1. ANTENNA Subthreshold Transmitter Implementation for Low Power Sensor Platform Gordon D. Burgett Joseph A. Duperre III Rajesh Garg, Ph.D. Student Dr. Sunil Khatri, Professor Department of Electrical and Computer Engineering Texas A&M University, College Station, TX 77843-3128 Electrical Engineering Research Applications to Homeland Security National Science Foundation Research Experiences for Undergraduates ABSTRACT Low power consumption is a chief constraint for many systems, particularly small sensor platforms. Subthreshold design allows for very low power operation of digital systems when increased signal delay is not an issue. The objective of this research is to create a low power, small transmitter utilizing a subthreshold IC in addition to off-the- shelf components. In the future, this transmitter will be used for a low power sensor platform carried by a cockroach. A subthreshold Binary Frequency Shift Keying (BFSK) transmitter chip was created by Dr. Sunil Khatri’s group for use in this project. This chip sends out two tones at 115kHz and 345kHz, corresponding to a binary data input. In this project, the output of this BFSK transmitter was mixed with an 80MHz tone to allow for a significant reduction in antenna size. The signal was transmitted through a small coil antenna to the receiver base station, up to 100 feet away. We also implemented the RF front end of the receiver base station. The base station amplifies and filters the signal, then mixes the signal back down to the baseband. This baseband signal was sent to the demodulator to retrieve the original transmitted binary data. SYSTEM DIAGRAM Digital BFSK Modulator Produces two tones 115khz if Input is LOW 345khz if Input is HIGH Binary Input Data @32kbps Bandpass Filter Bandpass Filter Low Noise Amplifier Demodulation Binary Output Data DAC Wireless Transmission Transmitter Side Receiver Side The input signal must be mixed with an 80MHz carrier signal to transmit efficiently. This process is called heterodyning or up-conversion, and is achieved by the SA602 mixer chip. The mixer takes a BFSK baseband signal and mixes that with a second local oscillator (LO) input to create a radio frequency (RF) output signal which is fed into the transmit antenna. The IF signal has peaks at the sum and difference of the RF signal and the LO signal. The BFSK signal input is provided by the subthreshold BFSK chip at 115kHz or 345kHz. The LO is provided as an input to the mixer by an 80Mhz crystal oscillator circuit. RESULTS ACKNOWLEDGEMENTS Tx FR signal from SA602 Mixer, differential output, input to Tx antenna. PCB ARCHITECHTURE 1 cm 3X Figure () – PCB layout at actual size and 3x zoom Rx The transmitted signal needs to be retrieved from the surrounding noise. A band pass filter immediately after the antenna passed our 80MHz transmissions and rejected noise including strong signals from the nearby FM band (90Mhz to 104MHz). The weak-by-design transmission is amplified by a low noise amplifier (LNA) with a gain of 20 dB. The high 3GHz bandwidth of the LNA calls for filtering both before and after amplification. Returning the heterodyned signal to its original form, superheterodyning, is achieved by the Rx mixer. The received signal is again mixed with a LO of 80MHz which produces the signal (along with harmonics, as well as a strong frequency component around 10- 20MHz, due to the mixing of the FM radio signals with the 80MHz LO). Additional filtering captures while rejecting unwanted high frequency components. FUTURE WORK The extreme low power consumption property of the subthreshold chip opens up an array of possibilities for extreme low power applications that are fully powered by scavenged ambient light. The small footprint may allow for the implementation of a robust, fieldable sensor. Initial ideas include the implementation of a subthreshold technology sensor paltform that can be carried by a cockroach. Applications are expansive and new ideas are always sought after. Figure () – actual size Tx board on cockroach Wireless transmission requires a precisely tuned antenna for both Tx and Rx. Antenna size limitation comes from the half-wave dipole equation (right). A lower frequency requires a longer antenna to match the wavelength of the signal. Mixing the 115kHz chip output with the 80MHz LO signal reduces the required antenna λ/2 length from 1.3 km to just 1.87 m. Several implementations of Tx antennas were examined and tested. Due to the size restrictions on this side, coiled wire antennas, and board style antennas were primarily considered. Omnidirectional properties must also be considered for our application. The Rx side antenna was not restricted by size. A standard FM band telescopic antenna was used to receive the 80MHz transmission. This work is supported by Lawrence Livermore National Labs and the National Center for MASINT Research, Texas A&M University, and the National Science Foundation. The authors of this work also gratefully acknowledge Professors K. Chang, K. Entesari, G. Huff, and J. Porter for valuable discussions and assistance. Tx Mixer 80 MHz LO Rx Mixer 80 MHz LO Test Setup - View of Tx from Rx POV Top-BFSK signal Middle-345 kHz attenuation Lower-115kHz attenuation BACKGROUND The previously designed subthreshold chip is tested to a 19.4X power consumption improvement compared to traditional chips of the same size. This project focuses on using this chip to prepare a very small size, low power transmitter that will eventually transmit data from the back of a cockroach. Small circuit and antenna size on the Tx side are crucial. The weight of the device must never exceed five grams in order to meet the project specifications. Sacrifices must be made for antenna efficiency in order to accommodate the small size. The Rx in this application is a simple base station and power consumption and circuit size are not crucial. The receiver can be adjusted and tuned for the ideal reception of the transmitted signal. The desired result of this work is to implement the transmitter and receiver and to use the system to accurately transmit data to a distance up to 100 feet. A functioning Subthreshold chip. Lower- binary input to chip Center- BFSK output from chip Top- Demodulation through FPGA Early results from demodulation circuit: Yellow- BFSK input to circuit Red- Amplified 115kHz signal Violet- Amplified 345kHz signal A transmitter was designed with a size and weight that met the specifications (<5g). The transmitter was capable of taking the binary input, BFSK modulating it, and mixing to prepare the signal for wireless transmission. Likewise, a receiver was designed for most efficient reception of our 80MHz frequency. The lack of size and power limitations allowed for the use of efficient active components. Antenna design resulted in antennae that could dependably produce a powerful enough signal for our receiver. The designs were omnidirectionally tested. There are several possible antennae that can meet both the size and efficiency requirements. Initial testing shows success in the implementation of the designed and manufactured system. Additional testing is currently taking place. Additional circuitry is being developed to improve the accuracy and compatibility of the entire system. Passes 70-83MHz RF signal from SA602 Mixer, single ended outputs, input to Tx antenna. FFT spectrum of Rx output, 20 dB spike at 115 kHz 115 kHz