A multichannel cardiovascular
telemetry system for recording neural signals
YOSHIHARU YONEZAWA, ISHIO NINOMIYA, AND NAOKI NISHIURA Department of Electrical Engineering, Hiroshima Institute of Technology, Itukaichi, Hiroshima 738, and Department of Cardiac Physiology, Research National Cardiovascular Center, Suita, Osaka, 565 Japan
YONEZAWA, NISHIURA.
YOSHIHARU,
ISHIO
NINOMIYA,
AND
NAOKI
A multichannel telemetry system for recording cardiouascuZar neuraZ signals. Am. J. Physiol. 236(3): H513-H518, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 5(3): H513-H518, 1979.-A multichannel telemetry system was developed for use with chronically instrumented unrestrained cats. This system can si’multaneously record efferent and afferent cardiovascular neural signals, bioelectrical noise arising near the electrode recording the neural signals, EEG, ECG, and a standard calibration signal. The miniature (18 cm’), lightweight (24 g), telemeter is a five-channel, time-multiplexed, pulse width modulation (PWM)/FM device employing a high frequency subcarrier (60 kHz) and two sampling frequencies (30 kHz and 6 kHz). The device is powered by two small 120 mA h silver oxide cells; it has an indoor transmission range of 10 m and can operate for 48 h. One channel transmits a standard signal (a square wave of 100 Hz and 200 mV,,.,) used to monitor and regulate the system’s performance. When the variationin either the amplitude or frequency of the standard signal is greater than 10% of the control value, the transmitted bioelectrical signals are automatically discarded.
Institute,
pV,-,, peak-to-peak voltage), a new multichannel telemetry system capable of recording neural signals of the cardiovascular system (NS) simultaneously with EEG, ECG, and bioelectrical noise (BEN) in freely moving animals had to be developed. The system described in this report is a five-channel, time-multiplexed, pulse width modulation (PWM)/FM device employing a high frequency subcarrier (60 kHz) and two sampling frequencies (30 kHz in the NS channel; 6 kHz in the other channels). This device, powered by two small 1.5 V silver oxide cells, is 30 x 30 x 20 mm in size and weighs 24 g.
l
aortic nerve activity; sympathetic cats; implantable electrode
nerve activity in unrestrained
MEASUREMENT OF the efferent and afferent nerve activity in the cardiovascular system is important for analyzing the cardiovascular neural control system in unanesthetized and freely moving animals. We recently reported that the sympathetic and aortic nerve activity in chronically instrumented cats and rabbits can be recorded with collagen electrodes (5, 8), silver wire electrodes (4, 6), or steel electrodes (3). In those studies, the implanted electrodes were connected to the recording apparatus by lightweight, flexible cables. The lead wires not only restrained the free movement of the animals, but in addition, their movement interrupted the continuity of the signal and often induced noise. The use of a multichannel telemetry system would resolve these problems. In fact, a number of such systems capable of recording bioelectrical signals of low frequency and large amplitude (e.g., EEG, ECG, and EMG) have been reported (1, 2, 7). However, the telemetering of autonomic nerve impulses has not previously been accomplished. Therefore, because the aortic and sympathetic nerve activity we wished to record contained high frequency components of small amplitude (i.e., 5-50 0363-6135/79/0000-000000$01.25
Copyright
0 1979 the American
Physiological
TELEMETRY
SYSTEM
Figure 1 is a block diagram of the telemeter. With this device, bioelectrical signals (NS, EEG, and ECG) and bioelectrical noise obtained by implantable electrodes are fed directly into each amplifier. The outputs from the amplifiers and the standard signal oscillator are sampled sequentially by a multiplexer. The multiplexer output provides pulse amplitude modulation (PAM) signals. A pulse width modulation (PWM) converter then converts the PAM signal to a PWM format; the PWM signals, in turn, frequency modulate (FM) the radio frequency (RF) transmitter. Figure 2 is a detailed circuit diagram of the device. Complementary metal oxide semiconductor (C-MOS) digital integrated circuits and low power operational amplifiers are used to reduce size, complexity, cost of fabrication, and power consumption. AMPLIFIERS
An amplifier of low noise and high gain is required for neural signals because the amplitude of NS ranges from about 5 to 50 $&-, (usually, lo-20 $&,). Therefore, a super beta monolithic dual NPN transistor of low noise and high hFE was selected for the first stage of the differential input. The second and third stages were constructed by a single L144 monolithic integrated circuit (IC) chip, having triple operational amplifiers. The second stage has two cross-coupled followers with gain circuits that track each other. The third stage acts as a differential input to a single-ended output converter and provides additional gain. The gain of the amplifier is set at 2436, in which the first, second, and third stages have Society
H513
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
H514
YONEZAWA, PWM
MULTIPLEXER
NINOMIYA,
AND
NISHIURA
CONVERTER
NS
BEN
ECG
1 DECAD;
FIG.
1. Block
COUNTER
j
diagram
(
of the multichannel
telemeter.
10K
N.S
4 15K
EEG
-.lSV
ZOOOP
270K
BEN d-15v
ECG SD46
ZOP
S.S.
mp
CD
C
4069
OP
AMPS
D 4069
: LlC4
FIG.
2. Circuit
diagram
of the multichannel
gains of 29, 21, and 4, respectively. This amplifier ‘was designed with a bandwidth of 50-3000 Hz because the power spectrum of the aortic (ca. 100-1500 Hz) and renal nerve (ca. 80-200 Hz) activities in cats lie within that range. The amplifier is bandlimited by capacitive coupling between the emitter of the input stage (low frequency cutoff) and the capacitors connected in parallel with the third stage feedback resistor (high frequency cutoff). The EEG, ECG, and BEN amplifiers were constructed
telemeter.
by L144 monolithic IC chips with circuits similar to that of the second and third stages of the NS amplifier. The low frequency cutoff of these amplifiers is set by a capacitive coupling between the second and third stages. The output of each amplifier is limited by a diode circuit to a range of &300 mV. The electrical characteristics of these amplifiers are summarized in Table 1. The standard signal (SS), a square wave of 100 Hz and 200 mV,-,, is generated by two C-MOS inverters and one operational amplifier.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
A MULTICHANNEL
TELEMETRY
H515
SYSTEM
TABLE 1. Electrical characteristics of amplifiers for recording NS, EEG, ECG, and BEN
not destroyed because the PAM signal is restricted in it IIL nqvpr ran range from -300 to +300 mV; therefor,, Iv exceed the triangle wave amplitude of k500 mV, P‘
FM
MULTIPLEXER
As shown in Fig. 2, the time division multiplexer is constructed of metal oxide semiconductor field effect transistor (MOSFET) switches (3SK35); the switches are controlled by a sequencer consisting of a decade counter (CD4017) and an OR gate (CD4075). The frequency of the sampling pulses must be at least 10 times the required signal bandwidth. For the NS channel, the high frequency of the aortic nerve activity is approximately 3 kHz, and therefore a sampling frequency of 30 kHz is required. The highest frequencies of the EEG, ECG, BEN, and SS signals are lower than that of neural signals and therefore a sampling frequency of 6 kHz was selected. The frequency (60 kHz) of the clock pulse oscillator is determined by an RC time constant of two C-MOS inverter gates (CD4069). The clock pulse oscillator output, as shown in Fig. 3A, is fed directly into the decade counter and converted to parallel pulses (pulse width, 16.6 ,us) to activate the MOSFET switches. The sampling pulse of the NS channel uses a serial pulse of 30 kHz obtained through the OR gate, which sums the odd segment pulses of the decade counter. For the other four channels, the sampling pulse of each channel is directly provided (6 kHz/channel) by the even segment pulses of the decade counter. The analog inputs from channels 1, 2, 3, 4, and 5 are sequentially sampled through the MOSFET switches, which are activated for each 16.6-ps period; and the time multiplexing output (PAM) is arranged in order of channels 1, 2, 1, 3, 1, 4, 1, 5, 1, and S (frame synchronization) (Fig. 3C). PWM
CONVERTER
TRANSMITTER
A Colpitts circuit is employed in the oscillator ‘U sed to transmit the PWM signal. It is commonly used ,as aFM transmitter in biotelemeters (1, 7). The transmit tii ng frequency was set at 81 MHz to comply with vario US ; technical problems and legal restrictions characterist 1c of our laboratory’s location and permits the use of a con1n iercial FM front end. The electrical field intensity is 7 E LV ‘/m at a distance of 3 wavelengths.
LOCK PUL.S E
RIANGLE
w‘AVE
ftttfutftttttttf
CH51S121314151S1213
1 PWM
WAVETRAIN
ttttttrttrtrrtttt
CH51S121314151S1213
The PAM signal is converted into a PWM format in a FIG. 3. Typical system wave forms of clock pulse, triangle wave, PWM converter by comparison with a triangle wave. PAM, and PWM wavetrain. In C and D, NS, SS, EEG, BEN, and ECG The PWM converter consists of a C-MOS comparator are sampled by channels 1, 2, 3,4, and 5, respectively. Time calibration, (MM74C909) and a triangle wave generator. The circuit 25 ps; vertical bar, IV for A, B, and D and 200 mV for C. of the triangle wave generator includes a C-MOS inverter (CD4069) and was designed to be used as an integrator -3 converting the clock pulse to a triangle wave, as shown o-o-o-o-o 0’ in Fig. 3B. The DC level of this triangle wave is adjusted v5 0’ to 0 V by a resistor (150 kQ) and by an inverter located 0’ between the clock pulse oscillator and the integrator. 3 7/ o/ Thus, the zero input voltage of each amplifier should ci L L 1 I /O , 1 I c I 1 represent a 50% duty cycle of the PWM signal. ,g loo 200 300 400 500 To provide frame synchronization of the PWM signal, -‘O” -400 -300 -*O” -‘“,o’ every 10th segment is blanked out by the lOth-segment o/o’ P.A (mV) -1 1 pulse of the decade counter (Fig. 30). The PAM-PWM 0’ conversion characteristics are illustrated in Fig. 4. Within 0’ o-o-o-o-o’ a range of t300 mV, PAM signals are converted almost linearly to PWM signals. The frame synchronization is FIG. 4. Characteristics of PAM-PWM conversion. Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
H516
YONEZAWA,
RECEIVER
The FM tuner is similar to a commercially available FM tuner (Fig. 5), but it cannot be used directly. Because the system has a high frequency subcarrier of 60 kHz, the FM tuner must have a good rise time to accommodate the PWM signal. The tuner was modified as follows: 1) the bandlimited IFT from a commercial available FM fro& end (Sank0 Denshi, SFM-6OF) is replaced with a resistor (ZOO a); 2) a new wide-band (10-11.4 MHz) highgain (110 dB) IF amplifier was designed; and 3) a frequency discriminator (LM373) including a balance mixer is employed. This modified FM tuner accurately demodulated the PWM signal (Fig. 30) from the received FM signal. After the PWM signal is amplified and shaped, the multiplexed signals are separated into individual outputs in a demultiplexer, which consists of a synchronous pulse generator, a decade counter, an OR gate and five AND gates (Fig. -, 5). Demodulation of the PWM signal back to original analog input wave forms is made by an integrator that provides a voltage proportional to the pulse width. The demodulated signal is separated into individual channels through C-MOS switches controlled by the demultiplexer, and each channel output is passed through a hold circuit and a low-pass filter. The demodulated standard signal is used to regulate the system performance and to improve the reliability of the data obtained from continuous, long-term recording. The variation in both amplitude and frequency of the standard signal induced by many factors (e.g., battery voltage variation, attachment of antenna to ground or other materials, mechanical artifacts during severe moving and disturbance from the external radio waves) is monitored continuously with the circuit of Fig. 6. When
LY
373
CA3140
NINOMIYA,
AND
NISHIURA
the variation is greater than 10% of the control value, recording of transmitted bioelectrical signals is interrupted for 1.5 s. Therefore, we assumed that maximum deviation of bioelectrical signals are within 10% of recorded data. The standard signal is also used to calibrate the amplitude of bioelectrical signal for each channel. Calibration is performed by comparison of the demodulated bioelectrical signal with the demodulated standard signal. The multiplexer input voltage (MIV) of each channel is given by the equation MIV = 200 (mV) X the voltage of demodulated bioelectrical signal/the voltage of demodulated standard signal As can be seen in Fig. 2, the MIV of each channel (i.e., output of the amplifier) is obtained by multiplication of the gain of each amplifier with the input of bioelectrical signal. Therefore, the actual input voltage of bioelectrical signals are estimated by the equation NS, BEN, EXPERIMENTAL
MIV gain of amplifier
ECG, or EEG = RECORDS
The PWM/FM telemeter was tested by recording bioelectrical signals from cats. With the animals under sodium pentobarbital anesthesia (35 mg/kg ip), a bundle of nerve fibers from either the renal or aortic nerve was isolated from the surrounding connective tissue. The nerve bundle (about 10 mm in length) was desheathed carefully at the site of the electrode implantation, and a collagen electrode was implanted. The bioelectrical noise coming from the region of the electrode recording neural signals was picked up by a pair of wires (Ag, 0.127 mm diam) placed in parallel on the outside of the collagen electrode. To record the EEG and ECG, a pair of wires
LM319
Lb4316
CO4066
741
.12v
.
N.S
-1
OUTPUT
ECG
FIG.
5. Circuit
diagram
of the telemetry
receiver.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
A MULTICHANNEL
TELEMETRY
H517
SYSTEM
N.S
OUTPUT BEN0
FIG. 6. Circuit
diagram
for monitoring
both
the amplitude
and frequency
of the standard
FIG. 7. Comparison of aortic nerve activities by direct lead wire and telemetry. by two methods in anesthetized condition. Time calibration, 150 ms; vertical bar, demonstrates linearity of this telemetry system.
(Ag, 0.5 mm diam) were implanted into the head and chest, respectively. The lead wires from each electrode are passed through the subcutaneous tissue and brought out to the surface of the body on the back, and these wires are connected to the telemeter. The telemeter is fixed on the leather sheet (10 x 10 cm) placed on the cat’s back. The leather sheet prevents the animal from cutting the lead wires by biting or scratching. To test the linearity of this telemetry system, the aortic nerve activity was recorded simultaneously by both the telemetry system and a direct lead wire connected to an amplifier with a band pass filter of 20-3000 Hz. Figure 7 shows the two recorded aortic nerve activities and their amplitude spectra. The results indicate that two record-
signal
and regulating
the system
performance.
In A, discharge patterns of original neurograms are recorded 10 pV. In B, close similarity of two relative amplitude spectra
ing methods are the same and that system is within +l% deviation. By system, typical data, including aortic and BEN, taken from a chronically strained cat in the first postoperative in Fig. 8.
the linearity of this using the telemetry nerve activity, ECG instrumented unreday, are illustrated
CONCLUSIONS
A multichannel telemetry system was developed for simultaneously recording cardiovascular neural signals, EEG, ECG, and bioelectrical noise in chronically instrumented unrestrained small animals. The special features of this system can be summarized as follows: 1) The high
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
H518
YQNEZAWA,
AND
NISHIURA
frequency cardiovascular neural signals and the low frequency EEG, ECG, and bioelectrical noise signals can be recorded simultaneously using two sampling frequencies of 30 kHz and 6 kHz, respectively. 2) The standard signal served to improve the reliability of data recorded continuously over long periods. 3) Power consumption was reduced to 7.5 mW by using C-MOS digital integrated circuits and low power operational amplifiers. 4) The use of IC devices simplified the fabrication of the system and made possible a miniature (18 cm”), lightweight (24 g) telemeter. 5) Bioelectrical signals in freely moving animals could be measured within a range of 10 m indoors.
ECG
A.N.A.
M.A.N.A. B.E.N.
FIG. 8. Typical MANA (integrated noise coming from were recorded by mented unrestrained for ECG; 10 PV for
NINOMIYA,
wave forms of ECG, ANA (aortic nerve activities), aortic nerve activities), and BEN (bioelectrical the region of the electrode recording neural signals) the radio telemetry system in a chronically instrucat. Time calibration, 150 ms; vertical bar, 200 PV ANA, 2 ,uV for MANA, and 100 PV for BEN.
The authors express their profound appreciation to Professor Sukiro Obata, Department of Electrical Engineering, University of Aoyama Gakuin, for his guidance in the conduct of this study. They also thank Professor Hiroshi Irisawa, Department of Physiology, Hiroshima University, for his encouragement and interest in these experiments, which were initially conducted at Hiroshima. This study was supported in part by research grants from the Asahi Shinbun, Ministry of Health and Welfare and Ministry of Education, Science and Culture. Address reprint requests to Dr. Ishio Ninomiya, Department of Cardiac Physiology, National Cardiovascular Center, Research Institute, 5-125 Fugishirodai, Suita, Osaka, 565, Japan.
Received
7 April
1978; accepted
in final
form
25 October
1978.
REFERENCES 1. FISCHLER, H., N. PELED, AND S. YERUSHALMI. FM/FM multiplex radio telemetry system for handling biological data. IEEE Trans. Bio-Med. Eng. 14: 30-39, 1967. 2. FRYER, T. B., H. SANDLER, W. FREUND, E. P. MCCUTCHEON, AND E. L. CARLSON. A multichannel implantable telemetry system for flow, pressure, and ECG measurements. J. A&. Physiol. 39: 31% 326, 1975. 3. KIRCHNER, F. Correlations between changes of activity of the renal sympathetic nerve and behavioral events in unrestrained cats. Basic Res. Cardiol. 69: 243-256, 1973. 4. NINOMIYA, I., W. V. JUDY, W. M. CALDWELL, AND M. F. WILSON. Sympathetic nerve activity in unanesthetized cats. Physiologist 12: 316, 1969.
5. NINOMIYA, I., Y. YONEZAWA, AND M. F. WILSON. Implantable electrode for recording nerve signals in awake animals. J. Appl. Physiol. 41: 111-114, 1976. 6. SCHAD, H., AND SELLER, H. A method for recording autonomic nerve activity in unanesthetized, freely moving cats. Brain Res. 100: 425-430, 1975. 7. SMITH, E. N., AND T. J. SALB, JR. Multichannel subcarrier ECG, respiration, and temperature biotelemetry system. J. AppZ. Physiol. 39: 331-334, 1975. 8. YONEZAWA, Y., AND I. NINOMIYA. Collagen fiber electrode for recording peripheral nerve activity. Jap. J. Med. EZec. BioZ. Eng. 14: 387-392, 1976.
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telemetry system for recording neural signals
YOSHIHARU YONEZAWA, ISHIO NINOMIYA, AND NAOKI NISHIURA Department of Electrical Engineering, Hiroshima Institute of Technology, Itukaichi, Hiroshima 738, and Department of Cardiac Physiology, Research National Cardiovascular Center, Suita, Osaka, 565 Japan
YONEZAWA, NISHIURA.
YOSHIHARU,
ISHIO
NINOMIYA,
AND
NAOKI
A multichannel telemetry system for recording cardiouascuZar neuraZ signals. Am. J. Physiol. 236(3): H513-H518, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 5(3): H513-H518, 1979.-A multichannel telemetry system was developed for use with chronically instrumented unrestrained cats. This system can si’multaneously record efferent and afferent cardiovascular neural signals, bioelectrical noise arising near the electrode recording the neural signals, EEG, ECG, and a standard calibration signal. The miniature (18 cm’), lightweight (24 g), telemeter is a five-channel, time-multiplexed, pulse width modulation (PWM)/FM device employing a high frequency subcarrier (60 kHz) and two sampling frequencies (30 kHz and 6 kHz). The device is powered by two small 120 mA h silver oxide cells; it has an indoor transmission range of 10 m and can operate for 48 h. One channel transmits a standard signal (a square wave of 100 Hz and 200 mV,,.,) used to monitor and regulate the system’s performance. When the variationin either the amplitude or frequency of the standard signal is greater than 10% of the control value, the transmitted bioelectrical signals are automatically discarded.
Institute,
pV,-,, peak-to-peak voltage), a new multichannel telemetry system capable of recording neural signals of the cardiovascular system (NS) simultaneously with EEG, ECG, and bioelectrical noise (BEN) in freely moving animals had to be developed. The system described in this report is a five-channel, time-multiplexed, pulse width modulation (PWM)/FM device employing a high frequency subcarrier (60 kHz) and two sampling frequencies (30 kHz in the NS channel; 6 kHz in the other channels). This device, powered by two small 1.5 V silver oxide cells, is 30 x 30 x 20 mm in size and weighs 24 g.
l
aortic nerve activity; sympathetic cats; implantable electrode
nerve activity in unrestrained
MEASUREMENT OF the efferent and afferent nerve activity in the cardiovascular system is important for analyzing the cardiovascular neural control system in unanesthetized and freely moving animals. We recently reported that the sympathetic and aortic nerve activity in chronically instrumented cats and rabbits can be recorded with collagen electrodes (5, 8), silver wire electrodes (4, 6), or steel electrodes (3). In those studies, the implanted electrodes were connected to the recording apparatus by lightweight, flexible cables. The lead wires not only restrained the free movement of the animals, but in addition, their movement interrupted the continuity of the signal and often induced noise. The use of a multichannel telemetry system would resolve these problems. In fact, a number of such systems capable of recording bioelectrical signals of low frequency and large amplitude (e.g., EEG, ECG, and EMG) have been reported (1, 2, 7). However, the telemetering of autonomic nerve impulses has not previously been accomplished. Therefore, because the aortic and sympathetic nerve activity we wished to record contained high frequency components of small amplitude (i.e., 5-50 0363-6135/79/0000-000000$01.25
Copyright
0 1979 the American
Physiological
TELEMETRY
SYSTEM
Figure 1 is a block diagram of the telemeter. With this device, bioelectrical signals (NS, EEG, and ECG) and bioelectrical noise obtained by implantable electrodes are fed directly into each amplifier. The outputs from the amplifiers and the standard signal oscillator are sampled sequentially by a multiplexer. The multiplexer output provides pulse amplitude modulation (PAM) signals. A pulse width modulation (PWM) converter then converts the PAM signal to a PWM format; the PWM signals, in turn, frequency modulate (FM) the radio frequency (RF) transmitter. Figure 2 is a detailed circuit diagram of the device. Complementary metal oxide semiconductor (C-MOS) digital integrated circuits and low power operational amplifiers are used to reduce size, complexity, cost of fabrication, and power consumption. AMPLIFIERS
An amplifier of low noise and high gain is required for neural signals because the amplitude of NS ranges from about 5 to 50 $&-, (usually, lo-20 $&,). Therefore, a super beta monolithic dual NPN transistor of low noise and high hFE was selected for the first stage of the differential input. The second and third stages were constructed by a single L144 monolithic integrated circuit (IC) chip, having triple operational amplifiers. The second stage has two cross-coupled followers with gain circuits that track each other. The third stage acts as a differential input to a single-ended output converter and provides additional gain. The gain of the amplifier is set at 2436, in which the first, second, and third stages have Society
H513
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
H514
YONEZAWA, PWM
MULTIPLEXER
NINOMIYA,
AND
NISHIURA
CONVERTER
NS
BEN
ECG
1 DECAD;
FIG.
1. Block
COUNTER
j
diagram
(
of the multichannel
telemeter.
10K
N.S
4 15K
EEG
-.lSV
ZOOOP
270K
BEN d-15v
ECG SD46
ZOP
S.S.
mp
CD
C
4069
OP
AMPS
D 4069
: LlC4
FIG.
2. Circuit
diagram
of the multichannel
gains of 29, 21, and 4, respectively. This amplifier ‘was designed with a bandwidth of 50-3000 Hz because the power spectrum of the aortic (ca. 100-1500 Hz) and renal nerve (ca. 80-200 Hz) activities in cats lie within that range. The amplifier is bandlimited by capacitive coupling between the emitter of the input stage (low frequency cutoff) and the capacitors connected in parallel with the third stage feedback resistor (high frequency cutoff). The EEG, ECG, and BEN amplifiers were constructed
telemeter.
by L144 monolithic IC chips with circuits similar to that of the second and third stages of the NS amplifier. The low frequency cutoff of these amplifiers is set by a capacitive coupling between the second and third stages. The output of each amplifier is limited by a diode circuit to a range of &300 mV. The electrical characteristics of these amplifiers are summarized in Table 1. The standard signal (SS), a square wave of 100 Hz and 200 mV,-,, is generated by two C-MOS inverters and one operational amplifier.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
A MULTICHANNEL
TELEMETRY
H515
SYSTEM
TABLE 1. Electrical characteristics of amplifiers for recording NS, EEG, ECG, and BEN
not destroyed because the PAM signal is restricted in it IIL nqvpr ran range from -300 to +300 mV; therefor,, Iv exceed the triangle wave amplitude of k500 mV, P‘
FM
MULTIPLEXER
As shown in Fig. 2, the time division multiplexer is constructed of metal oxide semiconductor field effect transistor (MOSFET) switches (3SK35); the switches are controlled by a sequencer consisting of a decade counter (CD4017) and an OR gate (CD4075). The frequency of the sampling pulses must be at least 10 times the required signal bandwidth. For the NS channel, the high frequency of the aortic nerve activity is approximately 3 kHz, and therefore a sampling frequency of 30 kHz is required. The highest frequencies of the EEG, ECG, BEN, and SS signals are lower than that of neural signals and therefore a sampling frequency of 6 kHz was selected. The frequency (60 kHz) of the clock pulse oscillator is determined by an RC time constant of two C-MOS inverter gates (CD4069). The clock pulse oscillator output, as shown in Fig. 3A, is fed directly into the decade counter and converted to parallel pulses (pulse width, 16.6 ,us) to activate the MOSFET switches. The sampling pulse of the NS channel uses a serial pulse of 30 kHz obtained through the OR gate, which sums the odd segment pulses of the decade counter. For the other four channels, the sampling pulse of each channel is directly provided (6 kHz/channel) by the even segment pulses of the decade counter. The analog inputs from channels 1, 2, 3, 4, and 5 are sequentially sampled through the MOSFET switches, which are activated for each 16.6-ps period; and the time multiplexing output (PAM) is arranged in order of channels 1, 2, 1, 3, 1, 4, 1, 5, 1, and S (frame synchronization) (Fig. 3C). PWM
CONVERTER
TRANSMITTER
A Colpitts circuit is employed in the oscillator ‘U sed to transmit the PWM signal. It is commonly used ,as aFM transmitter in biotelemeters (1, 7). The transmit tii ng frequency was set at 81 MHz to comply with vario US ; technical problems and legal restrictions characterist 1c of our laboratory’s location and permits the use of a con1n iercial FM front end. The electrical field intensity is 7 E LV ‘/m at a distance of 3 wavelengths.
LOCK PUL.S E
RIANGLE
w‘AVE
ftttfutftttttttf
CH51S121314151S1213
1 PWM
WAVETRAIN
ttttttrttrtrrtttt
CH51S121314151S1213
The PAM signal is converted into a PWM format in a FIG. 3. Typical system wave forms of clock pulse, triangle wave, PWM converter by comparison with a triangle wave. PAM, and PWM wavetrain. In C and D, NS, SS, EEG, BEN, and ECG The PWM converter consists of a C-MOS comparator are sampled by channels 1, 2, 3,4, and 5, respectively. Time calibration, (MM74C909) and a triangle wave generator. The circuit 25 ps; vertical bar, IV for A, B, and D and 200 mV for C. of the triangle wave generator includes a C-MOS inverter (CD4069) and was designed to be used as an integrator -3 converting the clock pulse to a triangle wave, as shown o-o-o-o-o 0’ in Fig. 3B. The DC level of this triangle wave is adjusted v5 0’ to 0 V by a resistor (150 kQ) and by an inverter located 0’ between the clock pulse oscillator and the integrator. 3 7/ o/ Thus, the zero input voltage of each amplifier should ci L L 1 I /O , 1 I c I 1 represent a 50% duty cycle of the PWM signal. ,g loo 200 300 400 500 To provide frame synchronization of the PWM signal, -‘O” -400 -300 -*O” -‘“,o’ every 10th segment is blanked out by the lOth-segment o/o’ P.A (mV) -1 1 pulse of the decade counter (Fig. 30). The PAM-PWM 0’ conversion characteristics are illustrated in Fig. 4. Within 0’ o-o-o-o-o’ a range of t300 mV, PAM signals are converted almost linearly to PWM signals. The frame synchronization is FIG. 4. Characteristics of PAM-PWM conversion. Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
H516
YONEZAWA,
RECEIVER
The FM tuner is similar to a commercially available FM tuner (Fig. 5), but it cannot be used directly. Because the system has a high frequency subcarrier of 60 kHz, the FM tuner must have a good rise time to accommodate the PWM signal. The tuner was modified as follows: 1) the bandlimited IFT from a commercial available FM fro& end (Sank0 Denshi, SFM-6OF) is replaced with a resistor (ZOO a); 2) a new wide-band (10-11.4 MHz) highgain (110 dB) IF amplifier was designed; and 3) a frequency discriminator (LM373) including a balance mixer is employed. This modified FM tuner accurately demodulated the PWM signal (Fig. 30) from the received FM signal. After the PWM signal is amplified and shaped, the multiplexed signals are separated into individual outputs in a demultiplexer, which consists of a synchronous pulse generator, a decade counter, an OR gate and five AND gates (Fig. -, 5). Demodulation of the PWM signal back to original analog input wave forms is made by an integrator that provides a voltage proportional to the pulse width. The demodulated signal is separated into individual channels through C-MOS switches controlled by the demultiplexer, and each channel output is passed through a hold circuit and a low-pass filter. The demodulated standard signal is used to regulate the system performance and to improve the reliability of the data obtained from continuous, long-term recording. The variation in both amplitude and frequency of the standard signal induced by many factors (e.g., battery voltage variation, attachment of antenna to ground or other materials, mechanical artifacts during severe moving and disturbance from the external radio waves) is monitored continuously with the circuit of Fig. 6. When
LY
373
CA3140
NINOMIYA,
AND
NISHIURA
the variation is greater than 10% of the control value, recording of transmitted bioelectrical signals is interrupted for 1.5 s. Therefore, we assumed that maximum deviation of bioelectrical signals are within 10% of recorded data. The standard signal is also used to calibrate the amplitude of bioelectrical signal for each channel. Calibration is performed by comparison of the demodulated bioelectrical signal with the demodulated standard signal. The multiplexer input voltage (MIV) of each channel is given by the equation MIV = 200 (mV) X the voltage of demodulated bioelectrical signal/the voltage of demodulated standard signal As can be seen in Fig. 2, the MIV of each channel (i.e., output of the amplifier) is obtained by multiplication of the gain of each amplifier with the input of bioelectrical signal. Therefore, the actual input voltage of bioelectrical signals are estimated by the equation NS, BEN, EXPERIMENTAL
MIV gain of amplifier
ECG, or EEG = RECORDS
The PWM/FM telemeter was tested by recording bioelectrical signals from cats. With the animals under sodium pentobarbital anesthesia (35 mg/kg ip), a bundle of nerve fibers from either the renal or aortic nerve was isolated from the surrounding connective tissue. The nerve bundle (about 10 mm in length) was desheathed carefully at the site of the electrode implantation, and a collagen electrode was implanted. The bioelectrical noise coming from the region of the electrode recording neural signals was picked up by a pair of wires (Ag, 0.127 mm diam) placed in parallel on the outside of the collagen electrode. To record the EEG and ECG, a pair of wires
LM319
Lb4316
CO4066
741
.12v
.
N.S
-1
OUTPUT
ECG
FIG.
5. Circuit
diagram
of the telemetry
receiver.
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A MULTICHANNEL
TELEMETRY
H517
SYSTEM
N.S
OUTPUT BEN0
FIG. 6. Circuit
diagram
for monitoring
both
the amplitude
and frequency
of the standard
FIG. 7. Comparison of aortic nerve activities by direct lead wire and telemetry. by two methods in anesthetized condition. Time calibration, 150 ms; vertical bar, demonstrates linearity of this telemetry system.
(Ag, 0.5 mm diam) were implanted into the head and chest, respectively. The lead wires from each electrode are passed through the subcutaneous tissue and brought out to the surface of the body on the back, and these wires are connected to the telemeter. The telemeter is fixed on the leather sheet (10 x 10 cm) placed on the cat’s back. The leather sheet prevents the animal from cutting the lead wires by biting or scratching. To test the linearity of this telemetry system, the aortic nerve activity was recorded simultaneously by both the telemetry system and a direct lead wire connected to an amplifier with a band pass filter of 20-3000 Hz. Figure 7 shows the two recorded aortic nerve activities and their amplitude spectra. The results indicate that two record-
signal
and regulating
the system
performance.
In A, discharge patterns of original neurograms are recorded 10 pV. In B, close similarity of two relative amplitude spectra
ing methods are the same and that system is within +l% deviation. By system, typical data, including aortic and BEN, taken from a chronically strained cat in the first postoperative in Fig. 8.
the linearity of this using the telemetry nerve activity, ECG instrumented unreday, are illustrated
CONCLUSIONS
A multichannel telemetry system was developed for simultaneously recording cardiovascular neural signals, EEG, ECG, and bioelectrical noise in chronically instrumented unrestrained small animals. The special features of this system can be summarized as follows: 1) The high
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
H518
YQNEZAWA,
AND
NISHIURA
frequency cardiovascular neural signals and the low frequency EEG, ECG, and bioelectrical noise signals can be recorded simultaneously using two sampling frequencies of 30 kHz and 6 kHz, respectively. 2) The standard signal served to improve the reliability of data recorded continuously over long periods. 3) Power consumption was reduced to 7.5 mW by using C-MOS digital integrated circuits and low power operational amplifiers. 4) The use of IC devices simplified the fabrication of the system and made possible a miniature (18 cm”), lightweight (24 g) telemeter. 5) Bioelectrical signals in freely moving animals could be measured within a range of 10 m indoors.
ECG
A.N.A.
M.A.N.A. B.E.N.
FIG. 8. Typical MANA (integrated noise coming from were recorded by mented unrestrained for ECG; 10 PV for
NINOMIYA,
wave forms of ECG, ANA (aortic nerve activities), aortic nerve activities), and BEN (bioelectrical the region of the electrode recording neural signals) the radio telemetry system in a chronically instrucat. Time calibration, 150 ms; vertical bar, 200 PV ANA, 2 ,uV for MANA, and 100 PV for BEN.
The authors express their profound appreciation to Professor Sukiro Obata, Department of Electrical Engineering, University of Aoyama Gakuin, for his guidance in the conduct of this study. They also thank Professor Hiroshi Irisawa, Department of Physiology, Hiroshima University, for his encouragement and interest in these experiments, which were initially conducted at Hiroshima. This study was supported in part by research grants from the Asahi Shinbun, Ministry of Health and Welfare and Ministry of Education, Science and Culture. Address reprint requests to Dr. Ishio Ninomiya, Department of Cardiac Physiology, National Cardiovascular Center, Research Institute, 5-125 Fugishirodai, Suita, Osaka, 565, Japan.
Received
7 April
1978; accepted
in final
form
25 October
1978.
REFERENCES 1. FISCHLER, H., N. PELED, AND S. YERUSHALMI. FM/FM multiplex radio telemetry system for handling biological data. IEEE Trans. Bio-Med. Eng. 14: 30-39, 1967. 2. FRYER, T. B., H. SANDLER, W. FREUND, E. P. MCCUTCHEON, AND E. L. CARLSON. A multichannel implantable telemetry system for flow, pressure, and ECG measurements. J. A&. Physiol. 39: 31% 326, 1975. 3. KIRCHNER, F. Correlations between changes of activity of the renal sympathetic nerve and behavioral events in unrestrained cats. Basic Res. Cardiol. 69: 243-256, 1973. 4. NINOMIYA, I., W. V. JUDY, W. M. CALDWELL, AND M. F. WILSON. Sympathetic nerve activity in unanesthetized cats. Physiologist 12: 316, 1969.
5. NINOMIYA, I., Y. YONEZAWA, AND M. F. WILSON. Implantable electrode for recording nerve signals in awake animals. J. Appl. Physiol. 41: 111-114, 1976. 6. SCHAD, H., AND SELLER, H. A method for recording autonomic nerve activity in unanesthetized, freely moving cats. Brain Res. 100: 425-430, 1975. 7. SMITH, E. N., AND T. J. SALB, JR. Multichannel subcarrier ECG, respiration, and temperature biotelemetry system. J. AppZ. Physiol. 39: 331-334, 1975. 8. YONEZAWA, Y., AND I. NINOMIYA. Collagen fiber electrode for recording peripheral nerve activity. Jap. J. Med. EZec. BioZ. Eng. 14: 387-392, 1976.
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