A RF TRANSCEIVER FOR WIRELESS MONITORING

SYSTEMS OF THE VERTEBRAL COLUMN BEHAVIOUR

J. P. Carmo and J. H. Correia

University of Minho, Dept. Industrial Electronics, Campus Azurem, 4800-058 Guimaraes, Portugal

Keywords: Wireless acquisition system, RF transceiver, Spine deceases, e-health systems.

Abstract: This paper presents a radio-frequency (RF) transceiver designed, using a standard 0.18 µm CMOS process,

for operation in the 2.4 GHz ISM band. The receiver has a sensivity of -64 dBm and consumes 3.5 mW

from a 1.5 V supply. The RF transmitter delivers an output power of +11 dBm (15 mW) with a power

consumption of 45 mW. These features make the RF transceiver suitable to be integrated in microsystems,

where the low-power is a major requirement. The application of these microsystems is to monitoring the

influence of heavy loads, in the behaviour of the vertebral column.

1 INTRODUCTION

The human posture has been an object of studies in

biomechanics, once some deviations of structural

and functional positions induce an unbalanced body.

These deviations usually, affects the vertebral

column and are caused by physical efforts, bad

postures in work, deficiency in sustentation muscles,

infections and congenital causes. The main

pathologies of vertebral column caused by the

referred deviations, are the scoliosis and lordosis.

Sometimes these pathologies appears in children

when they carry the heavy backpacks on the backs,

in this case, it’s very important monitoring the

influence of loads (backpack weight) in vertebral

column behavior. Figure 1 shows an adolescent

female with scoliosis in the vertebral column. Her

rib proeminence is most obvious upon her bending

forward. The radiograph demonstrates a right

thoracic scoliosis. The study of influence of

backpack weighs on the vertebral column of children

is an important issue, that has been worked by many

researchers for years (Palastranga et al, 2002). The

application cited in this work uses the following

approach: applying indirect information, using the

electrical potential generated by the muscles, when

they contract and when they are rest. The technique

which could measure this electrical potential is the

electromyography (EMG). Based on the results of

EMG, combined with the movements of the body,

measured by the accelerometers, it is possible to

know by numerical simulation, the displacement

occurred on the insertions points between the

muscles and the vertebral column (Pato et at, 2007).

Using these values of displacements in a finite

element code, like ANSYS, it is possible to compute

the value of stress field in the vertebral column,

especially it is possible to observe where are the

points more affected and the respective stress value.

Figure 1: An adolescent female with scoliosis.

The radio-frequency (RF) transceiver proposed in

this paper, was designed using a standard 0.18 μm

CMOS process. This process allows to have the

power supply of 1.5 V. The proposed low-

power/low-voltage transceiver, is intended for use in

wireless sensor networks, more specifically, for the

monitoring the influence of heavy loads, in the

behavior of the vertebral column.

281

Carmo J. and Correia J. (2009).

A RF TRANSCEIVER FOR WIRELESS MONITORING SYSTEMS OF THE VERTEBRAL COLUMN BEHAVIOUR.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 281-285

DOI: 10.5220/0001555802810285

 SciTePress

Local oscillator (LO)

@ 2.4/2.5 GHz

Envelope

detector

(100 MHz)

Filter

Switch

RXD

TXD

LNA

2.5 GHz

2.4 GHz

Figure 2: The block schematic of the RF transceiver.

2 TRANSCEIVER'S DESIGN

The transceiver has a receiver, a transmitter, an

antenna-switch and a phase-locked loop (PLL) as

frequency synthesiser. The Figure 2 shows the

architecture of the proposed transceiver, where the

reception is made by means of direct demodulation,

using the technique of heterodyne detection. The

final demodulation step is made with the use of a

envelope detector, applied for an intermediate

frequency of 100 MHz.

The quality requirement for the proposed RF

transceiver is a transmission with a bit error

probability less than 10

-6

(one error for each one

million bits transmitted) with a sensitivity of

-64 dBm, in a transmitted power of +11 dBm using

Amplitude Shift Keying (ASK)

modulation (Carlson et al, 2002). All of these

specifications are useful to make this transceiver

suitable for short range applications (e.g., between

fifty five and sixty meters - #55 / #60 meters), and

obviously, to the target biomedical application,

which will be further explained in the section 4.

2.1 Receiver

The Figure 3 shows the receiver’s front-end

schematic. This circuit has a low-noise amplifier

(LNA) that provides a 50 Ω input impedance, using

a tuned load to provide high selectivity. The

amplified RF signal is directly converted to an

intermediate frequency (IF) with a mixer, followed

by a low-pass filtering and a post-amplifier. The

final downconversion to the base-band is made with

envelope detection.

The low-noise amplifier (LNA) is the first gain

stage in the receiver path. In a LNA, the signal must

be amplified as much as possible, keeping the

signal-to-noise ratio (SNR) as low as possible. This

is achieved with the best noise figure (NF). The

LNA is an inductively degenerated common source

amplifier. The cascoding transistor M

is used to

increase the gain, to better isolate the output from

input and to reduce the effect of M

’s C

(Yao et al, 2007). The LNA enters in the sleeping

mode, when the current in the polarization stage is

switched off. The same principle applies to the all

subsystems.

Mixer circuit

Bias 4

Bias 1

Input

LNA

Bias 3

IF filter

Bias 2

Bias 5

Post-amplifier

Bias 6

Output

stream

Output buffer

Envelope

detector

Figure 3: The schematic of the receiver.

Class E

PA filter

Antenna

TDX

Class E Power amplifierPA driver

Figure 4: The schematic of the transmitter.

The downconversion to the IF uses a four-quadrant

multiplier Gilbert cell as a mixer. This mixer

performs well with local signals (LO) at 2.5 GHz

and a square shape. The main advantage is the

possibility to use ring-oscillators, rather than tuned

LC, thus, a big on-chip area saving is possible to be

achieved, without a severe degradation in the

phase-noise. Moreover, this don't poses special

cares, because the down-conversion from IF to the

base-base, is made by way of an envelope detection,

whose input signal comes from the output of a low-

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282

pass filter detector. The IF frequency is produced

from the 2.4 GHz RF and from the 2.5 GHz local

frequency. A minimum IF level at the envelope

detector defines the receiver’s sensitivity. This

envelope detector is of active type, e.g., it provides

gain, compared with the conventional topology

using a simple clamping circuit. Thus, the sensitivity

of the receiver is bigger, compared when a passive

detector is used.

2.2 Transmitter

Figure 4 shows the schematic of the transmitter. The

upconversion to the pass-band at 2.4 GHz, is made

by a cascade circuit, comprising transistors M

and

. This circuit combines the transmitted

bitstream (TXD) with local carrier generated in the

frequency synthesizer described further. This circuit

produces an AKS digital signal compatible with the

usage of a switched power amplifier of class E

(Sokal et al, 1975). The external filter that follows

the power amplifier is a typical class E network, and

removes the spectral components around the

2.4-GHz carrier frequency.

2.3 Frequency Synthesiser

The local generation of the frequency synthesiser is

a phase-locked loop (PLL) with integer division in

the feedback path. The Figure 5 shows the block

diagram of the PLL, which has a reference generator

circuit with a crystal based oscillator at 20 MHz,

followed by a phase-frequency difference

circuit (PFD) without dead zone, a current steering

charge pump (CP) and a third order passive filter.

The passive section output is connected to the VCO,

which generates the desired frequencies of 2.4 GHz

or 2.5 GHz. These frequencies must be divided by

120 or by 125 and connected to the PFD again,

closing the loop. For the TSPC logic, it is required a

rail-to-rail input to work properly. At these

frequencies, the power consumptions are lowest,

when compared with the SCL logic

(Pellerano et al, 2004). In real PFDs there is an

offset around the zero phase difference, and a gain

inversion region takes place for phase differences

higher than 2

-Δ rad. In this gain inversion region,

the PFD outputs the wrong control signals increasing

the phase and frequency differences between the

inputs, and the lock time takes a sudden turn for the

worse (Lee et al, 2003). The PFD has a linear gain in

the range [-

, +

] and a constant gain in the remain

interval. This constant gains increases the bandwidth

of the PLL, making it faster to lock, compared with

those containing other structures (Kim et al, 2005).

20 MHz XTAL

reference

generator

PFD

VCO

ref

div

Down

C1 C2

R3 V

control

High-frequency buffers

out

Static logic

30/32

TSPC

Down

div

ref

PFD

Down

CP output

Control

Charge-pump

Control

Bias

VCO

Bias

Tuning

VCO circuit

Control

Figure 5: a) The PLL structure, b) the schematic of the

PFD-CP, and c) the schematic of the VCO.

The charge pump (CP) is of current steering type

(Chih-Ming et al, 2002). This circuit avoids the

problems in conventional CPs. In spite of being

switched, the currents are routed from the load to an

alternative path, and from that path to the load. To

finish, the voltage controlled oscillator (VCO) is of

ring type, in order to save on-chip area and because

the phase and frequency errors are not critical in the

used modulation.

3 EXPERIMENTAL RESULTS

For the receiver, simulations shown a sensivity of

-64 dBm and a consumption of 3.5 mW from a 1.5 V

A RF TRANSCEIVER FOR WIRELESS MONITORING SYSTEMS OF THE VERTEBRAL COLUMN BEHAVIOUR

283

supply. The RF transmitter delivers an output power

of +11 dBm (15 mW) with a power consumption of

45 mW. The receiver has a total power consumption

of 3.5 mW for the receiver (1.5 mW for the LNA,

0.6 mW for the down-conversion mixer and 1.5 mW

to the post-amplifier and envelope detector). The

transmitter has the power consumptions in the

following blocks: 2.7 mW in the driver and

41.5 mW in the power amplifier.

2.1 2.2 2.3 2.4 2.5 2.6 2.72.0 2.8

Frequency [GHz]

fs(V

antenna

) [V]

0.5

1.0

0.0

1.5

MHz

= 2.4 GHz

fs(V

antenna

) = 1.229 e

+j59.9º

[V]

Figure 6: Amplitude spectrum of the amplified signal at

the antenna terminals.

The Figure 6 shows the amplitude spectrum of the

amplified signal at the terminals of an antenna, with

an input impedance of 50 Ω.

4 APPLICATION

The application for microsystems using the

transceiver proposed in this paper, is to make

wireless acquisition modules, in order to allow the

monitoring of heavy loads influence on vertebral

column’s behaviour. Each module makes the

electromyography (EMG), to measure the electric

potentials on the iliocostalis and longissimus

thoracis muscles, and use a dual-axis accelerometer

to get the movements of the body, in order to obtain

the complete behaviour of the vertebral column. The

acquired information is to be transmitted with the

maximum rate of 250 kbps, however, the

simulations shown, that the baud-rate can be

extended without jitter problems, for frequencies up

to 10 Mbps. An analog channel with differential

input connected to the electrodes, is required to

measure the EMG signal, while the remained

channel is to measurements of the patient’s

movements.

The analysis of the EMG signal must be made in the

amplitude domain, thus, before proceeding to the

ADC conversion, it is required a peak detection of

the amplified EMG signal, followed by an

integration (Robertson et al, 2004). This mandatory

process eliminates the fluctuations that characterize

the EMG signals.

The measurements of the motion and the

positioning of the patient’s body is made with the

use of a commercial dual-axis accelerometer of

MEMS type. This chip connects to a commercial

microController by way of an integrated Serial Port

Interface (SPI).

The Figure 7 shows the block diagram of a first

possible prototype, which contain the sensor

interface read-out, the electronics for data

acquisition (amplifications and analog-to-digital

conversion) a micro-controller, and the proposed RF

transceiver. A coin-sized 1.5 V battery will provide

the supply and a commercial DC/DC step-up

converter makes possible to supply the remaining

components of the prototype with different voltage

levels. As it can be seen in this Figure, the anti-static

(ESD) protections are provided by way of power-rail

connected diodes (Ker et al, 2005).

From EMG

electrode

ADC

transceiver

microControler

From

antenna

Interface

(ESD protections)

1.5 V

>1.5 V

DC/DC

step-up

2-axis

acelerometer

SPI

interface

Out-supply

Figure 7: Block diagram of the microsystems for use in

wireless modules.

5 CONCLUSIONS

This paper presented a low-power/low-voltage

radio-frequency (RF) transceiver at 2.4 GHz, for

working with a single 1.5 V coin-sized battery.

Simulations shown for this transceiver a

consumption of 3.5 mW in the receive mode and

15 mW of transmitted power, with a power

consumption of 45 mW in the transmitting mode.

These characteristics fulfil the requirements for

short-range communications.

The target application for this transceiver is in

wireless acquisition modules for monitoring of

heavy loads influence on vertebral column’s

behaviour, in order to understand the influence of

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284

heavy loads as a risk factors in the vertebral column,

such as the scoliosis and lordosis. These factors

normally associated to appears in children when they

carry the heavy backpacks on the backs. Thus, it’s of

extremely importance to characterise the influence

of heavy loads (backpack weight) in the vertebral

column behaviour. This solution fits the medical

doctors requirements for an easy placement and

removal of wireless modules. The main advantage of

this solution, is the maintenance of the mobility and

lifestyle of patients during the diagnosis.

The possibility to control the receiver and the

transmitter subsystems, allowing them to be

switched on and off is another advantage of this RF

transceiver. This is a specially important topic of

design, in applications where power efficient

algorithms are mandatory, e.g., in wireless sensors

networks (Enz et al, 2004).

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