Current technology in biomedical engineering has made it possible to remotely transmit, demodulate, store and accurately display a large suite of physiological electrical signals. By carefully selecting modern CMOS circuitry, the designer can fabricate micro-miniature, low power systems that ultimately can be hermetically sealed and implanted in a live, free-ranging animal. It was the lack of miniature, low power transmitter circuitry that has hampered some of the basic research questions on sleep patterns in free ranging animals, particularly birds and other small animals. We formulated a team of engineers in our Animal Sleep Research Laboratory to identify the critical technical problems and to design a system capable of transmitting at least two channels of electrophysiological information including the electroencephalogram (EEG) and electrocardiogram (ECG). In this paper we will present the design criteria and testing phase of this transmitter-demodulator system.
The transmitter design is based upon readily available, off-the-shelf CMOS chip circuitry. By adjusting a few components around the chips we can easily modify each channel's characteristics; the channels then can flexibly accommodate a broad spectrum of bandwidths and input voltages. Our presentation will demonstrate two basic channels of physiological information, namely the EEG and ECG. The EEG channel accommodates input voltages ranging between 10-300 V and a 0-30 Hz brainwave bandwidth. The ECG channel input voltages ranged between 10-500 mV and a 0-10 Hz bandwidth. These ranges fit well within the normal parameters of most of the birds and small reptiles we work with in the laboratory and field. Transmitter input channels were designed around the AD626 low noise instrumentation amplifier by Analog Devices. The next stage included a low-pass 5th order filter set at the high end of each physiological bandwidth (i.e. EEG = 30 Hz, ECG = 10 Hz). The filter stage used the Maxim 280 chip that attenuated the EEG signal -30 dB at 30 Hz and the ECG signal-77 dB at 10 Hz respectively. The resulting low-pass signal for each channel was conditioned by a voltage-controlled oscillator (ICL8038) made by Harris Semiconductor. The sub-carrier frequencies were selected in the audible range of 2 kHz and 9 kHz. This VCO chip was selected because it had excellent sine-wave output reproduction and used very little power. The two-channels were then summed and fed into a standard 151 MHz crystal controlled RF transmitter circuit used in our laboratory for several years.
A portable ICOM FM receiver (ICR100) received the transmitted signal and the audio portion was fed into the custom demodulator circuitry designed to extract information from individual channels. The first stage of the demodulator involves a 2 kHz low-pass filter and an 8.5-9.5 kHz band-pass filter to begin separation of the individual channels. A single Maxim 274 chip was used to create two 4th order filters, one for each channel. Each filtered signal was fed into a phase-locked loop (PLL) set at the specific sub-carrier frequency. The modulation of the sub-carrier by the physiological signal was detected by the PLL and then amplified ( 1.0 with 0.5 VDC adjustable offset) into usable signals for data storage and display on a portable polygraph. This research project was funded by NIMH grants RO1-MH37160 and RO1-MH42032 awarded to Charles J. Amlaner and Nigel Ball.
Available at: http://works.bepress.com/charles_amlaner/24/