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Phenotypic and Functional Characterization of CD8+T Cell Clones Specific for a Mouse Cytomegalovirus Epitope
Duty Cycle Shift Keying Data Transfer Technique for Bio-Implantable Devices S. A. Mirbozorgi, G. Nabovati, and M. Maymandi-Nejad Integrated Systems Lab., Department of Electrical Engineering, Ferdowsi University of Mashhad, Mashhad, I.R.Iran [email protected], [email protected], [email protected] Abstract—A novel technique for transferring data and power to biomedical implantable devices through an inductive link is presented. The data transfer technique is based on changing the duty cycle of the switching pulse of the class E power amplifier. Hence, we have called it Duty Cycle Shift Keying (DCSK). The modulator and demodulator of the proposed technique are simple and make it suitable for bio-implantable devices. The data rate to carrier frequency ratio can be as high as 100%. The proposed circuit is simulated by ADS simulator using the 0.18 µm CMOS technology. Moreover, in order to verify the effectiveness of the proposed technique, a test setup is implemented using off-the-shelf components.
I.
INTRODUCTION
Power transfer to an implantable device through an inductive link is a well known technique that has been used for many years [1]. Besides power, in many applications there is a need to transfer data to the implantable device wirelessly. It is possible to use a single inductive link to transmit data as well as power to the device [2]. Different modulation techniques have been used to transmit data to a bio-implantable device through the inductive link. In the case of biomedical implants low power consumption is one of the main design issues. Moreover, the maximum carrier frequency in such applications is limited due to the absorption of the RF signal by the tissue. If a high data rate is required the maximum carrier frequency becomes a limiting factor. In such applications a modulation technique that can provide a higher data rate to carrier frequency ratio is more desirable. Also, since the demodulator of such a system is typically placed in the implantable part, the simplicity of the demodulator circuit is an important consideration since it affects the power consumption of the circuit. Various types of digital modulation schemes have been reported for inductively coupled bio-implantable devices. The binary amplitude shift keying (BASK), binary frequency shift keying (BFSK), and binary phase shift keying (BPSK) are more common. The main advantage of the BASK is the simplicity of the circuit. However, it is susceptible to amplitude noise and the data transmission rate is typically limited to about 10% of the carrier frequency [3]. Also in the ASK modulation, the
978-1-4244-9474-3/11/$26.00 ©2011 IEEE
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quality factor (Q) of the LC tanks in the transmitter and receiver circuits should be high to have high powertransmission efficiency. In the BFSK modulation the binary data is sent with a constant amplitude sinusoidal carrier at two different frequencies, i.e. f1 and f0 representing logic “1” and logic “0”, respectively. Compared to BASK, BFSK provides a higher data rate to carrier frequency ratio (can be as high as 67% [4]). Moreover, in the BFSK the wireless link should have a relatively wide pass-band (low Q) so that both f1 and f0 frequencies can pass through the link. This is an advantage for BFSK in the bio-implantable chips since the Q of the on-chip coils is typically low; however, it reduces the power-transmission efficiency of this modulation scheme. In these applications, it is found that BPSK is a more suitable modulation than BASK or BFSK. This is due to the fact that BPSK uses a carrier with fixed amplitude and fixed frequency because it prevents the reduction of received power as a result of the data modulation [5]. This paper presents a novel technique for simultaneous data and power transmission to an implantable device using an inductive link. The parameter that is modulated by the binary data is the duty cycle of the pulse applied to the class E power amplifier of the transmitter. If the circuit is designed properly, this duty cycle variation can be detected in the receiver using an extra coil and a very simple circuit. As the proof of the concept, the proposed technique is implemented using off-theshelf components and the measurement results are provided in this paper. The proposed technique allows constant amplitude and constant frequency RF signal and is able to achieve a data rate to carrier frequency ratio of 100%. Moreover, the amount of power transferred to the implant can be changed without affecting the data transmission. The paper is organized as follows. Section II introduces the DCSK technique. The implemented circuits and simulation results are included in section III. The measurement results are provided in section IV. Finally, conclusions are drawn in section V. II.
THE PROPOSED DATA TRANSMISSION TECHNIQUE
In this section we present a novel data and power transfer technique. The proposed technique is based on the modulation of the duty cycle of a square wave. Hence, we call this
Fig. 1. Schematic block diagram of the proposed power and data transfer technique. Fig. 3. The three coils used for transferring power and data to the implant. TABLE I THE INDUCTANCES AND COUPLING COEFFICIENTS OF COILS
Fig. 2. Impact of duty cycle on the shape of the output voltage of the PA at Q=1.
modulation technique “Duty Cycle Shift Keying (DCSK)”. First the proposed technique will be explained. The design of the coils is critical in this work. The coil design considerations will be explained next. Finally, the demodulation and data extraction is explained. A. The proposed data transfer scheme When an inductive link is used to transfer power and data to an implant, an important issue is that how the amplitude of the induced voltage can be kept constant while the data is being transmitted. In this paper we propose a new technique that can be used to transmit data while the DC voltage on the implant can be controlled simultaneously. In the DCSK technique we use the duty cycle as a mean to transfer data. Moreover, the demodulation of the signal can be easily done in the DCSK technique, as will be explained later. Hence, the demodulation can be done in a very power efficient manner. Fig. 1 shows the schematic block diagram of the proposed scheme. The system has three coils. The external coil (L1) is in charge of transmitting power and data to the implant. There are two receiver coils, one for receiving power (L2) and the other one for receiving data (L3). The coil L2 has a parallel capacitor (C2) to tune the L2-C2 tank to the transmitter frequency so that maximum power transfer can be achieved. However, the coil L3 does not have any parallel capacitor. In fact the quality factor of this coil should be designed to be low. This is necessary if we want to detect the duty cycle variations. In order to reduce the quality factor of coil L3 a resistor (R3) is put in parallel to L3. An essential point that should be mentioned is that if the duty cycle of the square wave signals driving the PA changes, this duty cycle variation may not appear at the output of the PA. In order to be able to
918
Symbol
L1
L2
L3
L1 L2 L3 Coupling Coefficient
4.72 µH 0.108 µH 0.180 µH
0.108 µH 0.35 µH 0.069 µH
0.180 µH 0.069 µH 0.31 µH
k12=0.084
k13= 0.149
k23 = 0.212
observe these variations certain conditions should be fulfilled. When the quality factor (Q) of the circuit at the output of class E PA is low, the duty cycle variations are visible. Then in the implant side, it can be detected across L3. Fig. 2 shows the square wave driving voltage and the output voltage of the class E PA. Clearly, the shape of the output voltage of the PA changes considerably with the duty cycle of the driving voltage. The voltage induced on L3 will have almost the same shape as that of Fig. 2-b. These variations will be detected in the receiver to extract the data. This will be discussed further in the next subsection. B. Coil design The three coils needed in the proposed system should be designed carefully. Fig. 3 depicts the three coils schematically and their positions. All three coils are planar and the two receiver coils are concentric. L1 in this figure represents the transmitter coil and L2 and L3, placed on the same plane, are the receiver coils. L2 (the inner coil) is used for receiving power and L3 (the outer coil) is used for data. An important design issue of the coils for the DCSK technique is that the receiver coils (L2 and L3) should not have a large coupling factor while the coupling of each of them with the transmitter coil (L1) should be as strong as possible. The self- and mutual- inductances of the three coils can be calculated using the formulas provided in [6]. For the coils implemented in this project we used NL1=40 (number of turns of L1), NL2=10 (number of turns of L2), NL3=4 (number of turns of L3), d=1cm, and R=0.1mm (the radius of the wire). Using these values in the equations provided in [6] the self- and mutual- inductances as well as the coupling factors are calculated and shown in Table 1. C.
Data extraction In order to detect the transmitted data the voltage across L3 (Vdata) is used. As discussed above, the duty cycle of Vdata is an indication of the binary data. The data detector circuit should be able to distinguish these duty cycle variations and then
datain, V
Fig. 5. The circuit used to verify the proposed technique.
e)
919
Vdrain, V
0 2.5 1.5 0.5 -0.5 160 120 80 40
-50 -150 400 200 -200 -600 4
f)
g)
h)
i)
Vpower, V
In order to verify the function of the DCSK technique the circuit shown in Fig. 5 is simulated in ADS. The simulation results are explained in this part. In the circuit of Fig. 5, transistor Q1, choke Lch, capacitors C1 and Cp, resistor R1, and the inductor L1 form the class E power amplifier. In the modulator, the input digital data changes the duty cycle of the square wave that drives the gate of Q1. The simulated waveforms of different nodes of the circuit are depicted in Fig. 6. The input data (Fig. 6-a) represents a stream of “0” and “1”. The carrier frequency is 13.56 MHz. The modulated signal which appears at the gate of Q1 is shown in Fig. 6-b. When the input data is “0” the duty cycle is around 30% and when the data is “1” the duty cycle is around 70%. The voltage at the drain of Q1 and output of class E PA signal are shown in Fig. 6-c and 6-d respectively. As can be seen in Fig. 6-d, the voltage across L1 is not a square wave since the resonant circuit attenuates the harmonics. However, the effect of the duty cycle (and hence the input data) can be clearly seen in this waveform. On the implant, the voltage induced across L3 has almost the same shape as that of L1 (Fig. 6-e). However, the voltage across L2 is almost a sinusoidal because the capacitance C2 is placed in parallel to L2 and the tuned circuit eliminates the harmonics that is shown in Fig. 6f. Using C2 in parallel to L2 causes the voltage across L2 to be much larger compare to the case when C2 is missing. Note that a similar capacitor cannot be used across L3 otherwise the duty cycle variations could not be detected. The voltage across L2 is used to provide power for the implant.
Vtran, V
d)
2
0 100 50
Vdata-rec
SIMULATION RESULTS
c)
clkout, V
III.
b)
dataout
convert it to a binary data. In order to do this the circuit shown in Fig. 4 is used. A clock signal is produced from the voltage induced across L2 by a clock regenerator circuit. This clock is then used in a D flip flop (DFF). The waveforms at the output of the data regenerator and the clock signal are shown in Fig. 4-b. The clock signal passes through a delay element before being applied to the DFF to avoid metastable operation of the DFF and is added for reliability of data detection. The proper timing is shown in Fig. 4-b.
Vdata, mV
Fig. 4. Data detector circuit and its signals.
inputpulse, V
a)
5 4
2 0 -2 2.0 1.5 0.5
-0.5 2.0 1.5 0.5 -0.5 2.0 1.5 0.5 -0.5
1.6
1.9
2.2 time, usec
2.5
2.8
Fig. 6. Simulated waveforms of different nodes of the circuit in figure 8.
A clock signal is also extracted from this voltage using a clock regenerator circuit [3]. Generally speaking, the main function of this circuit is to convert a sinusoidal or a pseudosinusoidal signal to a square wave signal as well as data regenerator. The clock signal is used for demodulating the data signal and extracting the transmitted data. The outputs of the data and clock regenerators are shown in Fig. 6-g and 6-h. As can be seen in Fig. 6-g the duty cycle variations is detected.
Fig. 7. The prototype circuit of the proposed DCSK system.
Applying this voltage and the clock signal to a D flip flop extracts the transmitted data (Fig. 6-i). In order to detect the transmitted data two regenerators and a D flip flop are the only circuits needed. The simplicity of the demodulator is a main advantage of the DCSK technique. Comparing Fig. 6-a and 6-i it is clear that the proposed modulation technique has data tare to carrier frequency of 100% and operates accurately. Simulation results show that the demodulator part consumes 2µW in the 0.18 µm CMOS technology. IV.
MEASUREMENT RESULTS
As a proof of concept, the operation of the proposed technique is verified by a test setup using off-the-shelf components. In this section the measurement results are presented. Due to the limitations we have been facing using off-the-shelf components we were forced to limit the carrier frequency to 900 kHz. Although this frequency is much lower than that used in the previous section, it can justify the correct operation of the technique. Using state of the art technology, the carrier frequency can be increased without affecting the basic principles of the proposed simultaneous data and power transmission technique. The transmitter and receiver coils as well as the class E power amplifier used in this test are shown in Fig. 7. The digital input and the drain voltage of the MOSFET are shown in Fig. 8-a and 8-b, respectively. Clearly, the duty cycle of the drain voltage changes with the digital data. The digital data is a stream of consecutive “0” and “1”. This is chosen so in order to show that the DCSK technique can reach a data to carrier frequency ratio of 100%. The voltages across L3 and output data detected of the implemented circuit are illustrated in Fig. 8-c and 8-d, respectively. V.
CONCLUSIONS
A new technique to send data and power simultaneously to an implant through an inductive link is presented. The digital data that is to be sent to the implant modulates the duty cycle of the square wave driving the gate of the MOSFET in the class E power amplifier. The design issues are addressed in this paper. The DCSK technique can reach a data to carrier frequency ratio of 100%, the demodulation is simple and a low power circuit can be employed to detect the data. This is a very desirable feature since low power consumption and simplicity of the circuit is important in bio-implantable devices. Moreover, this technique allows the control of the transmitted power while the data is being sent. ADS simulation results are presented to illustrate the effectiveness
920
Fig. 8. a-the digital input data, b-the drain voltage of the MOSFET, c-the voltage across L3, and d-the detected data.
of the proposed technique. As a proof of concept, the proposed technique is implemented using off-the-shelf components and the measurement results are provided. The results of the measurement verify the correct operation of the proposed DCSK technique. Since the Q of the receiver coils does not have to be high, the DCSK technique is very suitable for CMOS integrated circuits. Moreover, due to the simplicity of the demodulator the power consumption of the receiver can be very low which makes this technique a desirable one for bio-implantable devices.
REFERENCES [1] [2] [3] [4]
[5] [6]
A. Harb, Y. Hu, and M. Sawan, “Low-power CMOS interface for recording and processing very low amplitude signals,” J. Analog Integr. Circuits Signal Process., vol. 39, pp. 39–54, 2004. Y. Hu and M. Sawan, “A fully integrated low-power BPSK demodulator for implantable medical devices,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 52, no. 12, pp. 2552–2562, Dec. 2005. M. Ghovanloo and K. Najafi, “A Wideband Frequency-Shift Keying Wireless Link for Inductively Powered Biomedical Implants,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 51, no. 12, Dec. 2004. M. Ghovanloo and K. Najafi, “A Tri-State FSK Demodulator for Asynchronous Timing of High-Rate Stimulation Pulses in Wireless Implantable Microstimulators,” Proceedings, 2nd Intl. IEEEIEMBS Conf on Neural Engineering, pp. 116-119, Mar. 2005. Z. Luo and S. Sonkusale, “A Novel BPSK Demodulator for Biological Implants,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 55, no. 6, July 2008. C. M. Zierhofer and E. S. Hochmair, “Geometric Approach for Coupling Enhancement of Magnetically Coupled Coils,” IEEE Trans. Biomed. Eng., vol. 43, no. 7, July 1996.
I.
INTRODUCTION
Power transfer to an implantable device through an inductive link is a well known technique that has been used for many years [1]. Besides power, in many applications there is a need to transfer data to the implantable device wirelessly. It is possible to use a single inductive link to transmit data as well as power to the device [2]. Different modulation techniques have been used to transmit data to a bio-implantable device through the inductive link. In the case of biomedical implants low power consumption is one of the main design issues. Moreover, the maximum carrier frequency in such applications is limited due to the absorption of the RF signal by the tissue. If a high data rate is required the maximum carrier frequency becomes a limiting factor. In such applications a modulation technique that can provide a higher data rate to carrier frequency ratio is more desirable. Also, since the demodulator of such a system is typically placed in the implantable part, the simplicity of the demodulator circuit is an important consideration since it affects the power consumption of the circuit. Various types of digital modulation schemes have been reported for inductively coupled bio-implantable devices. The binary amplitude shift keying (BASK), binary frequency shift keying (BFSK), and binary phase shift keying (BPSK) are more common. The main advantage of the BASK is the simplicity of the circuit. However, it is susceptible to amplitude noise and the data transmission rate is typically limited to about 10% of the carrier frequency [3]. Also in the ASK modulation, the
978-1-4244-9474-3/11/$26.00 ©2011 IEEE
917
quality factor (Q) of the LC tanks in the transmitter and receiver circuits should be high to have high powertransmission efficiency. In the BFSK modulation the binary data is sent with a constant amplitude sinusoidal carrier at two different frequencies, i.e. f1 and f0 representing logic “1” and logic “0”, respectively. Compared to BASK, BFSK provides a higher data rate to carrier frequency ratio (can be as high as 67% [4]). Moreover, in the BFSK the wireless link should have a relatively wide pass-band (low Q) so that both f1 and f0 frequencies can pass through the link. This is an advantage for BFSK in the bio-implantable chips since the Q of the on-chip coils is typically low; however, it reduces the power-transmission efficiency of this modulation scheme. In these applications, it is found that BPSK is a more suitable modulation than BASK or BFSK. This is due to the fact that BPSK uses a carrier with fixed amplitude and fixed frequency because it prevents the reduction of received power as a result of the data modulation [5]. This paper presents a novel technique for simultaneous data and power transmission to an implantable device using an inductive link. The parameter that is modulated by the binary data is the duty cycle of the pulse applied to the class E power amplifier of the transmitter. If the circuit is designed properly, this duty cycle variation can be detected in the receiver using an extra coil and a very simple circuit. As the proof of the concept, the proposed technique is implemented using off-theshelf components and the measurement results are provided in this paper. The proposed technique allows constant amplitude and constant frequency RF signal and is able to achieve a data rate to carrier frequency ratio of 100%. Moreover, the amount of power transferred to the implant can be changed without affecting the data transmission. The paper is organized as follows. Section II introduces the DCSK technique. The implemented circuits and simulation results are included in section III. The measurement results are provided in section IV. Finally, conclusions are drawn in section V. II.
THE PROPOSED DATA TRANSMISSION TECHNIQUE
In this section we present a novel data and power transfer technique. The proposed technique is based on the modulation of the duty cycle of a square wave. Hence, we call this
Fig. 1. Schematic block diagram of the proposed power and data transfer technique. Fig. 3. The three coils used for transferring power and data to the implant. TABLE I THE INDUCTANCES AND COUPLING COEFFICIENTS OF COILS
Fig. 2. Impact of duty cycle on the shape of the output voltage of the PA at Q=1.
modulation technique “Duty Cycle Shift Keying (DCSK)”. First the proposed technique will be explained. The design of the coils is critical in this work. The coil design considerations will be explained next. Finally, the demodulation and data extraction is explained. A. The proposed data transfer scheme When an inductive link is used to transfer power and data to an implant, an important issue is that how the amplitude of the induced voltage can be kept constant while the data is being transmitted. In this paper we propose a new technique that can be used to transmit data while the DC voltage on the implant can be controlled simultaneously. In the DCSK technique we use the duty cycle as a mean to transfer data. Moreover, the demodulation of the signal can be easily done in the DCSK technique, as will be explained later. Hence, the demodulation can be done in a very power efficient manner. Fig. 1 shows the schematic block diagram of the proposed scheme. The system has three coils. The external coil (L1) is in charge of transmitting power and data to the implant. There are two receiver coils, one for receiving power (L2) and the other one for receiving data (L3). The coil L2 has a parallel capacitor (C2) to tune the L2-C2 tank to the transmitter frequency so that maximum power transfer can be achieved. However, the coil L3 does not have any parallel capacitor. In fact the quality factor of this coil should be designed to be low. This is necessary if we want to detect the duty cycle variations. In order to reduce the quality factor of coil L3 a resistor (R3) is put in parallel to L3. An essential point that should be mentioned is that if the duty cycle of the square wave signals driving the PA changes, this duty cycle variation may not appear at the output of the PA. In order to be able to
918
Symbol
L1
L2
L3
L1 L2 L3 Coupling Coefficient
4.72 µH 0.108 µH 0.180 µH
0.108 µH 0.35 µH 0.069 µH
0.180 µH 0.069 µH 0.31 µH
k12=0.084
k13= 0.149
k23 = 0.212
observe these variations certain conditions should be fulfilled. When the quality factor (Q) of the circuit at the output of class E PA is low, the duty cycle variations are visible. Then in the implant side, it can be detected across L3. Fig. 2 shows the square wave driving voltage and the output voltage of the class E PA. Clearly, the shape of the output voltage of the PA changes considerably with the duty cycle of the driving voltage. The voltage induced on L3 will have almost the same shape as that of Fig. 2-b. These variations will be detected in the receiver to extract the data. This will be discussed further in the next subsection. B. Coil design The three coils needed in the proposed system should be designed carefully. Fig. 3 depicts the three coils schematically and their positions. All three coils are planar and the two receiver coils are concentric. L1 in this figure represents the transmitter coil and L2 and L3, placed on the same plane, are the receiver coils. L2 (the inner coil) is used for receiving power and L3 (the outer coil) is used for data. An important design issue of the coils for the DCSK technique is that the receiver coils (L2 and L3) should not have a large coupling factor while the coupling of each of them with the transmitter coil (L1) should be as strong as possible. The self- and mutual- inductances of the three coils can be calculated using the formulas provided in [6]. For the coils implemented in this project we used NL1=40 (number of turns of L1), NL2=10 (number of turns of L2), NL3=4 (number of turns of L3), d=1cm, and R=0.1mm (the radius of the wire). Using these values in the equations provided in [6] the self- and mutual- inductances as well as the coupling factors are calculated and shown in Table 1. C.
Data extraction In order to detect the transmitted data the voltage across L3 (Vdata) is used. As discussed above, the duty cycle of Vdata is an indication of the binary data. The data detector circuit should be able to distinguish these duty cycle variations and then
datain, V
Fig. 5. The circuit used to verify the proposed technique.
e)
919
Vdrain, V
0 2.5 1.5 0.5 -0.5 160 120 80 40
-50 -150 400 200 -200 -600 4
f)
g)
h)
i)
Vpower, V
In order to verify the function of the DCSK technique the circuit shown in Fig. 5 is simulated in ADS. The simulation results are explained in this part. In the circuit of Fig. 5, transistor Q1, choke Lch, capacitors C1 and Cp, resistor R1, and the inductor L1 form the class E power amplifier. In the modulator, the input digital data changes the duty cycle of the square wave that drives the gate of Q1. The simulated waveforms of different nodes of the circuit are depicted in Fig. 6. The input data (Fig. 6-a) represents a stream of “0” and “1”. The carrier frequency is 13.56 MHz. The modulated signal which appears at the gate of Q1 is shown in Fig. 6-b. When the input data is “0” the duty cycle is around 30% and when the data is “1” the duty cycle is around 70%. The voltage at the drain of Q1 and output of class E PA signal are shown in Fig. 6-c and 6-d respectively. As can be seen in Fig. 6-d, the voltage across L1 is not a square wave since the resonant circuit attenuates the harmonics. However, the effect of the duty cycle (and hence the input data) can be clearly seen in this waveform. On the implant, the voltage induced across L3 has almost the same shape as that of L1 (Fig. 6-e). However, the voltage across L2 is almost a sinusoidal because the capacitance C2 is placed in parallel to L2 and the tuned circuit eliminates the harmonics that is shown in Fig. 6f. Using C2 in parallel to L2 causes the voltage across L2 to be much larger compare to the case when C2 is missing. Note that a similar capacitor cannot be used across L3 otherwise the duty cycle variations could not be detected. The voltage across L2 is used to provide power for the implant.
Vtran, V
d)
2
0 100 50
Vdata-rec
SIMULATION RESULTS
c)
clkout, V
III.
b)
dataout
convert it to a binary data. In order to do this the circuit shown in Fig. 4 is used. A clock signal is produced from the voltage induced across L2 by a clock regenerator circuit. This clock is then used in a D flip flop (DFF). The waveforms at the output of the data regenerator and the clock signal are shown in Fig. 4-b. The clock signal passes through a delay element before being applied to the DFF to avoid metastable operation of the DFF and is added for reliability of data detection. The proper timing is shown in Fig. 4-b.
Vdata, mV
Fig. 4. Data detector circuit and its signals.
inputpulse, V
a)
5 4
2 0 -2 2.0 1.5 0.5
-0.5 2.0 1.5 0.5 -0.5 2.0 1.5 0.5 -0.5
1.6
1.9
2.2 time, usec
2.5
2.8
Fig. 6. Simulated waveforms of different nodes of the circuit in figure 8.
A clock signal is also extracted from this voltage using a clock regenerator circuit [3]. Generally speaking, the main function of this circuit is to convert a sinusoidal or a pseudosinusoidal signal to a square wave signal as well as data regenerator. The clock signal is used for demodulating the data signal and extracting the transmitted data. The outputs of the data and clock regenerators are shown in Fig. 6-g and 6-h. As can be seen in Fig. 6-g the duty cycle variations is detected.
Fig. 7. The prototype circuit of the proposed DCSK system.
Applying this voltage and the clock signal to a D flip flop extracts the transmitted data (Fig. 6-i). In order to detect the transmitted data two regenerators and a D flip flop are the only circuits needed. The simplicity of the demodulator is a main advantage of the DCSK technique. Comparing Fig. 6-a and 6-i it is clear that the proposed modulation technique has data tare to carrier frequency of 100% and operates accurately. Simulation results show that the demodulator part consumes 2µW in the 0.18 µm CMOS technology. IV.
MEASUREMENT RESULTS
As a proof of concept, the operation of the proposed technique is verified by a test setup using off-the-shelf components. In this section the measurement results are presented. Due to the limitations we have been facing using off-the-shelf components we were forced to limit the carrier frequency to 900 kHz. Although this frequency is much lower than that used in the previous section, it can justify the correct operation of the technique. Using state of the art technology, the carrier frequency can be increased without affecting the basic principles of the proposed simultaneous data and power transmission technique. The transmitter and receiver coils as well as the class E power amplifier used in this test are shown in Fig. 7. The digital input and the drain voltage of the MOSFET are shown in Fig. 8-a and 8-b, respectively. Clearly, the duty cycle of the drain voltage changes with the digital data. The digital data is a stream of consecutive “0” and “1”. This is chosen so in order to show that the DCSK technique can reach a data to carrier frequency ratio of 100%. The voltages across L3 and output data detected of the implemented circuit are illustrated in Fig. 8-c and 8-d, respectively. V.
CONCLUSIONS
A new technique to send data and power simultaneously to an implant through an inductive link is presented. The digital data that is to be sent to the implant modulates the duty cycle of the square wave driving the gate of the MOSFET in the class E power amplifier. The design issues are addressed in this paper. The DCSK technique can reach a data to carrier frequency ratio of 100%, the demodulation is simple and a low power circuit can be employed to detect the data. This is a very desirable feature since low power consumption and simplicity of the circuit is important in bio-implantable devices. Moreover, this technique allows the control of the transmitted power while the data is being sent. ADS simulation results are presented to illustrate the effectiveness
920
Fig. 8. a-the digital input data, b-the drain voltage of the MOSFET, c-the voltage across L3, and d-the detected data.
of the proposed technique. As a proof of concept, the proposed technique is implemented using off-the-shelf components and the measurement results are provided. The results of the measurement verify the correct operation of the proposed DCSK technique. Since the Q of the receiver coils does not have to be high, the DCSK technique is very suitable for CMOS integrated circuits. Moreover, due to the simplicity of the demodulator the power consumption of the receiver can be very low which makes this technique a desirable one for bio-implantable devices.
REFERENCES [1] [2] [3] [4]
[5] [6]
A. Harb, Y. Hu, and M. Sawan, “Low-power CMOS interface for recording and processing very low amplitude signals,” J. Analog Integr. Circuits Signal Process., vol. 39, pp. 39–54, 2004. Y. Hu and M. Sawan, “A fully integrated low-power BPSK demodulator for implantable medical devices,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 52, no. 12, pp. 2552–2562, Dec. 2005. M. Ghovanloo and K. Najafi, “A Wideband Frequency-Shift Keying Wireless Link for Inductively Powered Biomedical Implants,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 51, no. 12, Dec. 2004. M. Ghovanloo and K. Najafi, “A Tri-State FSK Demodulator for Asynchronous Timing of High-Rate Stimulation Pulses in Wireless Implantable Microstimulators,” Proceedings, 2nd Intl. IEEEIEMBS Conf on Neural Engineering, pp. 116-119, Mar. 2005. Z. Luo and S. Sonkusale, “A Novel BPSK Demodulator for Biological Implants,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 55, no. 6, July 2008. C. M. Zierhofer and E. S. Hochmair, “Geometric Approach for Coupling Enhancement of Magnetically Coupled Coils,” IEEE Trans. Biomed. Eng., vol. 43, no. 7, July 1996.