Logarithmic resistance-to-frequency converter

Claim 2

A logarithmic resistance-to-frequency converter which produces a square-wave signal whose period is proportional to the natural logarithm of a resistance ratio comprising, a comparator amplifier having two input terminals and two output terminals, a first resistor, a second resistor connected in series with said first resistor to form first and second arms of a bridge circuit with said resistance ratio being determined by the resistance ratio of said first and second resistors, a third resistor, a fourth resistor, a capacitor, means connecting the third resistor in circuit to form a third arm of the bridge and the series combination of said fourth resistor and said capacitor as the fourth arm of the bridge, means connecting the bridge input terminals to the comparator amplifier output terminals and the bridge output terminals to the comparator amplifier input terminals so that the junction of the first and second bridge arms supplies a positive feedback voltage to the comparator amplifier input terminal to which it is connected whereby the circuit arrangement will self-oscillate to produce a square-wave signal at the comparator amplifier output terminals whose period varies as the natural logarithm of the resistance ratio of said first and second resistors.

Claim 3

Converter as claimed in claim 1 wherein the third and fourth resistors have equal resistance values and the first resistor comprises a measuring-value transducer.

Claim 4

Converter as claimed in claim 2, characterized in that the measuring-value transducer comprises a temperature transducer the resistance value R 1 of which satisfies the relation R 1 = R 0 e K/T , where R 0 and K are constants and T is the absolute temperature in degrees Kelvin.

Claim 5

A converter as claimed in claim 2, characterized in that a counter is connected to the output terminals of the comparator amplifier and a counter reading recorded in a predetermined time interval is a measure of the measuring value.

Claim 6

A converter as claimed in claim 1 wherein said first resistor is of the type that exhibits a non-linear resistance variation as a function of a physical parameter.

Claim 7

A converter as claimed in claim 1 wherein said first resistor exhibits an exponential resistance variation as a function of a physical parameter and the bridge components are selected so that the period of the square-wave signal is determined by the absolute values of the third and fourth resistors and the capacitor and the natural logarithm of the resistance ratio of the first and second resistors.

Claim 8

A converter as claimed in claim 1 wherein the bridge forms a positive feedback circuit for the comparator amplifier to sustain oscillations and the bridge output terminals are connected to the comparator amplifier input terminals so that the junction of said first and second resistors is connected to the non-inverting input of the comparator amplifier and the junction of the third and fourth bridge arms is connected to the inverting input of said comparator amplifier.

Claim 9

A logarithmic converter which produces a square wave signal whose period is proportional to the natural logarithm of a resistance ratio comprising, a voltage comparator having two input terminals and two output terminals and means responsive to the zero passage of the voltages at the two input terminals for switching the voltage at the two output terminals between first and second fixed values of voltage, a first resistor, a second resistor connected in series with said first resistor to form first and second arms of a bridge circuit with said resistance ratio being determined by the resistance ratio of said first and second resistors, third and fourth resistors having the same value of resistance, a capacitor, means connecting the third resistor in circuit to form a third arm of the bridge and the series combination of said fourth resistor and said capacitor as the fourth arm of the bridge, means connecting the bridge input terminals to the voltage comparator output terminals and the bridge output terminals to the voltage comparator input terminals so that the circuit arrangement will self-oscillate to produce a square-wave signal at the voltage comparator output terminals whose period varies as the natural logarithm of the resistance ratio of said first and second resistors.

Claim 10

A logarithmic converter which produces a square wave signal whose period is proportional to the natural logarithm of a resistance ratio comprising, a voltage comparator having two input terminals and two output terminals and means responsive to the zero passage of the voltages at the two input terminals for switching the voltage at the two output terminals between first and second fixed values of voltage, a first resistor, a second resistor connected in series with said first resistor to form first and second arms of a bridge circuit with said resistance ratio being determined by the resistance ratio of said first and second resistors, third and fourth resistors having the same value of resistance, a capacitor, means connecting the third resistor in circuit to form a third arm of the bridge and the series combination of said fourth resistor and said capacitor as the fourth arm of the bridge, means connecting the bridge input terminals to the voltage comparator output terminals and the bridge output terminals to the voltage comparator input terminals so that the circuit arrangement will self-oscillate to produce a square-wave signal at the voltage comparator output terminals whose period varies as the natural logarithm of the resistance ratio of said first and second resistors.

Claim 11

A converter as claimed in claim 8 wherein said voltage comparator comprises differential amplifier means for comparing the voltages at said two input terminals and for producing at its output terminals said first and second fixed values of voltage, said first and second voltages being of opposite polarity as a function of the relative magnitudes of the voltages at the two input terminals, and the voltage at the bridge output terminal formed by the junction of said first and second bridge arms provides positive feedback and operates as a reference voltage for the zero passage of the voltage at the other bridge output terminal and which varies exponentially with the charge and discharge of said capacitor.