Wireless link testing

The transmitter was powered directly from the chip’s supply voltage, which ranges from 1.4V-1.2V. The antenna was implemented as a single loop coil with a diameter of 6mm. To test the functionality of the transmitter and, for the sake of simplicity and since the development of the CC2500 receiver module was out of the scope of this thesis, the receiver was implemented using a GNU software define radio instead of the CC2500 module. The code for the GNU radio was already written by Dr.Stuart Bowyer and hence only a minor modification was required. The packet format was compatible with the CC2500 module and the latter could be coded and used in future work. The GNU software radio was implemented on a computer connected to an antenna. The antenna received the RF signal from the transmitter and the GNU radio decoded that signals. Two codes were written in the GNU radio software. The first one displays the spectrum of the received signal in the frequency domain as shown in Fig. 6.19 and the second code displays the received raw data packet as shown in Fig. 6.20. The transmission frequency was at 2483.32 MHz and the received power was -39dB when the transmitter was placed 15cm away from the reciever. The received raw data packets are shown in Fig. 6.20. A zoomed view on an individual packet to show the individual recived bits is illustrated in Fig. 6.21. Each packet of data

630mV pk-pk

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contains 46×8-bits (46 bytes) of data sampled at 10 KHz. This implies that each two consecutive bytes represent 0.1ms of data and hence each whole received packet represents 4.5ms of data.

Fig. 6.19. Spectrum of the received signal at the receiver

Fig. 6.20. Received raw data packets

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Fig. 6.21. Zoomed view on a received packet

A 500Hz sinusoidal signal with 10mV peak-peak was applied to the system. The transmitted packets were received by the receiver and then uploaded into MATLAB to decode and recover the data in order to test the full functionality of the system. As explained earlier, each packet of data represented 4.5ms of real time data and hence 2 packets were decoded to obtain 9ms of data as shown in Fig. 6.22.

Fig. 6.22. Comparison between the transmitted and recovered 500Hz sinusoidal signal with 10mV peak-peak magnitude

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It can be seen how the transmitted and recovered data were identical. The red spikes that can be noted on the transmitted data are due to the data being sampled and switched to a sampling capacitor. The percentage error between the transmitted and recovered signal is plotted in Fig. 6.23 and it shows a maximum error of less than 0.7%.

Fig. 6.23. Percentage error between the transmitted and recovered signal in Fig. 6.22

To verify the full system functionality for a range of input frequencies, a 50mv peak-peak sinusoidal sweep from 100Hz-1.5KHz was applied to the system. The transmitted packets were received by the receiver and then uploaded into MATLAB to decode and recover the data. Seven packets of data were decoded to obtain 31.5ms of data. This is shown in the blue graph in Fig. 6.24. The solid red graph represents the signal at the output of the AFE, which was digitized, formatted and then transmitted. The broken blue line represents the recovered data. A very good match can be seen between the transmitted and received data, indicating a good functionality of the full system. The percentage error between the transmitted and recovered signal is plotted in Fig. 6.25 and it shows a maximum error of less than 0.5%.

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Fig. 6.24. Comparison between the transmitted and recovered sinusoidal sweep from 100 Hz- 1.5KHz with 50mV peak-peak magnitude

Fig. 6.25. Percentage error between the transmitted and recovered signal in in Fig. 6.24

6.5 Conclusion

This chapter has described the ADC architecture and the implementation of the transmitter, together with the measurements results of the full integrated breathing monitoring system. The final system’s PCB had a diameter of 23mm, and the system and consumed 560µW of power at 1.3V supply voltage including the microphone. Lab measurements showed that the system can

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operate continuously for more than two weeks with a 0.83g P13 zinc air battery. Testing the functionality of the system showed good results and great potential for realising a complete remote diagnostic process.

References

[1] N. Verma and A. Chandrakasan, “An Ultra Low Energy 12-bit Rate-Resolution Scalable

SAR ADC for Wireless Sensor Nodes,” IEEE Journal of Solid-State Circuits, vol. 42, no. 6, pp. 1196-1205, 2007.

[2] D. Ham and A. Hajimiri, “Concepts and methods in optimization of integrated LC VCOs,”

IEEE Journal of Solid-State Circuits, vol. 36, no. 6, pp. 896-909, 2001.

[3] R. S. Elliott, Electromagnetics: History, Theory, and Applications, Wiley-IEEE Press, 1999. [4] B. Razavi, Design of Analog CMOS Integrated Circuits, McGraw-Hill Education, 2001. [5] D. Yates, A. Holmes and A. Burdett, “Optimal Transmission Frequency for Ultralow-Power

Short-Range Radio Links,” IEEE Transactions on Circuits and Systems I: Regular Papers,

vol. 51, no. 7, pp. 1405-1413, 2004.

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