US 5,499,392 A · Filter having a variable response time for filtering an input signal

Abstract

A tunable filter (25) is described for use in loop control circuits. The filter (25) has a time constant which is determined by a resistor (40) and a capacitor. The capacitor is simulated by the series combination of an impedance converter (41) and a variable resistor, such as a field effect transistor (42). The resistance of the transistor (42) is determined by a control signal present on a control line (32) and the impedance converter (41) converts the resistance into an equivalent capacitive reactance. Therefore, the control signal on the control line (32) effectively controls the capacitance present at a node (43) and, in conjunction with the resistor (40), determines the time constant, and therefore the response time and bandwidth, of the filter (25). Multiple pole, lowpass, bandpass, highpass, and bandstop filters can be constructed. The impedance converter (41) uses a very small capacitor to simulate a large capacitance value at the node (43). This small capacitance value also allows the filter (25) to be fabricated as part of an integrated circuit.

Claims14

Dependent claims

A filter apparatus having a variable response time for filtering an input signal to provide a filtered output signal, comprising:

a resistor having a first terminal connected to said input signal and a second terminal for providing said filtered output signal;
a series circuit connected between said second terminal and a signal return point, said series circuit comprising a variable resistor connected in series with an impedance converter, said variable resistor having a control terminal and having a resistance responsive to a response control signal;
and
means for providing said response control signal to said control terminal of said variable resistor.

1 more element

A receiver apparatus for processing a received signal, comprising:

a mixer having a first input functionally connected to said received signal and a second input functionally connected to an oscillator signal, and an output for providing an intermediate frequency (IF) signal responsive to said received signal and to said oscillator signal;
an amplifier connected to said output of said mixer and having an output for providing an amplified IF signal;
a detector connected to said output of said amplifier for demodulating said amplified IF signal to provide a demodulated output signal;
an oscillator for providing a reference signal;
a variable frequency oscillator having an input for receiving an error signal and an output for providing said oscillator signal, said oscillator signal having a frequency responsive to said error signal;
a phase comparator having a first input functionally connected to said reference signal, a second input functionally connected to said oscillator signal, and an output for providing a phase comparison signal;
means for providing a response control signal;
and
a filter having a variable cutoff frequency for filtering said phase comparison signal to provide said error signal, said filter comprising:
a resistor having a first terminal connected to said output of said phase comparator and a second terminal connected to said input of said variable frequency oscillator;
and
a series circuit connected between said second terminal and a signal return point, said series circuit comprising a variable resistor connected in series with an impedance converter, said variable resistor having a control terminal connected to said response control signal, and having a resistance responsive to said response control signal.

9 more elements

Dependencies are inferred from the claim text.

Filter having a variable response time for filtering an input signal

Life of this patent

Filter having a variable response time for filtering an input signal

Life of this patent

Abstract

Claims14