Low Power Modulator for the Application of

Capsule Endoscope#1 #2 *3

Lioe De Xing , Suhaidi Shafie , Harikrishnan Ramiah

#Department of Electrical and Electronic Engineering, Universiti Putra Malaysia

43300 Serdang, Malaysia1

dexing@ieee.org2

suhaidi@eng.upm.edu.my*Department Electrical Engineering, University of Malaya,

50603 Kuala Lumpur, Malaysia3

hrkhari@um.edu.my

Abstract— This paper presents the radio frequency (RF)

modulator for high data rate medical imaging for capsule

endoscope. The RF modulator consists of a mixer and a ring

oscillator. The ring oscillator provides carrier frequency of

433MHz and mix with the mixer to produce modulated signal.

The modulator is designed using Silterra 0.13µm CMOS

process. For supply voltage of 1.2 V, data rate of 3.5Mbps the

mixer has current consumption of 594µA , IIP3 of 2dBm and at

output power of -14.6 dBm. The ring oscillator consumes 740µA

with phase noise of -81 dBc/Hz @ 160kHz offset.

Keywords— mixer, modulator, low power, capsule endoscope

I. INTRODUCTION

The advancement of technology has lead to modern and effective application of medical instrument, one of them being capsule endoscope. The important characteristic of this wireless medical imaging application is low power consumption and high data transmission rate. In 1999, the Federal Communications Commission (FCC) allocated a frequency band of 402-405 MHz for Medical Implant Communications Service (MICS) [1]. This band is widely used for implantable medical devices. The requirement of the size, power, antenna performance and receiver design have to be fully satisfied. However, the requirement of MICS of only allowing transmission up to 500kbps does not support high frame rate and high resolution image. Low frame rate will cause certain part of the interest area not being captured, while physicians may not be able to confirm a disease with the low resolution image. There is no standard been established yet for this particular application of capsule endoscope of high data rate wireless medical imaging. For this battery operated capsule endoscope, low power consumption is extremely important. The battery need to last for at least 8 hours to prevent the battery to die out before full operation of capsule endoscope is done. Several researches have been done on the communication of capsule endoscope [2]-[6]. The issues in [2]-[4] is the insufficient data rate or frame rate to transmit highly reliable image back to the receiver. The circuit size in [5] is very large that it will limit additional circuit to be added to the capsule for increasing functionality. Power consumption in [6] in high even though it claim to last for eight hours but it is not energy efficient.

This paper presents the idea and design of a RFmodulator for the application of capsule endoscope. The modulator is to be realized with a mixer and a ring oscillator. The design considerations are presented, followed by circuit design, simulation results and conclusion.

II. DESIGN CONSIDERATION

The MICS band of 402-405 MHz (New Zealand allocated 402-406 MHz) is created by FCC for implantable medical devices. But the stringent regulation of MICS band especially in the aspect of bandwidth restrict MICS to be used for high data rate applications. Besides MICS, there are several other frequency bands which can be considered for this application such as Industrial, Scientific, Medical (ISM), Wireless Medical Telemetry Services (WMTS) and general telemetry. Each of the frequency has their pros and cons in terms of propagation characteristics, bandwidth, crowded spectrum etc [7].

Figure 1 shows the relationship between radiation loss,

attenuation loss, antenna loss and total loss with frequency

[8]. From the graph the frequency between 400MHz and

900MHz result in lowest total loss. MICS band and WMTS

(608-614 MHz) falls into this range, however MICS has only

limited bandwidth so it is not suitable for high data rate

transmission application. WMTS is only applicable in the

United States and it has limited spectrum and heavily used.

One of the ISM band falls into 433.05- 434.79 MHz, which

fits in the lower loss frequency range according to [8]. No

transmission bandwidth issue, no restriction of modulation

method and carrier frequency for this ISM band. Therefore

the ISM band is chosen for this paper. Considering a good image resolution, QVGA which has

320x240 effective pixels is adopted. Frame rate of 5 frames per second (fps) would be decent for not to miss certain interested part in the evaluation. For a 8 bit colour image, the bit rate would be 320x240x8x5 which is equal to 3Mbps for raw data image transmission, equivalent to 15-20 fps if appropriate image compression is applied. The modulator would be designed to have data rate of 3.5Mbps

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Fig. 1: Loss against frequency for in body wireless communications [8]

III. CIRCUIT DESIGN

In this section, the design of mixer is described while the VCO design is discussed. Figure 2 shows the block diagram of the modulator. Basically the modulator consists of a mixer and a local oscillator (LO) with the input of digital data from the capsule camera. The digital data is of non-return-to-zero (NRZ) type.

Fig. 2: Block diagram of modulator

A. Mixer

As shown in Figure 3, the mixer consists of switch pairs (M1 M4), transconductance stages (M5 M8) and output load (M9, M10 and RL) which is a common mode feedback (CMFB) structure. The switch pairs which are the inverters, switched between two signal paths. The transconductance stage converts the voltage signal to the current signal. In the end the output load convert the current signal back to the voltage signal. CMFB structure is important in industrial devices to ensure the devices are more stable in PVT variation even though CMFB structure adds some amount of noise into the circuit. CMFB also increases the conversion gain where the two PMOS load transistor absorb the common mode current, and at the same time provide larger headroom. The tail M11 is to enhance the stability of the mixer.

BB+

Vdd

M5 M6 M7 M8

M9

Vdd

M1 M2

M3 M4

M10

RL RL

M11

LO+

LO-

BB+

BB-

Vb

Fig. 3: Schematic of the proposed mixer

During the negative half cycle of the LO, the switch pair or the inverter on the right (M3 & M4) has low voltage level output. The transconductance pair of M7 and M8 is therefore switched on, while the other pair is off. The situation is vice versa for positive half of the LO cycle.

The switches of this mixer topology generate common mode output noise current. This is due to the two transconductor pair are working at the same state. The noise will then cancels in the differential output. This characteristic has advantage over the conventional Gilbert cell mixer where each of the switch transistors contributes to the significant output noise [10].

B. VCO

The Voltage Controlled Oscillator (VCO) used in this paper is a 3 stages ring oscillator. Ring oscillator does not

– – need inductor thus largely reduce the chip area where the size of capsule does matter. Capsule size reduction helps to ease the uncomfortable experience of patients swallowing the capsule endoscope while at the same time provide spaces for other functions to be added into the capsule. The current consumption of ring oscillator is also lower than that of LC type VCO which contribute to low power consumption of the system.

In general, communication circuit application rarely use

ring oscillator as the VCO compared to LC tank oscillator

due to their poor phase noise. However, the phase noise

requirement for this application is not too strict because of

low regulated power and short communication range [9].

Linearity of ring oscillator is also better than LC VCO for the

trade off of poorer phase noise and supply noise sensitivity.

The ring oscillator has to have supply noise sensitivity as low

as possible. Figure 4 shows the schematic of the delay stage

of ring oscillator.

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Vdd

M5

M7

M6

M1M2

M3 M4

Vdd

Vb

Vin+ Vin-

Vout- Vout+Vctrl

Fig. 4: Schematic of ring oscillator delay stage

IV. SIMULATION RESULTS

The circuits are designed using Silterra 0.13-µm CMOS process. The 433MHz LO is supplied for simulation of mixer performance. The mixer alone has current consumption of 594µA with a voltage supply of 1.2V. The output power from the mixer is -14.6 dBm which gives a power gain of 5.4 dBm.The 1dB compression point is – 5.4 dBm.

Figure 5, 6 and 7 illustrates the simulated input

referred third-order intercept point (IIP3) to be -0.811 dBm,

the dynamic range of the mixer by 1dB compression plot, and

single side band noise figure (NF) respectively. The

simulation of result shows the SSB NF is about 23dB.

Fig. 5: Simulated IIP3

Fig. 6: 1dB Compression Point

Fig. 7: SSB Noise Figure

The three stages ring oscillator achieves low current consumption of 740µA from 1.2V of supply voltage. The phase noise at 160kHz offset is -81.92 dBc/Hz, as shown in Figure 8.

Fig. 8: Phase noise simulation

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V. CONCLUSIONS

A 433MHz RF modulator for the application of capsule

endoscope was proposed. The mixer of the proposed

modulator was simulated and achieved low power

consumption of 712µW for bit rate of 3.5Mbps and output

power of -14.6dBm. Ring oscillator keeps the power

consumption of the modulator low with 888µW of power

consumption. The low power and high frame rate

characteristic of the proposed modulator is suitable for the use

of capsule endoscope.

REFERENCES

[1] FCC Rules and Regulations, “MICS Band Plan”, Part 95, Jan. 2003.

[2] Bradley, P.D., “An Ultra Low Power, High Performance Medical

Implant Communication System (MICS) Transceiver for ImplantableDevices”, Proceeding, IEEE Biomedical Circuits and Systems

Conference, 2006. pp. 158-161

[3] Itoh, S.; Kawahito, S.; Terakawa, S., “A 2.6mW 2fps QVGA CMOS One-chip Wireless Camera with Digital Image Transmission Function

for Capsule Endoscopes”,. Proceedings of the 2006 IEEE International Symposium on Circuits and Systems, ISCAS 2006 (2006), Page(s):4

pp. 3353 – 3356.

[4] B. Chi, J. Yao, S. Han, X. Xie, G. Li, Z. Wang, “Low power transceiver analog front-end circuit for bidirectional high data rate

wireless telemetry in medical endoscopy applications”, IEEE

Transaction On Biomedical Engineering, Vol. 54, No. 7, 2007, pp. 1291-1299.

[5] J. Ryu; M. Kim; J. Lee; BS. Kim; MQ. Lee; S. Nam, “Low Power

OOK Transmitter for Wireless Capsule Endoscope!, IEEE/MTT-SInternational Microwave Symposium, 2007, pp: 855 – 858

[6] S. Diao; Y. Zheng; Y. Gao; CH. Heng; M. Je, “A 7.2mW 15Mbps ASK CMOS Transmitter for Ingestible Capsule Endoscope”, IEEE Asia

Pacific Conference on Circuit and System, 2010, pp. 512 – 515[7] Patel. M., Jianfeng Wang, “Applications, Challenges, and Prospective

in Emerging Body Area Networking Technologies”, IEEE Wireless

Communications, Volume 17, Issue 1, 2010, pp. 80-88.[8] https://mentor.ieee.org/802.15/dcn/08/15-08-0447-02-0006-

considerations-on-high-data-rate-in-bodycommunications-forwban.ppt

9] A. Tekin, M.R. Yuce, W. Liu, “Integrated VCOs for Medical Implant Transceivers”, VLSI Design, Volume 2008, Issue 4, January2008.

[10] H. Darabi and A.A. Abidi, “Noise in current-commutating CMOS

mixers,” IEEE Journal Solid-State Circuits, Vol. 35, Jan 2000, pp. 15

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