Demodulator for frequency-burst-duration modulated signals

Claim 2

In a system for transmission of analog data in the form of a tone signal of duration proportional to the amplitude of an analog signal from a sensor, apparatus for demodulating said tone signal comprising means for rectifying said tone signal, a capacitor for filtering the output of said rectifying means, amplifying means connected to said capacitor for producing at an output terminal thereof a signal of predetermined amplitude while a charge is stored in said capacitor above a set minimum level, and means in parallel with said filter capacitor for automatically discharging said filter capacitor to below said set minimum level while said tone signal is not present, and for permitting said capacitor to charge the moment said tone signal is detected by said rectifying means.

Claim 3

The combination of claim 1 wherein said means for discharging said filter capacitor includes a field-effect transistor having a source, a drain, and a gate, said transistor being connected with its source drain circuit in parallel with said capacitor, said source being connected to one side of said capacitor and said drain and gate being connected to the other side of said capacitor, where said one side is selected of a polarity which provides a high source-gate voltage to bias current through said source drain circuit to a level inversely proportional to the source-gate voltage.

Claim 4

The combination of claim 2 wherein said rectifying means is comprised of a diode bridge connected directly in parallel with said capacitor, and said discharging means includes means for providing initial discharge current for said capacitor such that, upon termination of said tone burst, high initial discharge current from said capacitor is provided.

Claim 5

The combination of claim 3 including means for integrating said output signal for the period of said output signal thereby producing an output proportional to the duration of said Tone burst.

Claim 6

The combination of claim 1 wherein said means for discharging said filter capacitor includes a transistor having input and output terminals and a control terminal, said transistor having its input terminal connected to one side of said capacitor and its output terminal connected to the other side of said input terminal to provide a direct discharge path for said capacitor while a control voltage at said control terminal biases said transistor into a high current state, a unidirectional conducting means coupling said rectifying means to said filter capacitor, a voltage dividing means connected in parallel with said unidirectional conducting means and filter capacitor in series, said voltage dividing means having an output terminal, and means connected between said output terminal of said voltage dividing means and said control electrode of said transistor to bias said transistor into a high current state when the output of said voltage divider is below said set minimum level.

Claim 7

The combination of claim 5 including means for integrating said output signal for the period of said output signal thereby producing an output proportional to the duration of said tone burst.

Claim 8

The combination of claim 6 wherein said integrating means provides said output in digital form representing a number proportional to the duration of said tone burst.

Claim 9

The combination of claim 1 including an active low-pass filter coupling said rectifying means to said filtering capacitor, said low-pass filter comprising a high-gain operational amplifier having a feedback resistor and capacitor in parallel between input and output terminals thereof.

Claim 10

The combination of claim 8 including an input amplifier coupling said tone signal to said filtering means, said input amplifier comprising a high-gain operational amplifier and a feedback resistor between input and output terminals thereof, said operational amplifer having a gain control terminal for voltage control of said gain, and a resistor connected between said gain control terminal of said input amplifer and a junction between said input terminal of said low-pass filter amplifier and said feedback resistor and capacitor of said low-pass filter.

Claim 11

The combination of claim 9 wherein said rectifying means is an active full-wave rectifier comprising a high-gain operational amplifier having two unidirectional current paths between output and input terminals thereof, one of said two paths having a diode poled for current of one polarity and the other of said two paths having a diode poled for current of the other polarity and a feedback resistor in series, and wherein the output of said full-wave rectifier is derived from a junction between said diode and resistor in the other of said two paths.

Claim 12

The combination of claim 10 wherein said means for discharging said filter capacitor includes a transistor having input and output terminals and a control terminal, said transistor having its input terminal connected to one side of said capacitor and its output terminal connected to the other side of said input terminal to provide a direct discharge path for said capacitor while a control voltage at said control terminal biases said transistor into a high current state, a unidirectional conducting means coupling said rectifying means to said filter capacitor, a voltage dividing means connected in parallel with said unidirectional conducting means and filter capacitor in series, said voltage dividing means having an output terminal, and means connected between said output terminal of said voltage dividing means and said control electrode of said transistor to bias said transistor into a high current state when the output of said voltage divider is below said set minimum level.

Claim 13

The combination of claim 11 including means for integrating said output signal for the period of said output signal thereby producing an output proportional to the duration of said tone burst.