U.S. patent number RE32,361 [Application Number 06/399,638] was granted by the patent office on 1987-02-24 for implantable telemetry transmission system for analog and digital data.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Stephen R. Duggan.
United States Patent |
RE32,361 |
Duggan |
February 24, 1987 |
Implantable telemetry transmission system for analog and digital
data
Abstract
An improved telemetry transmission system for transmitting
electrocardiographic information, indications of the occurrence of
the pacing pulse and for transmitting digitially encoded
information from an implanted pacemaker, an implanted drug
dispensing device, or other implanted device, to a remote receiver.
Digital data transmitted by the implanted system may include all
programmed parameters as well as power source status and self test
indications, or other programmed digital data such as the device
serial number and lot number.
Inventors: |
Duggan; Stephen R. (Rosemount,
MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
26715551 |
Appl.
No.: |
06/399,638 |
Filed: |
July 19, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
038805 |
May 14, 1979 |
04281664 |
Aug 4, 1981 |
|
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Current U.S.
Class: |
600/508; 128/903;
607/27; 607/32 |
Current CPC
Class: |
A61B
5/0031 (20130101); G08C 19/12 (20130101); A61N
1/3727 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61N 1/372 (20060101); G08C
19/12 (20060101); A61B 005/00 () |
Field of
Search: |
;128/696-697,903,70,419PT |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"A Totally Implantable Multi--Channel Telemetry System Using Custom
Integrated Circuits", Knutti, et al, pp. 63-66. .
"Single Frequency RF Powering", Ko, et al, pp. 67-70. .
"An Inductively Powered Implantable Multichannel Telemetry System
For Cardiovascular Data", McCutcheon, et al. pp. 71-74;
BIOTELEMETRY III, Academic Press, Inc., N.Y. 1976..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Beck; Robert C. Breimayer; Joseph
F. Rooney; John L.
Claims
Having described the invention by way of the above examples and
general description, the subject matter in which exclusive rights
are claimed is defined as follows:
1. In a telemetry system for transmitting data from at least one
location within a living body to an external receiver, a
transmitter comprising:
input means for receiving a signal from said location and providing
an electrical signal representative of said data;
modulator means for receiving said electrical signal and generating
an alternating signal having a frequency which differs from a
predetermined nominal frequency by an amount determined by some
characteristic of said electrical signal;
antenna means;
current source means;
capacitance means; and
switching means controlled by the alternating signal from said
modulator means to connect said capacitance means to said current
source means during half of the period of said alternating signal
from said modulator means, and to connect said capacitance means to
said antenna means during the other half of said alternating
signal.
2. The invention of claim 1 wherein said antenna means is tuned to
radiate damped sinusoidal signals having a frequency of
approximately 10 kHz when said switching means connects to said
capacitance means to said antenna means.
3. The invention of claim 1 wherein said antenna means has a
resonant frequency of 175 kHz, the resonant frequency of said
antenna means being reduced by the connection of said capacitance
means to 10 kHz.
4. The invention of claim 1 wherein the predetermined nominal
frequency of the output of said modulator means is approximately
1500 Hz.
5. The invention of claim 1 wherein said modulator means comprises
a voltage controlled oscillator having a minimum current at which
it may be operated and further comprising current limiting means
for limiting the current delivered to power said voltage controlled
oscillator to an amount slightly exceeding the minimum current at
which said voltage controlled oscillator may be operated.
6. In the telemetry system of claim 1, a receiver comprising:
receiving antenna means;
band pass filter means connected to said receiving antenna means to
receive a signal from said receiving antenna means and said filter
delivering an output signal representative of received signals
within the pass band; and
a phase lock loop circuit connected to said band pass filter to
receive the output therefrom and said circuit producing a
demodulated analog output signal representative of said data.
7. A telemetry system for transmitting data from at least one
location within a living body to an external receiver, a
transmitter comprising:
input means for receiving a signal from said location and providing
an electrical signal representative of said data;
modulator means for receiving said electrical signal and generating
an alternating signal having a frequency which differs from a
predetermined nominal frequency by an amount determined by some
characteristic of said electrical signal;
transmitting antenna means;
current source means;
capacitance means; and
switching means controlled by the alternating signal from said
modulator means to connect said capacitance means to said current
source means during half of the period of said alternating signal
from said modulator means, and to connect said capacitance means to
said transmitting antenna means during the other half of said
alternating signal, said transmitting antenna means radiating a
signal during said other half of said alternating signal;
a receiver comprising:
receiving antenna means adapted to receive a signal radiated by
said transmitting antenna means;
band pass filter means connected to said receiving antenna means to
receive a signal from said receiving antenna and said filter
delivering an output signal representative of received signals
within the pass band; and
a phase lock loop circuit connected to said band pass filter to
receive the output therefrom and said circuit producing a
demodulated analog output signal representative of said data.
.Iadd.
8. A transmitter for transmitting signals from an implantable
medical device which are representative of either analog or digital
values comprising a signal controlled variable frequency oscillator
means having a control terminal, signal application means for
selectively presenting information signals representative of said
digital and analog values to said control terminal to vary the
frequency of said oscillator means in response thereto relative to
a nominal output frequency of said oscillator means that exists
when said information signals are not being supplied, tank circuit
and antenna means having a predetermined ringing frequency of
oscillation when pulsed with energy, and drive circuit means
coupled to said oscillator means and to said tank circuit and
antenna means for supplying pulses of energy to said tank circuit
and antenna means at a rate that is proportional to the output
frequency of said oscillator means whereupon said pulses are
radiated from said tank circuit and antenna means as damped ringing
signals wherein said drive circuit means comprises a storage
capacitor and discharge means for discharging said capacitor into
said tank circuit and antenna means at a rate proportional to the
frequency of said oscillator means so as to cause bursts of energy
to be expended in said tank circuit and antenna means and
transmitted thereby. .Iaddend. .Iadd.9. A transmitter as claimed in
claim 8 wherein said discharge means comprises switching means for
controlling the charging and discharging rate of said storage
capacitor under the control of the output of said oscillator means.
.Iaddend. .Iadd.10. A transmitter as claimed in claim 9 wherein the
frequency of said tank circuit and antenna means is higher than
said nominal frequency of oscillation of said signal-controlled
oscillator means. .Iaddend. .Iadd.11. A transmitter as claimed in
claim 10 wherein said tank circuit and antenna means comprises
inductance means and capacitance means connected in parallel and
said inductance means comprises a radiation coil. .Iaddend.
.Iadd.12. A transmitter as claimed in claim 8 wherein said
discharge means comprises switching means for controlling the
supply of energy pulses to said tank circuit and antenna means
under the control of the output of said signal-controlled
oscillator means. .Iaddend. .Iadd.13. A transmitter as claimed in
claim 12 wherein said tank circuit and antenna means comprises
inductance means and capacitance means connected in parallel and
said inductance means comprises a radiation coil. .Iaddend.
.Iadd.14. A transmitter as claimed in claim 12 wherein the
frequency of said tank circuit and antenna means is higher than
said nominal frequency of oscillation of said signal-controlled
oscillator means. .Iaddend.
Description
DESCRIPTION
BACKGROUND OF THE INVENTION
The present invention relates to a telemetry system for
transmitting information detected by or relating to electronic
devices such as implanted cardiac pacemakers or implanted
medication dispensing devices to an external receiver for recording
and analysis or for retransmission over phone lines to a remote
location.
Adequate evaluation of the operation of implanted electronic
prosthetic devices such as pacemakers is necessary to verify their
proper operation and to avoid undetected premature performance
degradation so that corrective steps may be taken promptly.
Although some systems have been previously described which claim to
have achieved adequate monitoring of one or more variables in
connection with the operation of an implanted pacemaker, those
systems have proven to be cumbersome in providing usable
information to an external terminal.
In addition, the prior art systems have not generally attempted to
solve the problems of transmitting information from a metal encased
implantable device. Prior art systems have also failed to consider
the compatibility of the telemetry system with a programmable
pacemaker or other remotely programmed implantable device which
operates at some times in response to an externally generated
programming signal.
A telemetry patent, U.S. Pat. No. 4,026,305, to Tyers, relates to a
telemetry system for transmitting a signal indicating the battery
voltage of a pacemaker to an external monitor. The system disclosed
in Tyers is not usable for transmission of electrocardiogram
information or for digital data. The Tyers system uses a low pass
system which includes 60 Hz and 120 Hz.
Further, the problem of minimizing the power consumption of a
two-way telemetry system capable of transmission through a metal
shield has not been adequately addressed.
The present invention accordingly provides a two-way telemetry
system utilizing an improved ultra-low power circuit for
transmission of pacemaker EKG, analog data, or stored digital data
to an external terminal. The system operates to permit monitoring
of the electrical activity on the lead of an implantable pacemaker
without attaching external or catheter electrodes to the patient.
The receiver uses a bandpass system which operates above the common
noise frequencies of 60 Hz and 120 Hz.
To receive transmitted data, a receiving coil antenna is placed
over the implantable pacemaker and the pacemaker is commanded by
applying a programming signal to its circuitry to cause it to send
out the electrogram, the pacing pulse, or other data to the remote
receiver.
The transmitting circuitry of the present invention is compatible
with the antenna and receiving circuitry shown in co-pending
application entitled Digital Cardiac Pacemaker, filed Nov. 6, 1978,
under Ser. No. 957,958, now U.S. Pat. No. 4,230,128 naming Ray S.
McDonald as the inventor. That application discloses a pacemaker
pulse generator which can be remotely programmed.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the present invention is hereafter
described with specific reference being made to the following
figures in which:
FIG. 1 is a simplified block diagram of the entire telemetry
system;
FIG. 2 shows the type of damped sinusoid signals transmitted by the
system;
FIG. 3 is a detailed schematic diagram of the transmitter
circuitry;
FIG. 4 is a detailed schematic of the receiving circuitry.
BLOCK DIAGRAM OF TRANSMITTER
Referring now to FIG. 1, there is shown a block diagram of the
implantable transmitter 10 and of the receiver 12 used therewith.
The dashed line 13 generally designates the skin of the patient in
which the transmitter 10 is implanted. The block diagram of the
transmitter 10, which is enclosed within a sealed metallic case 13
made of titanium or some similar material. A solid state switch
module 14 which receives analog data at an input 16 and digitial
data at an input 18 is shown within case 13. The sources of the
analog and digital data provided to inputs 16 and 18 are discussed
more fully below.
The analog signal may, in cardiac pacemaker applications, be
derived from any of a number of sources. Through appropriate
conventional switching, analog signals indicative of the endo or
myoelectrogram or pacing artifact may be provided. Typical pacing
artifacts are the actual pacing pulse, a voltage indicative of the
charging of the pacemaker output stage capacitor or a signal
indicative if lead electrode repolarization. Any low voltage 0.1 Hz
to 80 Hz bandwidth analog signal lends itself to transmission
utilizing the system disclosed.
Digital data suitable for transmission is required to be in
non-return to zero digital pulses shifted at 10 msec per bit. In
digital pacemakers such as the one disclosed in my co-pending
application, continuation-in-part Ser. No. 127,308, filed Mar. 5,
1980, entitled Multimode Adaptable, Implantable Pacemaker, data
relating to the operation of the device is located in a number of
memory locations as 8 data bit words. Such digital data can be
readily put into asynchronous non-return to zero form with a start
bit, 7 or 8 data bits, a parity bit, and a stop bit. The formatting
of the data can be accomplished with commercially available UART
circuits such as the CDP 1854 UART sold by RCA and other
manufacturers.
The digital data input terminal 18 may also be used to generate a
calibrating signal for use in calibrating the EKG channel in a
recorder connected to the telemetry receiver.
Solid state switch 14 acts in response to a digital/analog control
signal on terminal 20 to select either analog or digital data for
transmission. Such a signal can be internally generated within a
programmable digital pacemaker. The output of the solid state
switch 14 is connected to the input of a voltage controlled
oscillator 22 through a conductor 24. The voltage controlled
oscillator 22 also receives a controlled current on conductor 26
which is regulated by a constant current source 28 driven by a
voltage source 30. In the embodiment shown, the supply voltage is
nominally 5 volts. Use of low threshold CMOS circuitry in the
transmitter 10 would allow use of an even lower nominal voltage for
the power source.
The output of the voltage controlled oscillator 22 is connected to
control the operation of a further solid state switch 32 through
conductor 34. Solid state switch 32 receives a controlled current
through conductor 36. The controlled current is supplied by a
current regulating circuit 38 which is in turn connected to voltage
source 30. The output of the solid state switch 32 is connected to
an antenna coil 40 across which is connected a capacitor 42.
The magnitude of the voltage at output of the solid state switch 14
linearly modulates the free running frequency of the voltage
controlled oscillator or VCO 22 as a function of the input voltage.
In the preferred embodiment shown, the free running frequency of
VCO 22 is 1500 Hz and the modulation scale factor is 10 Hz per
millivolt. The 1500 Hz free running frequency was selected to
correspond to the center of the band width of a standard telephone
system, and the scale factor was selected for compatibility with
typical .+-.20 mvolt EKG signals.
The output of voltage controlled oscillator 22 on line 34 is used
to control the action of the solid state switch 32. When the
voltage controlled oscillator 22 has a low output voltage
representative of a logic zero, switch 32 is in the position shown
in FIGS. 1 and 3, and connected to receive current from the
constant current source 38 and charge capacitor 44. In the specific
embodiment shown, the increase of voltage across capacitor 44
during the charging cycle is typically in the vicinity of 18
millivolts. When the output of the voltage controlled oscillator 22
is a high voltage corresponding to a logic 1, the solid state
switch 32 switches to the other position connecting the capacitor
44 to dump its energy into the tuned parallel combination of
capacitor 42 and the antenna coil 40.
It should be pointed out that capacitor 42 tunes antenna coil 40 to
the resonant frequency of input programming pulses as discussed in
the above-identified Digital Cardiac Pacemaker patent application
to Ray S. McDonald. Thus, the antenna coil functions not only as a
transmitting antenna as described herein, but also as a receiving
antenna when switch 32 is in a position to charge capacitor 44.
The tuned combination of capacitors 42 and 44 and coil 40
oscillates at a resonant frequency of 10 kHz. It is important to
limit the damped sinusoid frequency F1 to approximately 10 kHz or
less to minimize the attenuation of the signal by the titanium case
used to enclose the antenna and circuitry. The damping factor of
the equivalent parallel tuned circuit comprising capacitors 42 and
44 and conductor 40 should not exceed 0.2 to assure oscillation of
the tuned circuit.
The damped sinusoidal voltage created across antenna 40 is
important in the operation of the telemetry system and creates a
distinct advantage in performance over systems, such as Tyers,
which drive the transmitting antenna with a fixed level voltage. In
the present invention, use of a damped sinusoidal voltage across
antenna 40 creates an electromagnetic field whose maximum strength
occurs at the frequency F1, which in the preferred embodiment
occurs at a frequency of approximately 10 kHz. In contrast, a fixed
voltage impressed across an antenna such as 40 would cause the
maximum energy to be concentrated at zero frequency and diminish at
higher frequencies in accordance with an envelope of sine F divided
by F as is commonly known in the art. All things being equal, a
receiver such as 12 which detects this radiated energy must be
tuned to a very low frequency to recover the transmitted energy.
This is undesirable since the prevalent noise frequencies of 60 Hz
and 120 Hz are included in the frequencies.
In the preferred embodiment discussed, a receiver such as 12 which
is receiving radiated evergy from an antenna driven with damped
sinusoidal voltage would be tuned at a frequency F1 of 10 kHz to
avoid reception of the noise frequencies of 60 and 120 Hz. Use of
damped sinusoidal frequency modulated signals as disclosed thereby
results in a desirable noise-free operation and a more
cost-effective system since costly noise filters are not
necessary.
FIG. 2 illustrates the nature of the voltage waveform across the
antenna coil 40 as a function of time. The frequency F1 is the 10
kHz resonant frequency of the antenna 40, its associated capacitor
42 and capacitor 44, while the frequency F2 represents the 1.5 kHz
frequency modulated signal which appears at the output of the
voltage controlled oscillator 22.
SCHEMATIC OF TRANSMITTER
FIG. 3 is a detailed schematic of the transmitter circuitry 10
shown in FIG. 1 with the components which are shown in FIG. 1
similarly numbered. The analog input at terminal 16 is connected to
the input terminal of solid state switch 14 through a DC blocking
capacitor 46 which has a sufficiently large capacitance to provide
a low frequency response sufficient to pass a 0.1 Hz sine wave and
large enough to prevent distortion of an EKG signal due to
differentiation.
The signal after passing through blocking capacitor 46, is
connected to the input terminals of the solid state switch 14. The
indicated terminal designations for solid state switches 14 and 44
and the terminals for the voltage controlled oscillator 22 are all
as specified by the manufacturer. Switch 14 operates to connect
either the analog input received at terminal 16 or the digital
input received at terminal 18 to the input of terminal 9 of the
voltage controlled oscillator 22.
Switch 47 is shown in FIG. 3 with its wiper tied to terminals 3 and
6 of switch 14, which are grounded when an analog input is
selected. When a digital input signal or a calibration signal is to
be received at terminal 18, switch 47 is moved to position B with
terminals 3 and 6 tied to a positive voltage. Switch 47 or an
associated logic signal such as 20, shown in FIG. 1, can be
generated by the prosthetic device control logic to cause the
transmitter to select a digital or analog input. Alternatively,
switch 47 can be actuated from outside of the body in which the
transmitter is implanted by using a magnetic reed switch to switch
between input signals.
In the specific circuit shown in FIG. 3, the digital input port 18
is connected to an N channel transistor switch module 14. When the
voltage at terminal 18 is high, the transistor 48 places a low
impedance path across resistor 50, and the voltage change across
resistor 50 which is caused by the switching of transistor 48, is
coupled through a capacitor 52 through the solid state switch
14.
When solid state switch 14 is in position B to receive digital
input data from terminal 18, a path through the switch is provided
from the constant current cirucit 70 used to provide input bias to
the voltage controlled oscillator 22. The bias circuit 70 provides
a current of approximately 0.5 microamperes along conductor 54.
Since in the preferred embodiment, bias resistors 56 and 58 have a
total impedence of approximately 1.5 megohms, and resistor 50 has a
resistance of approximately 20 K ohms, the switching of transistor
48 across resistor 50 does not measurably alter the input bias to
the voltage controlled oscillator 22 unless the switch of the solid
state switch 14 is in position B as shown to select digital
data.
The ten millivolt voltage swing across resistor 50 is transmitted
through capacitor 52 and switch 14 to the input of the voltage
controlled oscillator 22 with a rapid rise time. The digital input
can thus be used to calibrate an EKG recorder receiving the signal
from the transmitter to provide an output signal indicative of a 10
mv input signal.
FIG. 3 also shows a current regulator 28 connected to the power
supply 30. In the preferred embodiment shown, the current regulator
comprised of PNP transistors 60 and 62 and resistors 64 and 66 is
adjusted to provide a constant current of approximately 3.5
microamperes nominal from the collector of transistor 62. Variable
resistor 66 is used to adjust the current delivered.
The output of current regulator 28 is delivered to the positive
supply voltage terminal of the voltage controlled oscillator 22 and
further to the current regulator 70 comprised of transistors 72 and
74 and resistors 76 and 78 which establish a controlled current
output from the collector of transistor 74 of 0.5 microamps nominal
as a bias current to the input of the voltage controlled oscillator
22. Capacitor 80 is connected to the output of the current
regulator 28 to smooth the supply voltage applied to the voltage
controlled oscillator 22. Capacitor 81 is connected between pins 6
and 7 of VCO 22, while resistor 83 is connected between pin 11 and
ground.
The regulation and limiting of the current delivered to the voltage
controlled oscillator 22 limits the current consumption of the
oscillator to a value preset by the constant current source to
prevent voltage controlled oscillator 22 from drawing excessive
current. By limiting the current supplied to voltage controlled
oscillator 22 and to the output switch 32 to an amount slightly
above the minimum current at which they will operate the current
drain can be minimized and held constant as the voltage 22 of the
voltage source 30 decays with time. Since the minimum operating
voltage at which devices can operate varies between devices, it is
not practical to minimize the current drawn by the various circuits
by adjusting the voltage of voltage source 30.
The output of voltage controlled oscillator 22 at pin 4 of
oscillator 22 is a 50 percent duty cycle, frequency modulated 1500
Hz square wave, which is converted to a damped 10 kHz damped
sinusoid by the output stage 32 of the transmitter 10 as described
below.
Current source 38, which is comprised of transistors 82 and 84 and
resistors 86 and 87, supplies a regulated 13 microampere charging
current to a 0.22 microfarad capacitor 44 through solid state
switch 32 when the switch is in position A as shown in FIG. 3.
Since the switch 32 is driven from conductor 32 by the 50 percent
duty cycle output of VCO 22, the average current is 6.5
microamperes nominal. The current from current regulator 38 charges
capacitor 44 to approximately 18 millivolts during the half period
of the 1500 Hertz drive signal from the voltage controlled
oscillator output 34 when the voltage is low.
During the half period when the VCO 22 has a logic one at its
output terminal 4, the switch in solid state switch 32 is in the B
position, which turns the field effect transistor 92 on, providing
a low impedance path for capacitor 44 to discharge the energy
stored in the previous half cycle into the parallel combination of
capacitor 42 and antenna 40. The action of dumping the stored
charge in capacitor 44 into the parallel combination of capacitor
42 and inductor antenna 40 causes the antenna voltage to oscillate
as a damped sinusoid whose frequency and amplitude are readily
controlled by the selection of capacitor values 42 and 44,
inductance and resistance of antenna 40 and the charging current
from current source 38 according to formulae known to those skilled
in the art. The capacitance of capacitor 42 and the inductance of
antenna 40 are selected to resonate at a frequency of 175 K Hertz,
which is the receiving frequency of the antenna of the device
disclosed in the above-identified patent application for Digital
Cardiac Pacemaker of Ray S. McDonald.
In the preferred embodiment shown, capacitor 42 has a capacitance
of 330 picofarads, capacitor 44 has a value of 0.22 microfarads,
and the inductance of the antenna 40 is approximately one
millihenry. Since capacitor 44 is much larger than capacitor 42,
inductor 94 and capacitor 42 resonate as they receive the stored
energy in capacitor 90 as a damped sinusoid at 10 kilohertz. The
frequency of the damped sinusoidal oscillation is controlled
primarily by capacitor 44 and inductor 40, while the repetition
rate of the pulses is determined by the VCO 22. Note also, that
when a switch 32 is in position A, the solid state switch 32 and
the field effect transistor 92 are connected in such a way as to
isolate capacitor 44 from the parallel tuned circuit consisting of
capacitor 42 and inductor 40. This design allows the inductor 40
and capacitor 42 to function as a receiving parallel tuned circuit
without appreciable attenuation caused by other components in the
circuit. Inductor antenna 40 therefore functions not only as a
transmitting antenna, but also as a receiving antenna, which may be
connected to receiving circuitry at terminal 43, thereby achieving
benefits of lower cost and fewer components by having a dual
function. A suitable receiving circuit is shown in the McDonald
application identified above.
The transmitter 10, when not in operation, does not place a load on
antenna 40 which has a significant effect on its ability to receive
externally transmitted information at 175 kHz. Tests of the unit
indicate that the loading of the transmitter on the antenna is less
than 0.5 db when the transmitter is not active.
RECEIVER CIRCUITRY
Referring again to FIG. 1, a block diagram of the receiver section
12 is shown. The receiver consists of an antenna section 100 and a
tuning capacitor 102 which are connected to a band pass filter 103
which delivers its signal to a phase lock loop circuit 104 which
provides its output signal to an amplifier 106 which drives a loud
speaker 108 or some other indicating means.
A more detailed schematic of the circuitry of the receiving section
12 is shown in FIG. 4. The antenna in a preferred embodiment is
wound from 1500 turns of #30 AWG wire in a loop having a diameter
of approximately 10 centimeters. The inductance of the coil is
approximately 0.396 henrys, and the coil was tuned with a shunt
capacitor 102 to resonate at 10 kHz.
The antenna output is connected through a coupling capacitor 110
and a resistor 112 to the input of the first stage 114 of the band
pass amplifier shown in FIG. 1, as block 103. Each stage of the
amplifier provides independent control of low frequency cutoff and
high frequency cutoff frequencies and provides a single order or
six decibels per octave attenuation beyond the respective cutoff
frequencies. The low frequency break point is determined by the
values of resistor 112 and capacitor 110 while the high pass break
point is determined by feed back resistor 116 and capacitor 118 for
the first amplifier 114. Similarly, the low frequency break point
is determined by the values of resistor 122 and capacitor 120 while
the high frequency break point is determined by the values of
resistor 126 and its stray capacitance for amplifier stage 124.
The particular advantage of the band pass amplifier configuration
shown in FIG. 4 is that the amplifier's pulse response does not
produce an oscillatory output because the poles of the amplifier
Bode plot are always real rather then imaginary. This
characteristic of the band pass amplifier is important in the
disclosed receiver, since the received information signal from the
transmitter is a damped sinusoid pulse.
The output of amplifier 114 is connected through a capacitor 120
and a resistor 122 to the input of the second band pass amplifier
stage 124 which has a feed back resistor 126 connected between its
output and its noninverting input terminals. The output of the
second stage 124 of the band pass amplifier 103 is connected
through a capacitor 128 to the input terminal of the phase lock
loop circuit 104 and to one end of a grounded resistor 130.
The phase lock loop circuit is connected in an FM demodulation
circuit arrangement. When the input 2 to the phase lock loop
circuit 104 is grounded, the output frequency at terminals 4 and 5
is 1500 Hz. The pin designations are those indicated by the
manufacturer. The FM demodulator circuit and its characteristics
are described in further detail in Signetic Analog Manual, dated
1976, at Page 623.
Resistor 132 is an adjustable resistor used to set the phase lock
loop circuit at 1500 Hz with its input at terminal 2 grounded. The
oscillator output of the phase lock loop 104 is connected to a
resistor 140 which provides the base drive to a grounded emitter
NPN transistor 142, which in turn, drives a loud speaker or similar
transducer 144 through a resistor 146. The capture range of the VCO
as configured in FIG. 4 is plus or minus 434 Hz, while the lock
range is plus or minus 2 kHz. Since the transmitter 10 has a
voltage controlled oscillator scale factor of 10 Hz per millivolt
deviation, the receiver circuit has the ability to receive an EKG
signal of plus or minus 43 millivolts.
The output stage, including transistor 142, amplifies the 1500
Hertz FM square wave at the output of the voltage controlled
oscillator to drive a small transducer 144 such as a one inch
speaker which can then be positioned near a conventional telephone
handset to transmit the 1500 Hz frequency modulated signal to a
remote EKG machine or a digital data recorder.
Speaker 144 is connected to a telephone to transmit the frequency
modulated signal over telephone lines, a receiver such as the Model
9401 Teletrace R receiver manufactured by Medtronic, Inc. can be
used to demodulate the signal and provide an EKG trace.
Alternatively, an analog signal representation can be directly
obtained from the phase lock loop circuit on terminal 148.
Digital data can be received and converted to non-return to zero
pulses which may then be decoded into numerals and characters using
known terminal devices.
The preferred embodiment of the telemetry system disclosed above
has been found to work well with the following values or part
designations.
______________________________________ Resistors Ohms
______________________________________ 50 20K 56 1.3 M 58 200K 64,
78, 86 10 M 66 100K variable in series with 75K fixed 76 820K 83,
112, 122, 130 100K 116 2.2 M 126 47 M 132 10K variable 140 15K 146
100 87 100K variable Capacitors Microfarads 42 0.000330 44 0.22 46,
80 1.0 102 118 0.000005 110, 120 0.000200 52, 128 0.1 134 0.47 136
.001 81 0.000170 138 0.047 Transistors 60, 62, 72, 74, 82, and 84
2N3799 48 3N171 92 2N6661 142 2N2222 Circuits switch 14 TA 6178 RCA
switch 32 CD 4007 RCA voltage controlled oscillator 22 CD 4046 RCA
amplifiers 114, 124 LM 318 National phase lock loop 104 NE 565
Signetics ______________________________________
* * * * *