A LOW-POWER WIRELESS TRANSCEIVER FOR DEEPLY IMPLANTED BIOMEDICAL DEVICES by STEVE MAJERUS Submitted in partial fulfillment of the requirements for the degree of Master of Science Thesis Advisor: Dr. Steven L. Garverick Department of Electrical Engineering and Computer Science CASE WESTERN RESERVE UNIVERSITY August, 2008 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis/dissertation of _____________________________________________________ candidate for the ______________________degree *. (signed)_______________________________________________ (chair of the committee) ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ (date) _______________________ *We also certify that written approval has been obtained for any proprietary material contained therein. Table of Contents 1 INTRODUCTION..................................................................................1 1.1 Motivation...............................................................................................1 1.2 Background.............................................................................................2 1.2.1 Existing Transceivers..........................................................................3 1.2.1.1 An RF-powered neural recording device: the Utah Array........3 1.2.1.2 Battery Powered Bion……………………………………..….5 1.2.1.3 A wide bandwidth, battery powered recorder: the SmartPill....7 2 BAND SELECTION AND DESIGN CONSIDERATIONS.................9 2.1 Band Analysis............................................................................................9 2.1.1 FCC Regulations................................................................................9 2.1.2 Power Considerations on Band Selection.........................................11 2.1.2.1 Receiver Power Savings..........................................................13 2.1.2.2 Transmitter Power Savings…………………………………..18 2.2 Band Selection..........................................................................................19 2.3 Design Goals.............................................................................................19 3 ARCHITECTURE & DESIGN.............................................................22 3.1 Architecture..............................................................................................22 3.1.1 OOK Receiver……………………………………………………...23 3.1.1.1 Differential Amplifier………………………………………..25 3.1.1.2 Schmitt Trigger………………………………………………28 3.1.1.3 Activity Detector……………………………………………..32 3.1.2 Manchester Decoder……………………………………………….34 3.1.2.1 Method for Decoding………………………………….……..35 3.1.2.2 All-Digital Manchester Decoder……………………….…….37 3.1.3 FSK Transmitter……………………………………………………40 3.1.3.1 LC Tank Modeling…………………………………………...41 3.1.3.2 LC Oscillator…………………………………………………43 3.1.3.3 Digital Power Control………………………………………..46 4 TEST RESULTS.....................................................................................48 4.1 Test Setup.................................................................................................48 4.2 OOK Receiver..........................................................................................49 4.2.1 Fully-Differential Amplifier……………………………………….49 4.2.2 Schmitt Trigger…………………………………………………….51 4.2.3 Activity Detector…………………………………………………..54 4.2.4 Entire Receiver…………………………………………………….56 4.3 Manchester Decoder................................................................................58 4.4 FSK Transmitter......................................................................................60 5 CONCLUSIONS AND FUTURE WORK............................................65 6 REFERENCES.......................................................................................68 i List of Tables Table 1-1 Performance summary of the Utah Transceiver, as described in [2]....................................................................................................................5 Table 1-2 Performance summary of the BPB in [3]................................................6 Table 1-3 Performance summary of the wireless endoscope in [6].........................7 Table 2-1 Summary of the transceiver design specifications................................21 Table 3-1 Transistor sizes and bias currents for the DA........................................27 Table 3-2 Transistor sizes used in the Schmitt Trigger.........................................30 Table 3-3 Transistor sizes for the activity detector................................................34 Table 4-1 Comparison of designed and estimated actual bias currents and output resistance for the differential amplifier.......................................50 Table 4-2 Measured Schmitt trigger thresholds.....................................................53 Table 5-1 Comparison of Measured Results to Design Goals...............................68 ii List of Figures Figure 1-1 Block diagram of a multi-channel, dual-band implantable neural recorder (from [2])...............................................................................4 Figure 1-2 Photograph of a packaged BPB (from [3])............................................6 Figure 1-3 Conceptual drawing of the SmartPill as it traverses the gastro- intestinal tract (from [5])................................................................................7 Figure 2-1 Table of FCC limits for maximum permissible exposure to electrical and magnetic fields at various frequencies (from [7])..................10 Figure 2-2 Plot of the attenuation by distance versus frequency for electric fields propagating in pure water and seawater (after [3]).............................12 Figure 2-3 A two-stage differential amplifier (from [9]).......................................14 Figure 2-4 The small-signal model for the output stage of the amplifier in Figure 1-5, used to determine the frequency response (after [9]).................14 Figure 2-5 The typical frequency response of a bandpass filter, such as a parallel-resonant LC tank (from [10])...........................................................16 Figure 3-1 Simplified block diagram of the low-power transceiver......................23 Figure 3-2 Block Diagram of the OOK Receiver..................................................24 Figure 3-3 High-level circuit diagram of the OOK receiver..................................24 Figure 3-4 Binary approximation of an OOK-modulated signal...........................25 Figure 3-5 Circuit schematic of one of the DAs used in the OOK receiver..........25 Figure 3-6 Half-circuit model of the DA: (a) schematic; (b) small-signal model.........................................................................................................26 Figure 3-7 Transistor-level schematic of the Schmitt trigger used in the OOK receiver.............................................................................................28 Figure 3-8 Detailed simulation showing switching thresholds of ±53 mV...........31 Figure 3-9 The activity detector used to demodulate the OOK signal to baseband.....................................................................................................33 Figure 3-10 Timing diagram showing Manchester-Encoded Data........................35 Figure 3-11 Timing diagram for a delay-based Manchester Decoder, using MED from Figure 3-10....................................................................36 Figure 3-12 Block diagram of a delay-based Manchester Decoder.......................37 Figure 3-13 SPICE simulation of the Manchester Decoder...................................39 Figure 3-14 Block diagram of a simple FSK modulator........................................40 Figure 3-15 Transformation of lossy LC tank with ESR to equivalent parallel RLC circuit....................................................................................42 Figure 3-16 NMOS cross-coupled pair, with equivalent negative resistance value, from [16].........................................................................................43 Figure 3-17 Transmitter schematic with frequency-switching network and off-chip LC tank.........................................................................................44 Figure 3-18 Digitally-controlled bias network for the FSK transmitter................47 Figure 4-1 Die photo of the fabricated transceiver circuits...................................48 Figure 4-2 Measured transfer function of the differential amplifiers....................49 Figure 4-3 Schematic of the differential amplifier................................................51 Figure 4-4 Schematic of the test setup used to measure the Schmitt trigger thresholds...................................................................................................52 iii Figure 4-5 Measurement of Schmitt positive threshold.........................................53 Figure 4-6 Oscilloscope plot showing the activity detector input (top trace) and output (bottom trace)...........................................................................55 Figure 4-7 Activity detector release delay measurement showing activity detector response (bottom trace) to a step input (top trace).......................57 Figure 4-8 OOK receiver output (top trace) when responding to a 50-mVpp input (bottom trace)...................................................................59