Active filter for selecting and controlling signals

Claim 2

A signal control circuit which derives a sinusoidal output signal having a substantially constant amplitude from a particular sinusoidal input signal which has a predetermined normal frequency and an amplitude that is subject to variation, such signal control circuit including in combination: resonant circuit means having input, control and output terminals, said resonant circuit means normally having a first resonant frequency which is substantially equal to the normal frequency of the particular input signal, said input terminal of said resonant circuit means being connected to receive the particular input signal, said resonant circuit Means developing the output signal at said output terminal thereof in response to the particular input signal, said output signal having an amplitude that tends to vary in response to the amplitude variations of the particular input signal; amplitude responsive means having input and output terminals, first circuit means connecting said input terminal of said amplitude responsive means to said output terminal of said resonant circuit means so that said amplitude responsive means receives the output signal, said output terminal of said amplitude responsive means being connected to said control terminal of said resonant circuit means, said amplitude responsive means providing control signals to said control terminal in response to and having durations proportional to portions of the output signal that have instantaneous values exceeding a threshold value, said durations of said control signals thereby varying with the amplitude of the particular input signal; and said resonant circuit means being responsive to the existence of each of said control signals to operate at a second resonant frequency during said portions, said resonant circuit means decreasing the amplitude of the output signal derived from the particular input signal in response to being operated at said second resonant frequency, said second resonant frequency being chosen with respect to the predetermined normal frequency of the particular input signal to cause the amplitude of the output signal to remain substantially constant in the presence of the variations in the amplitude of the particular input signal.

Claim 3

The signal control circuit of claim 1 wherein said amplitude responsive means includes: first electron control means having first, second and third electrodes, said first electrode being connected to receive the output signal, second circuit means being adapted to connect said second electrode to reference potential supply means; load resistor means having first and second ends, said first end of said load resistor means being connected to said third electrode, third circuit means being adapted to connect said second end of said load resistor means to a bias supply means; said first electron control means being normally nonconductive and having a turn on characteristic which establishes said threshold value whereby said control signals are developed at said first end of said load resistor means; and normally conductive means being coupled between said first end of said load resistor means and said control terminal of said resonant circuit means.

Claim 4

The signal control circuit of claim 2 wherein: said normally conductive means includes second electron control means having first, second and third electrodes, fourth circuit means being adapted to connect said first electrode of said second electron control means to reference potential supply means, said second electrode of said second electron control means being connected to said first end of said load resistor means, said third electrode of said second electron control means being coupled to said control terminal of resonant circuit means; said second electron control means being rendered normally conductive by voltage and current supplied through said load resistor means so that a first control signal is coupled to said control terminal of said resonant circuit means, said first control signal operating said resonant circuit means to said first resonant frequency; said first electron control means being rendered conductive by said portions of the output signal which exceed said threshold value; and said second electron control means being rendered nonconductive by said first electron control means being rendered conductive so that a second control signal is coupled to said control terminal of said resonant circuit means, said second control signal operating said resonant circuit means to said second resonant frequency during said portions.

Claim 5

The signal control circuit of claim 1 wherein said reSonant circuit means has frequency response characteristics similar to those of a parallel tuned circuit.

Claim 6

The signal control circuit of claim 4 wherein said amplitude responsive means includes: amplifying means having an input terminal connected to receive the output signal and an output terminal, said amplifying means having said threshold value and increasing the amplitude of said portions of the output signal which exceed said threshold value to cause said portions to be a predetermined amount greater than said threshold value so that said control signals have durations which approach one-fourth of the period of the output signal, said control signals operating said resonant circuit means to said second resonant frequency during said durations so that the phase difference between the particular input signal and the output signal of said resonant circuit means is virtually constant even though the frequency of the particular input signal varies.

Claim 7

The signal control circuit of claim 5 wherein said phase difference between the particular input signal and the output signal is maintained at substantially 70* to facilitate damping of the output signal within a predetermined time in response to a damping input signal having a selected phase difference with respect to the phase of the particular input signal.

Claim 8

The signal control circuit of claim 1 wherein said resonant circuit means includes: active filter means; component means for determining the resonant frequency of said filter means, said filter means operating at said first resonant frequency in response to said component means being connected thereto and at said second resonant frequency in response to said component means being disconnected therefrom; and normally conductive means having first and second terminals connected in series with said component means and said filter means, said normally conductive means also having a control terminal connected to said amplitude responsive means so that said normally conductive means receives said control signals, and said normally conductive means being rendered nonconductive between said first and second terminals thereof only during and in response to said control signals, said normally conductive means acting to thereby disconnect said component means from said filter means and operate said filter means at said second resonant frequency during the existence of said control signals and at said first resonant frequency during the nonexistence of said control signals.

Claim 9

The signal control circuit of claim 1 wherein said resonant circuit means includes: active filter means; component means for determining the resonant frequency of said filter means, said filter means operating at said second resonant frequency in response to said component means being connected thereto and at said first resonant frequency in response to said component means being disconnected therefrom; and normally nonconductive means having first and second terminals connected in series with said component means and said filter means, said normally nonconductive means also having a control terminal connected to said amplitude responsive means so that said normally nonconductive means receives said control signals, said normally nonconductive means being rendered conductive between said first and second terminals thereof only during and in response to said control signals, said normally nonconductive means acting to thereby connect said component means to said filter means and operate said filter means to said second resonant frequency during the existence of said control signals and at said first resonant frequency during the nonexistence of said control signals.

Claim 10

The signal control circuit of claim 7 wherein said active filter means includes: first operational amplifier means arranged to operate as a phase inverter and having first and second inputs and an output, said first input of said first operational amplifier means being connected to reCeive the particular input signal; second operational amplifier means arranged to operate as a first integrator and having an output and an input, said input of said second operational amplifier means being coupled to said output of first operational amplifier means; third operational amplifier means arranged to operate as a second integrator and having an input coupled to said output of said second operational amplifier means and an output coupled to said second input of said first operational amplifier means; and the sinusoidal output signal occurring at said output of said second operational amplifier means in response to the particular input signal being applied to said first input of said first operational amplifier means.

Claim 11

The signal control circuit of claim 7 wherein said amplitude responsive means and said normally conductive means are comprised of: bias supply means providing first and second bias potentials at first and second output terminals thereof and a reference potential at a third output terminal thereof, said reference potential having a value between the values of said first and second bias potentials; differential amplifier means having first and second differential inputs and a differential output, said first differential input being connected to receive the output signal; potentiometer means having a first terminal connected to said first output terminal of said bias supply means, a second terminal connected to said second output terminal of said bias supply means and an adjustable terminal connected to said second differential input, said adjustable terminal providing a selected portion of the potential difference between said first and second bias potentials to establish said threshold value at said second differential input; switchable means having first, second and third terminals, said first terminal being connected to said differential output, said second terminal being connected to said third output terminal of said bias supply means, said third terminal of said switchable means being connected to said component means; and said differential amplifier means normally providing a first output signal state at said differential output which renders said switchable means normally conductive so that said component means is normally connected to said active filter means to operate said filter means at said first resonant frequency, said differential amplifier means responding to said portions of the output signal of said active filter means during which the instantaneous magnitudes thereof exceed said threshold value to provide a second output signal state at said differential output, said switchable means being rendered nonconductive by said second output signal state and thereby disconnecting said component means from said filter means to operate said filter means to said second resonant frequency during said portions.

Claim 12

The signal control means of claim 7 wherein said component means is a resistor means.

Claim 13

In a selective calling system providing a first selected audio frequency input signal having varying frequency and amplitude, and a second selected audio frequency input signal which has the same frequency as the nominal frequency of the first selected input signal and which is out-of-phase with respect to the first selected input signal, with the second selected input signal being developed by the system immediately upon the termination of the first selected input signal, a signal control means for providing a sinusoidal output signal which has a predetermined substantially constant amplitude and phase shift with respect to the first selected input signal thereby enabling the output signal to be damped out within a predetermined time by the second selected input signal, such signal control means including in combination: active filter means having a frequency response characteristic similar to the frequency response characteristic of a tuned circuit and input, control and output teRminals, said active filter means normally having a first resonant frequency which is substantially equal to the frequency of the first selected input signal; means for applying the first and second selected input signals to said input terminal of said active filter means, said active filter means developing the output signal at said output terminal thereof which has an amplitude that tends to vary slightly with the amplitude variations of the first selected input signal; amplitude sensing means having input and output terminals; means for applying the output signal to said input terminal of said amplitude sensing means; means connecting said output terminal of amplitude sensing means to said control terminal of said active filter means; said amplitude sensing means providing control signals to said control terminal in response to and during the portions of alternate half cycles of the output signal that have instantaneous magnitudes greater than a threshold level; said active filter means changing its resonant frequency from said first resonant frequency to a second resonant frequency in response to said control signals so that the selected input signal is attenuated in proportion to its amplitude to develop the output signal having an amplitude maintained at the predetermined substantially constant value; and said threshold level being selected to be less than the usual amplitude of the output signal so that the phase difference between the first selected input signal and the output signal is substantially constant, said predetermined substantially constant amplitude and phase of the output signal enabling the second selected input signal to damp out the output signal within the predetermined time after the first selected input signal ceases.

Claim 14

In a selective calling system providing a first selected audio frequency input signal having varying frequency and amplitude, and a second selected audio frequency input signal which has the same frequency as the nominal frequency of the first selected input signal and which is out-of-phase with respect to the first selected input signal, with the second selected input signal being developed by the system immediately upon the termination of the first selected input signal, a signal control means for providing a sinusoidal output signal which has a predetermined substantially constant amplitude and phase shift with respect to the first selected input signal thereby enabling the output signal to be damped out within a predetermined time by the second selected input signal, such signal control means including in combination: active filter means having a frequency response characteristic similar to the frequency response characteristic of a tuned circuit and input, control and output teRminals, said active filter means normally having a first resonant frequency which is substantially equal to the frequency of the first selected input signal; means for applying the first and second selected input signals to said input terminal of said active filter means, said active filter means developing the output signal at said output terminal thereof which has an amplitude that tends to vary slightly with the amplitude variations of the first selected input signal; amplitude sensing means having input and output terminals; means for applying the output signal to said input terminal of said amplitude sensing means; means connecting said output terminal of amplitude sensing means to said control terminal of said active filter means; said amplitude sensing means providing control signals to said control terminal in response to and during the portions of alternate half cycles of the output signal that have instantaneous magnitudes greater than a threshold level; said active filter means changing its resonant frequency from said first resonant frequency to a second resonant frequency in response to said control signals so that the selected input signal is attenuated in proportion to its amplitude to develop the output signal having an amplitude maintained at the predetermined substantially constant value; and said threshold level being selected to be less than the usual amplitude of the output signal so that the phase difference between the first selected input signal and the output signal is substantially constant, said predetermined substantially constant amplitude and phase of the output signal enabling the second selected input signal to damp out the output signal within the predetermined time after the first selected input signal ceases.

Claim 15

The signal control means of claim 12 wherein said phase difference between the first selected input signal and the output signal is on the order of 70* and said threshold level is on the order of one-fourth the amplitude that the output signal otherwise would have if the signal control means was not developing said control signals.

Claim 16

The combination of claim 12 wherein said active filter means further includes: first operational amplifier means arranged to operate as a phase inverter and having first and second inputs and an output, said first input of said first operational amplifier means being connected to receive the first and second selected input signals; second operational amplifier means arranged to operate as a first integrator and having an output and an input, said input of said second operational amplifier means being coupled to said output of said first operational amplifier; equivalent resistance means including a first resistor having first and second ends, a second resistor, and a third resistor, said second resistor being connected to said output of said second operational amplifier, said third resistor being connected in series with said second resistor forming a junction therebetween, said first end of said first resistor being connected to said junction between said second and third resistors, said second end of said first resistor forming said control terminal of said active filter means; third operational amplifier means arranged to operate as a second integrator and having an input coupled to said third resistor means, and an output coupled to said second input of said first operational amplifier means; and the sinusoidal output signal occurring at said output of said second operational amplifier means in response to the first selected input signal.