Transcript Slide 1

Subthreshold Transmitter Implementation for Low Power Sensor Platform
Gordon D. Burgett
Joseph A. Duperre III
Rajesh Garg, Ph.D. Student
Dr. Sunil Khatri, Professor
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-theshelf components. In the future, this transmitter will be used
for a low power sensor platform carried by a cockroach.
BACKGROUND
•The previously designed subthreshold
chip is tested to a 19.4X power
consumption improvement compared
to traditional chips of the
same size.
ANTENNA
• 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.
• Wireless transmission requires a precisely tuned antenna for both Tx and Rx.
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.
• 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.

c
length  
2 f
• 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.
• 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.
• 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 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 Rx side antenna was not restricted by size. A standard FM band telescopic
antenna was used to receive the 80MHz transmission.
•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.
Receiver Side
Transmitter Side
Binary
Input
Data
@32kbps
Rx Mixer
Tx Mixer
Digital BFSK Modulator
Produces two tones
115khz if Input is LOW
345khz if Input is HIGH
Wireless
Transmission
DAC
Bandpass
Filter
Low Noise
Amplifier
Passes 70-83MHz
80 MHz LO
80 MHz LO
SYSTEM DIAGRAM
Rx
PCB ARCHITECHTURE
3X
1 cm
• The transmitted signal needs to be retrieved from the surrounding
noise.
• 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.
• 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).
RF signal from SA602 Mixer, single ended outputs,
input to Tx antenna.
115 kHz
•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.
FR signal from SA602 Mixer, differential output, input
to Tx antenna.
RESULTS
A functioning Subthreshold chip.
Lower- binary input to chip
Center- BFSK output from chip
Top- Demodulation through FPGA
• 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.
Department of Electrical and Computer Engineering
Texas A&M University, College Station, TX 77843-3128
Demodulation
Passes 70-83MHz
Tx
Figure () – PCB layout at
actual size and 3x zoom
Bandpass
Filter
Binary
Output
Data
FFT spectrum of Rx output, 20 dB spike at 115 kHz
• 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 1020MHz, due to the mixing of the FM radio signals with the 80MHz LO).
Additional filtering captures while rejecting unwanted high frequency
components.
• 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.
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 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.
Test Setup - View of Tx from Rx POV
ACKNOWLEDGEMENTS
FUTURE WORK
Early results from demodulation circuit:
Yellow- BFSK input to circuit
Red- Amplified 115kHz signal
Violet- Amplified 345kHz signal
Top-BFSK signal Middle-345 kHz attenuation Lower-115kHz attenuation
The authors of this work also gratefully acknowledge
Professors K. Chang, K. Entesari, G. Huff, and J.
Porter for valuable discussions and assistance.
Figure () – actual size Tx board on cockroach
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