achieves a power efficiency of about 30% at the load resistor of

20k at input power of −5dBm. The measured results of the

YU et al.: PASSIVE AND SEMI-PASSIVE WIRELESS TEMPERATURE AND HUMIDITY SENSORS 2409

TABLE I

PERFORMANCE SUMMARY AND COMPARISON

NVM show that the write power consumption is only 4.3μW

@1.2V supply voltage, which is acceptable in the wireless

sensors. The NVM also shows good endurance performance

of more than 105 write/erase times, and it is fully compatible

with standard CMOS process.

To test the temperature sensing performance, the wireless

temperature sensor, the interrogator antenna and a reference

temperature sensor are placed inside the temperature chamber

with a temperature constancy of ±0.3 °C. Fig. 15 and Fig. 16

show the typical measurement results of the on-chip passive

temperature sensor and the off-chip semi-passive temperature

sensor. The inaccuracy of the on-chip temperature sensor is

−1.0 °C/0.8 °C over operating range from −20 °C to 50 °C

with a resolution of 0.02 °C. The commercial off-chip temperature

sensor also shows good performance, and can achieve a

resolution of 0.04 °C over a range of −40 °C to 125 °C.

2410 IEEE SENSORS JOURNAL, VOL. 15, NO. 4, APRIL 2015

Fig. 16. Measured temperature error of the off-chip temperature sensor.

Fig. 17. Measured humidity error of the off-chip humidity sensor.

To test the humidity sensing performance, the wireless

sensors, the interrogator antenna and a reference humidity

sensor are placed inside a chamber in which the humidity

is adjustable. The commercial off-chip humidity sensor is

energized by a 3V lithium battery. Fig. 17 shows the typical

measurement results of the off-chip humidity sensor. The

off-chip humidity sensor can monitor the humidity with a

resolution of 0.4% over a range of 20% to 98%. Although

the sensing error is limited by the reference sensor and other

non-ideal effects, the wireless sensors show good performance.

Table I summarizes and compares the performances of the

wireless sensors with those of other works. The wireless sensors

successfully measured data using commercial interrogator

with 4W EIRP. It achieved a reading/sensing distance of over

9.5/6.0 m with on-chip sensor, and 2.7 m with off-chip sensors.

The maximum wireless sensitivity is −15.1dBm for reading

operation, −11.2dBm for on-chip passive temperature sensing

operation and −4.2dBm for off-chip semi-passive temperature

and humidity sensing operation, respectively.

Compared with the sensors in [30]–[32], our proposed

wireless sensors integrate the sensor interface for operating

off-chip sensors to expand their application area. Our wireless

sensors also have longer operating distance because of the

ultra-low power NVM, on-chip temperature sensor, baseband

circuit and high efficiency RF rectifier. Compared with the

sensors in [24], our wireless sensors can perform temperature

sensing operation in passive mode without battery power.

Compared with the sensors in [25], our wireless sensors can

perform sensing operation without MCU. This can reduce

the power consumption and cost. Furthermore, our wireless

sensors are equipped with only one UHF antenna designed

and patterned directly on FR4 substrate to perform both

energy-harvesting and communication. This can reduce the

size and cost of the sensors.

V. CONCLUSION

This paper proposed passive and semi-passive wireless

temperature and humidity sensors based on EPC Generation-2

UHF protocol. A sensor key chip and off-chip temperature

and humidity sensors were integrated in the wireless sensors.

The sensor interface in sensor key chip was designed for

connecting the off-chip sensors. The wireless sensors can

be operated in two modes: passive mode and semi-passive

mode. Power management method and clock gating method

were adopted to reduce the power consumption. The sensor

key chip was fabricated in 0.18μm CMOS process occupying

an area of 1.44mm2 . The experimental results indicate

that the wireless sensors can measure temperature by the

on-chip temperature sensor passively, and temperature and

humidity by the commercial off-chip sensors semi-passively.

In passive mode, the maximum wireless sensitivity of the sensor

is −15.1/−11.2dBm for reading and sensing operation of

on-chip sensor, respectively. It achieves a reading/sensing

distance of over 9.5/6.0m with 4W EIRP. In semi-passive

mode, the wireless sensitivity is −4.2dBm for sensing operation

of off-chip sensors, and the sensing distance is 2.7 m.

The wireless sensors can be operated by the commercial

interrogator.

ACKNOWLEDGMENT

The authors would like to thank S. Zhang for his help in

testing.

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Shuang-Ming Yu was born in Liaoning,

China, in 1987. He received the B.S. degree in

electronic science and technology from the Beijing

University of Posts and Telecommunications,

Beijing, China, in 2010. He is currently pursuing

the Ph.D. degree at the State Key Laboratory

for Superlattices and Microstructures, Institute of

Semiconductors, Chinese Academy of Sciences,

Beijing. His current research interest includes

ultralow-power circuit design, digital circuits

design, and radio frequency identification tags.

Peng Feng was born in Chongqing, China, in 1983.

He received the B.S. degree in electronics from

Sichuan University, Chengdu, China, in 2006, and

the Ph.D. degree in microelectronics and solidstate

electronics from the Institute of Semiconductors,

Chinese Academy of Sciences, Beijing, China,

where he has been an Assistant Professor with the

Institute of Semiconductors since 2011. His current

research interests include radio frequency energy

harvesting, embedded CMOS nonvolatile memory,

CMOS-embedded sensors, and RFID tags.

Nan-Jian Wu (M’05) was born in Zhejiang, China,

in 1961. He received the B.S. degree in physics from

Heilongjiang University, Harbin, China, in 1982, the

M.S. degree in electronic engineering from Jilin

University, Changchun, China, in 1985, and the

D.Sc. degree in electronic engineering from the

University of Electronic-Communications, Tokyo,

Japan, in 1992. In 1992, he joined the Research

Center for Interface Quantum Electronics and

the Faculty of Engineering, Hokkaido University,

Sapporo, Japan, as a Research Associate. In 1998, he

was an Associate Professor with the Department of Electro-Communications,

University of Electronic Communications, Tokyo. Since 2000, he has been a

Professor with the Institute of Semiconductors, Chinese Academy of Sciences,

Beijing, China. In 2005, he visited the Research Center for Integrated Quantum

Electronics, Hokkaido University, as a Visiting Professor. In 2009, he

became an Honorable Guest Professor at the Research Institute of Electronics,

Shizuoka University, Shizuoka, Japan. His research is in the field of mixedsignal

LSI design and semiconductor quantum devices