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A circuit for coupling a transducer having a reactive sensing element to equipment providing either AC or DC drive voltage and equipped with strain gauge amplifiers in which the sensing impedance varies the gain of the feedback loop of an oscillator, an AGC circuit is coupled to the loop so as to de

A circuit for coupling a transducer having a reactive sensing element to equipment providing either AC or DC drive voltage and equipped with strain gauge amplifiers in which the sensing impedance varies the gain of the feedback loop of an oscillator, an AGC circuit is coupled to the loop so as to derive a control signal related to the amplitude of oscillations therein and a gain control means is included in the loop that responds to the control signal in such manner as to restore the loop gain to unity after it has been changed therefrom by changes in the value of the sensing impedance. If the gain control means is a voltage divider having a variable resistor as the operative element, a matched variable resistor is used as the unbalancing element of a resistive bridge, and the values of the circuit are chosen such that the output signal from the bridge which may be applied to strain gauge amplifiers is linearly or desirably nonlinearly related to changes in the value of the reactive sensing impedance. Operating potentials for the oscillator are derived by a rectifier from the drive voltage. If the control signals is to be used as a linear output signal, the variable resistor of the voltage divider is made to have a suitably nonlinear relation with respect to the value of the control signal. The variable resistor can be a nonlinear digitally controlled resistor in which event the control signal can be digitized without using an A/D converter.

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1. A transducer circuit for producing an output signal in response to variations in the value of a reactive sensing impedance, comprising amplifying means having an output and an input, a loop coupling said output to said input so as to cause oscillations to occur therein, said loop having circ

1. A transducer circuit for producing an output signal in response to variations in the value of a reactive sensing impedance, comprising amplifying means having an output and an input, a loop coupling said output to said input so as to cause oscillations to occur therein, said loop having circuit components for determining the frequency at which the oscillations occur, a reactive sensing impedance coupled to said loop so that changes in the value of the reactive sensing impedance effect a change in the gain of the loop, means responsive to an applied signal for controlling the gain of said loop, an automatic gain control circuit applied to said loop for providing a control signal at its output, said control signal having a value corresponding to the amplitude of oscillations occurring in said loop, means for applying the control signal from said automatic gain control circuit to said means for controlling the gain of said loop so as to return the gain of the loop to unity after the gain is made to depart from unity by a change in the value of said reactive sensing impedance, and an output coupled to the output of said automatic gain control circuit. 2. A transducer circuit as set forth in claim 1 further comprising rectifying means having an input to which a drive voltage may be applied and an output, said output being coupled to provide operating potential for said amplifying means, a bridge circuit coupled to said output, means for applying a drive voltage, when present, across one diagonal of said bridge, said bridge circuit having a variable resistive means connected between given points thereof for controllably unbalancing the bridge in response to a signal applied to said variable resistive means whereby the resistance of said variable resistive means is controlled by the control signal and an output signal is provided by said bridge across its other diagonal. 3. A transducer circuit as set forth in claim 2 wherein said means for controlling the gain of said loop is a resistive voltage divider having a variable resistive means connected between predetermined points thereof, and wherein a first ratio of the resistance of said bridge as viewed from said given points where said variable resistive means is connected, with said variable resistive means removed, to the resistance presented by said voltage divider at said predetermined points where said variable resistive element is connected, with said variable resistive element removed, is the same as a second ratio of the resistance of said variable resistance means in said bridge circuit to the resistance of said resistive element in said voltage divider for different values of said control signal. 4. A transducer circuit as set forth in claim 3 wherein said first ratio is different from said second ratio so as to make the output signal provided by the bridge nonlinear. 5. A transducer circuit as set forth in claim 3 wherein the ratio of the voltage between said points in the bridge circuit when said variable resistive means is removed therefrom and the voltage between said points when said variable resistive means is connected between said points is inversely proportional to the gain introduced into the loop by said gain control means. 6. A transducer circuit as set forth in claim 1 wherein said gain control means for said loop responds to the gain control signal in such manner that the gain control signal varies linearly with changes in the value of said reactive sensing impedance so that said gain control signal is the output signal. 7. A transducer circuit as set forth in claim 1 in which a comparator having one input connected to a point of reference potential, the other input coupled to said automatic gain control circuit and its output connected to the up/down terminal of a counter, a clock connected to a clock terminal of said counter, a decoder coupled between the output of said up/down counter and the control input of said digitally controlled resistive element, the decoder being such that the variation in resistance it causes said digitally controlled resistor to have is such that the digital output of said up/down counter represents a value that varies linearly with changes in the value of said reactive sensing impedance. 8. A transducer circuit, comprising a first operational amplifier having positive and negative inputs and an output, said positive input being connected to a point of reference potential, a first capacitor connected between the output of said amplifier and its negative terminal, and one side of a second capacitor connected to said negative terminal, one of said capacitors being a sensing impedance such that its value can be changed by a phenomenon of interest, a second operational amplifier having positive and negative inputs and an output, a third capacitor and a resistor connected in series between the output of said first amplifier and the negative input ot said second amplifier, the positive input of said second amplifier being connected to a point of reference potential, and a parallel combination of a fourth capacitor and a resistor connected between the output of said second amplifier and its negative input, an attenuator comprised of first and second resistors and a variable resistive element connected in series in the order named between the output of said second amplifier and a point of reference potential, the junction of said first and second resistors being connected to the other side of said second capacitor so as to form an oscillatory loop that oscillates at a frequency determined by said third and fourth capacitors and the resistors with which they are connected, an automatic gain control circuit coupled to said oscillating loop so as to produce a control signal having an amplitude that varies in accordance with the amplitude of oscillations occurring in said loop, means coupled to said automatic gain control circuit for causing the resistance of said variable resistive element to vary in accordance with the amplitude of the control signal, a resistive bridge circuit having a variable resistive means that can unbalance the bridge, means for applying a drive voltage, when present, across said bridge, rectifying means for deriving operating potentials for said amplifiers from said drive voltage, when present, and means for changing the resistance of said resistive means with the control signal so as to cause the bridge to produce an output signal having an amplitude variation that is related to changes in the value of the capacitor that serves as a sensing impedance. 9. A transducer circuit as set forth in claim 8 wherein said variable resistive element in said attenuator and said variable resistive means in said bridge circuit are matched. 10. A transducer circuit as set forth in claim 9 wherein said resistive bridge is comprised of third, fourth, fifth and sixth resistors connected in series between first and second terminals connected to opposite sides of said drive voltage, when present, the junction of said fourth and fifth resistors being connected to a point of reference potential and said variable resistive means being connected between the junction of said third and fourth resistors and the junction of said fifth and sixth resistors, seventh and eighth resistors connected in series between said terminal that is connected to said third resistor and the junction of said fourth and fifth resistors, ninth and tenth resistors connected between said terminal that is connected to said sixth resistor and the junction of said third and fourth resistors, whereby an output signal appears between the junction of said seventh and eighth resistors and the junction of said ninth and tenth resistors. 11. A circuit for coupling a reactive sensing impedance to a bridge circuit in such manner that the relationship between the output signal of the bridge has a desired degree of non-linearity with respect to changes in the value of the reactive sensing impedance caused by a phenomenon of interest, comprising a loop including amplifying means contributing a gain of G 1 to the loop, means for coupling said reactive sensing impedance to said loop in such manner as to contribute a gain of G 2 to the loop, there being a given value of G 2 when the sensing impedance is free from the effect of the phenomenon of interest, and gain control means having a variable element connected to predetermined points thereof, said gain control means contributing a gain of G 3 to the loop, there being a given value of G 3 when said reactive sensing impedance is free from the effect of the phenomenon of interest, the product of G 1, the given value of G 2 and the given value of G 3 being unity so that said loop oscillates at a frequency determined by circuit components included in said loop, an automatic gain control circuit coupled to said loop for producing a gain control signal related to the amplitude of the oscillations in the loop, means coupling said gain control signal to said variable element of said gain control means so as to cause it to change G 3 in such direction as to compensate for any changes that may be brought about in G 2 by changes in the value of the sensing impedance and thereby keep the gain of the loop at unity, a resistive bridge circuit having variable resistance means connected between points therein in such manner that changes in the value of the said variable resistance unbalance the bridge, means coupling said gain control signal to said variable resistance so as to vary its value, and the ratio of the voltage between said points in the bridge circuit when said variable resistive means is removed from the circuit and the voltage between said points for a value of the gain control signal when the variable resistance means is connected in the bridge circuit is 1/G 3, the latter value of G 3 occurring in response to the latter gain control signal. 12. A circuit for coupling a reactive sensing impedance to a bridge circuit in such manner that the relationship between the output signal of the bridge has a desired degree of nonlinearity with respect to changes in the value of the reactive sensing impedance caused by a phenomenon of interest, comprising a loop including amplifying means contributing a gain of G 1 to the loop, means for coupling said reactive sensing impedance to said loop in such manner as to contribute a gain of G 2 to the loop, there being a given value of G 2 when the sensing impedance is free from the effect of the phenomenon of interest, and gain control means having a variable element connected to predetermined points thereof, said gain control means contributing a gain of G 3 to the loop, there being a given value of G 3 when said reactive sensing impedance is free from the effect of the phenomenon of interest, the product of G 1, the given value of G 2 and the given value of G 3 being unity so that said loop oscillates at a frequency determined by the circuit components included in said loop, an automatic gain control circuit coupled to said loop for producing a gain control signal related to the amplitude of the oscillations in the loop, means coupling said gain control signal to said variable element of said gain control means so as to cause it to change G 3 in such direction as to compensate for any changes that may be brought about in G 2 by changes in the value of the sensing impedance and thereby keep the gain of the loop at unity, a resistive bridge circuit having variable resistance means connected between given points therein in such manner that changes in the value of the said variable resistance can imbalance the bridge, means coupling said gain control signal to said variable resistance so as to vary its value, the ratio of the input impedance of said bridge viewed from said given points where the variable resistance means is connected, with the variable resistance means removed from the circuit, to the impedance of said gain control means viewed from the predetermined points where its variable element is connected, with the variable element removed from the circuit, is the same as the ratio of the impedance of said variable resistance to the impedance of the variable element in said gain control means for different values of the control signal. 13. A circuit as set forth in claim 12 wherein said variable element of said gain control means and said variable resistance means of said bridge circuit are matched. 14. A circuit as set forth in claim 12 wherein said variable element of said gain control means and said variable resistance means of said bridge circuit are matched FETs. 15. A circuit as set forth in claim 12 wherein said gain control means is a voltage divider comprised of a series combination of first and second resistors and a variable resistance element, said gain control means being inserted into said loop such that the oscillatory voltage at one point in the loop is applied across said series combination and the output of said gain control means is taken across said variable resistance element and the one of said resistors to which it is connected, and wherein the two ratios are different.