FIELD OF INVENTION

This invention relates to inductance or inductive reactance to frequency converter circuits particularly suited for use in monitoring biological processes.

BACKGROUND OF THE INVENTION

Electronic circuitry is widely used to monitor var¬ ious biological processes in critically ill patients and others. In monitoring such biological processes, minor changes therein can be critical. Accordingly, it is desirable to have electronic circuits which are highly sensitive to such small static and dynamic parameter changes and at the same time are relatively insensitive to ambient temperature variations and power supply varia- tions. These type of circuits are normally required to be portable and, as such, utilize batteries as the power source. Thus, it is also desirable for the circuit to have low current consumption.

One biological process which is monitored using elec- tronics is breathing volume. As disclosed in Watson et al U. S. Patent 4,308,872, issued January 5, 1982, assigned to the assignee hereof and incorporated herein by reference, clinically accurate data on breathing volumes for human subjects can be derived from continuous mea- suremeπt of the cross sectional areas of the upper chest and the lower abdomen. The Watsor et al patent discloses that changes in these cross sectional ^{^} areas can be moni¬ tored by measuring, as the patient breathes, the changes in the inductance of extensible conductors looped about these torso portions.

Another biological process which can be monitored by measuring changes in the inductance of an extensible con¬ ductive loop is disclosed in commonly assigned copending application Serial No. 317,418 filed November 2, 1981 and entitled "Surface Inductance Pulthysmography". As discus¬ sed in this application, a conductive loop is disposed on a surface of the subject, and movement of the surface is monitored by detecting changes in the area enclosed by the loop, as reflected by changes in the loop inductance. The contents of said application Serial No. 317,418 are hereby incorporated by reference in their entirety.

One way to measure small changes in the inductance of a loop, as disclosed in the above mentioned commonly owned references, is to use the loop as the inductance element

in an LC resonant circuit. When this is done, the resonance frequency of the circuit varies in response to changes to the inductance of the loop. While this techni¬ que is generally satisfactory under most circumstances, difficulties may arise in attempting to detect relatively small changes in the inductance of the extensible loop. Accordingly, it is desired to provide a circuit which overcomes such potential difficulties.

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DISCLOSURE OF THE INVENTION

As used throughout the specification and claims her¬ ein, the terms inductance and inductive reactance or inductive impedance are used with equal force and effect. The present invention is an improved inductive impedance to frequency converter circuit for producing an oscillat¬ ing electronic output signal whose frequency is capable of variation in response to a variable inductive input impe¬ dance and which is sensitive to small variations in this input impedance. In a particular embodiment of the pre¬ sent invention the improved inductive impedance to fre¬ quency converter circuit comprises a differential gain means and an LC resonant circuit element connected to the differential gain means whose resonance frequency deter- mines the frequency of the output signal. The improvement comprises a transformer comprising a secondary coil con¬ nected in parallel with the-capacitance element in the LC circuit, and a primary coil inductively coupled to the secondary coil and capable of being connected with the variable inductive input impedance, which illustratively may be an extensible conductor loop of the type previously described for use in monitoring biological processes, whereby a relatively small variation in the variable inductive input impedance is measurable as a significant variation in the resonance f equency of the resonant cir¬ cuit.

In a presently preferred embodiment of the invention, the differential gain means comprises a pair of emitter coupled bipolar transistors, the collector of the first transistor being coupled through a capacitor to the base of the second transistor and the collector of the second transistor being coupled through a capacitor to the base of the first transistor. In this case the output signal is taken at the collector of one of the transis- tors. In addition, the capacitance element of the LC cir¬ cuit and the secondary coil of the transformer, are con¬ nected between the collectors of the transistors. In alternative embodiments of the invention, the differential gain means may comprise a pair of cross coupled FETs or an

operational amplifier. In each of these instances, the LC resonant circuit is preferably connected between the high voltage terminals of the active elements forming the dif¬ ferential gain means. It should be noted that the above-described cross coupling arrangement results in an increased insensitivity to supply voltage variations.

In the above-described presently preferred embo¬ diment of the invention, a small change in the variable inductive input impedance connected to the primary coil of the transformer will cause a relatively large change in the effective inductive impedance of the LC resonant cir¬ cuit as seen across the terminals of the capacitance ele¬ ment in the LC circuit and, consequently, will cause a relatively large change in the resonance frequency which is essentially determined by the product of the capac¬ itance and the effective inductance in the LC resonant circuit. Typically, the ratio betwen a change in the input inductance and the corresponding change in effective inductance in the resonant circuit is dependent upon the turn ratio and core permeability of the transformer.

Thus, the improved inductive impedance to frequency con¬ verter circuit employing a transformer exhibits a much greater frequency change in response to a small change in inductive input impedance than is the case in prior art circuits in which no such transformer is employed.

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BRIEF DESCRIPTION OF THE DRAWING

The drawing schematically illustrates an improved inductive impedance to frequency converter circuit in accordance with an illustrative emodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An improved inductive impedance to frequency con¬ verter circuit for producing an oscillating electronic output signal whose frequency is variable in response to a variable input inductance is designated 10 in the FIGURE and is shown within the dotted lines. The circuit 10 shown in the FIGURE is intended to be illustrative only and numerical parameters stated in connection therewith are not intended to limit the scope of the invention. In the embodiment of the invention shown in the

FIGURE, the inductive reactance to frequency converter 10 includes differential gain means 40 comprising identical emitter coupled n-p-n transistors 42 and 44. The collector of transistor 42 is coupled to the base of transistor 44 through a capacitor 46 and the collector of transistor 44 is coupled to the base of transistor 42 through another capacitor 48. Resistors 50, 52, 54, 56 and 58 are used to establish the operating points of transistors 42 and 44. Illustratively, transistors 42 and 44 are 2N3904 transis- tors and resistors 50, 52, 54, 56 and 58 are 15K, 33K, 33K, 4K7, and 2K7 resistors, respectively.

The frequency of the oscillating output signal, which is taken at the collector of transistor 44, is determined by the resonance frequency of a resonant circuit 20 having an effective capacitance C and an effective inductance L. Illustratively, effective capacitance C may comprise a pair of capacitors 60 and 62 connected in parallel. By way of example, capacitor 60 may be a 2200-5500pf NP0 capacitor and capacitor 62 may be a 390pf N750 temperature compensating capacitor having a negative linear tempera¬ ture coefficient.

Effective inductance L is preferably determined by a transformer 64 comprising a diagrammatically illustrated magnetic core 66, a secondary coil 68 surrounding the core 66 and connected in parallel with the effective capac¬ itance C, and a primary coil 70 also surrounding the core 66. The primary coil 70 is preferably capable of being connected to a variable inductive input impedance, which is illustratively shown as a conventional extensible

O PI

conductor loop 30 such as shown in the aforementioned references. Illustratively, core 66 is a 905P A60 3B9 manufactured by Ferrox Cube Inc. and having a 10 mil air gap. The secondary coil 68 may illustratively comprise about _ turns of #36 wire and the primary coil 70 may illustratively comprise about 10_ turns of #31 wire. The circuit 10 is preferably operable on a standard 12V volt¬ age supply, which has been omitted for purposes of clar¬ ity. The standard 12V voltage supply is illustratively shown as being connected to the secondary coil 68 of the transformer 64 at about the 35th turn through an optional diode 72 whose purpose will be discussed below. The power supply is also connected to the base of transistor 44 through resistors 50 and 54 and to the base of transistor 42 through resistors 50 and 52. With the aforementioned choice of components, effective capacitance C has a nega¬ tive linear temperature coefficient and effective induc¬ tance L has a positive linear temperature coefficient, resulting in the product of L and C, and consequently the resonance frequency, having a decreased temperature sensi¬ tivity.

As previously indicated, one possible application of the preferred circuit 10 is for monitoring movement of a surface by measuring inductance changes resulting from changes in the cross-sectional area of an extensible con¬ ductor loop, such as loop 30, disposed on the surface.

This is accomplished by preferably connecting the ends of the loop 30 across the terminals of primary coil 70. When this is done, changes in the cross sectional area enclosed by the loop 30, and hence in its inductance, result in correspondingly large changes in the effective inductance L of the resonant circuit 20, thereby causing significant changes in the frequency of the oscillator output signal taken at the collector of transistor 44. In this and similar applications, the output signal from the preferred circuit 10 of the present invention is preferably amplified using, for example, a common emitter amplifier 10 of the present invention. Such a common emitter amplifier 80 is shown by way of example in the

FIGURE and comprises p-n-p transistor 82 and biasing resistors 84 and 86. The voltage drop across the diode 72, such as 0.7V, preferably insures that transistor 82 is always on. Preferably, the output signal of the preferred "5 circuit 10 is amplified since at relatively small values of input inductive reactance, which are typical for some applications here contemplated, the amplitude of the oscillating output signal is relatively small. Illustratively, transistor 82 is a 2N3906 transistor and 0 resistors 84 and 86 are 470 and 22K ohm resistors, respec¬ tively.

A capacitor 88 is provided as an AC coupling capac¬ itor which couples the output of the common emitter ampli¬ fier 80 into a voltage divider 90 formed from resistors 92 5 and 94, each of which illustratively has a resistance of 220K. The voltage divider 90 insures that the amplified oscillating signal from the common emitter amplifier 80 oscillates around a base voltage of 6 volts, by way of example. This is often advantageous for subsequent signal 0 processing operations.

In a typical application, the amplified oscillating electronic signal is processed by a commercially available divide-by-16 frequency divider 96, such as a CD4040. A resistance 98 which illustratively is a 4K7 resistor, is 5 preferably used to convert the output of the frequency divider 96 from a voltage signal to a current signal to * minimize interference during subsequent signal transmis¬ sion. The output signal from the frequency divider 96 may then preferably be processed by a conventional frequency to voltage converter (not shown) so that small changes in the variable input inductance are detectable as changes in a voltage output. In a typical application, the preferred circuit 10, the common emitter amplifier 80, the frequency divider 96, and the resistor 98, are all preferably packaged together and would preferably be situated near the extensible conductor loop 30 which serves as the var¬ iable inductive input impedance.

The circuit 10 has a number of advantages which make it well suited for medical applications. Illustratively,

the temperature sensitivity is about 20 parts per million per degree Centigrade. The circuit 10 has a wide dynamic frequency range and is highly sensitive to small changes in input inductance or inductive reactance. When the above mentioned illustrative components are used to form the circuit 10, the output frequency is about 200 kilohertz for an open primary coil 70 and about 900 kilohertz when the primary coil 70 is shorted. The illus¬ trative circuit 10 shown in the FIGURE has been empiri- cally optimized using the above-mentioned components to achieve these results.

The above-described circuit 10 is intended to be illustrative only, and numerous alternative embodiments of the invention may be devised by those skilled in the art without departing from the spirit and scope of the present invention as set forth in the following claims.