Notch filter and apparatus for receiving and transmitting radio-frequency signals incorporating same

1. A notch filter suitable for attenuating certain frequencies of a radio-frequency signal, said filter comprising:

an input for receiving said radio-frequency signal and an output for the output of a portion of said radio-frequency signal;

first and second capacitive means;

at least one inductor and a negative resistance circuit for compensating the resistive losses of said at least one inductor, said inductor and said first and second capacitive means being placed to produce a resonator; and

a control device suitable for controlling said negative resistance circuit, the input impedance of said filter comprising a pole and a zero, wherein said pole depends on said second capacitive means and said zero depends on both said first and second capacitive means, said first and second capacitive means are variable and said control device is operative for controlling said first and second capacitive means, wherein said control device comprises:

means for detecting the peak of said portion of signal in output from the filter;

an amplitude comparator for comparing the value of the signal in output from the peak detector with a reference signal; and

a frequency comparator for comparing the frequency of the portion of signal in output from the filter with a reference frequency, said comparators being operative for sending signals representative of said comparisons to a logic means operative for sending control signals to said first and second capacitive means and to said negative resistance circuit until the amplitude and the frequency of said portion of signal in output from the filter are substantially equal to said reference signal and to said reference frequency.

2. The notch filter according to claim 1 , wherein said control device is operative for controlling said first and second capacitive means independently from each other.

3. The notch filter according to claim 1 , wherein said control device is operative for varying said first capacitive means to vary the frequency associated to said zero.

4. The notch filter according to claim 1 , wherein said first and second capacitive means can be controlled digitally and said control device comprises digital control means.

5. The notch filter according to claim 1 , wherein said first and second capacitive means are formed by a digitally programmable array of capacitors.

6. An apparatus for receiving and transmitting radio-frequency signals comprising an antenna, a transmitter and a receiver, a duplexer for switching the signal to be transmitted towards the antenna and the signal received by means of the antenna towards the receiver, said receiver comprising a first low-noise amplifier, a second low-noise amplifier and a notch filter placed between said first and said second amplifier, said notch filter comprising:

an input for receiving said radio-frequency signal and an output for the output of a portion of said radio-frequency signal;

first and second capacitive means;

at least one inductor and a negative resistance circuit for compensating the resistive losses of said at least one inductor, said inductor and said first and second capacitive means being placed to produce a resonator; and

a control device suitable for controlling said negative resistance circuit, the input impedance of said filter comprising a pole and a zero, wherein said pole depends on said second capacitive means and said zero depends on both said first and second capacitive means, said first and second capacitive means are variable and said control device is operative for controlling said first and second capacitive means, wherein said control device of the notch filter comprises:

means for detecting the peak of said portion of signal in output from the filter;

an amplitude comparator for comparing the value of the signal in output from the peak detector with a reference signal; and

a frequency comparator for comparing the frequency of the portion of signal in output from the filter with a reference frequency, said comparators being operative for sending signals representative of said comparisons to a logic means operative for sending control signals to said first and second capacitive means and to said negative resistance circuit until the amplitude and the frequency of said portion of signal in output from the filter are substantially equal to said reference signal and to said reference frequency.

7. The apparatus according to claim 6 , wherein said first amplifier comprises an output terminal and said second amplifier comprises an input terminal, said notch filter is stably coupled to the output terminal of the first amplifier and to the input terminal of the second amplifier.

8. The apparatus according to claim 6 , wherein said filter and said first and second amplifier can be integrated into the same semiconductor chip.

9. The apparatus of claim 6 , wherein said control device of the notch filter is operative for controlling said first and second capacitive means independently from each other.

10. The apparatus of claim 6 , wherein said control device of the notch filter is operative for varying said first capacitive means to vary the frequency associated to said zero.

11. The apparatus of claim 6 , wherein said first and second capacitive means of the notch filter can be controlled digitally and said control device comprises digital control means.

12. The apparatus of claim 6 , wherein said first and second capacitive means of the notch filter are formed by a digitally programmable array of capacitors.

13. A notch filter suitable for attenuating certain frequencies of a radio-frequency signal, said filter comprising:

an input for receiving said radio-frequency signal and an output for the output of a portion of said radio-frequency signal;

first and second capacitors;

at least one inductor and a negative resistance circuit for compensating the resistive losses of said at least one inductor, said inductor and said first and second capacitors being placed to produce a resonator; and

a control circuit for controlling said negative resistance circuit, the input impedance of said filter comprising a pole and a zero, wherein said pole depends on said second capacitor and said zero depends on both said first and second capacitors, said first and second capacitors being variable, wherein said control circuit controls said first and second capacitors, and wherein said control circuit comprises:

a peak detector for detecting the peak of said portion of signal output from the filter;

an amplitude comparator for comparing the value of the signal output from the peak detector with a reference signal; and

a frequency comparator for comparing the frequency of the portion of signal output from the filter with a reference frequency, said comparators being operative for sending signals representative of said comparisons to a logic circuit operative for sending control signals to said first and second capacitors and to said negative resistance circuit until the amplitude and the frequency of said portion of signal output from the filter are substantially equal to said reference signal and to said reference frequency.

14. The notch filter according to claim 13 , wherein said control circuit controls said first and second capacitors independently from each other.

15. The notch filter according to claim 13 , wherein said control circuit varies said first capacitor to vary the frequency associated with said zero.

16. The notch filter according to claim 13 , wherein said first and second capacitors can be controlled digitally and said control circuit comprises a digital control circuit.

17. The notch filter according to claim 13 , wherein said first and second capacitors are formed by a digitally programmable array of capacitors.

18. A notch filter comprising:

a pair of cross-coupled transistors having a first node, a second node, and a third node coupled to a current source;

a first capacitor circuit coupled between the first node and an input;

a second capacitor circuit coupled between the second node and ground;

a third capacitor circuit coupled across the first and second nodes; and

an inductor circuit coupled across the first and second nodes, wherein the first and second nodes form a differential output.

19. The notch filter of claim 18 wherein the current source is digitally controlled.

20. The notch filter of claim 18 wherein the inductor circuit is coupled to a source of supply voltage.