Narrowband interference detector

1. A narrow band interference detector for a receiver of a communication system, the narrow band interference detector comprising:

a numerically-controlled oscillator (NCO), for generating an oscillating signal according to a time-variable frequency;

a mixer, for mixing an input signal with the oscillating signal, to generate a mixed signal;

a phase lock loop (PLL), for locking the mixed signal during a detection period according to the time-variable frequency;

a first switch, for controlling a connection between the NCO and the PLL according to a desirable narrow band interference signal type of the at least one narrow band interference signal; and

a narrow band determination unit, for determining at least one narrow band interference signal according to a low frequency component of the mixed signal, the time-variable frequency and the desirable narrow band interference signal type of the at least one narrow band interference signal.

2. The narrow band interference detector of claim 1 , wherein the time-variable frequency is fixed during a detection period, and the time-variable frequency during the detection period is lower than the time-variable frequency during a next detection period.

3. The narrow band interference detector of claim 1 , wherein the desirable narrow band interference signal type is a modulated signal or an unmodulated signal.

4. The narrow band interference detector of claim 3 , wherein the first switch does not conduct the connection between the NCO and the PLL when the desirable narrow band interference signal type of the narrow band interference is a modulated signal.

5. The narrow band interference detector of claim 1 , wherein the PLL comprising:

a low pass filter, for passing the low frequency component of the mixed signal;

a phase error detection unit, for detecting a phase error according to the low frequency component of the mixed signal; and

a loop filter, for compensating the phase error.

6. The narrow band interference detector of claim 5 , wherein when the desirable narrow band interference signal type of the narrow band interference is a modulated signal, the narrow band determination unit determines a second frequency, a bandwidth and a second level of a second narrow band interference signal of the at least one narrow band interference signal if a level of the low frequency component of the mixed signal is higher than a first threshold.

7. The narrow band interference detector of claim 1 , wherein the first switch conducts the connection between the NCO and the PLL when the desirable narrow band interference signal type of the at least one narrow band interference signal is an unmodulated signal.

8. The narrow band interference detector of claim 7 , wherein the narrow band determination unit determines a first frequency and a first level of a first narrow band interference signal of the at least one narrow band interference signal if the PLL locks the mixed signal according to the time-variable frequency during the detection period.

9. The narrow band interference detector of claim 1 , wherein the receiver further comprises a narrow band interference canceller, for cancelling the at least one narrow band interference signal from the input signal to output a filtered input signal according to at least one level of the at least one narrow band interference signal and a second threshold.

10. The narrow band interference detector of claim 9 , wherein the narrow band interference canceller cancels a third narrow band interference signal of the at least one narrow band interference signal from the input signal if a third level of the third narrow band interference signal is higher than the second threshold.

11. The narrow band interference detector of claim 9 , wherein the narrow band interference canceller comprises:

at least one notch filter, each for filtering one of the at least one narrow band interference signal to generate one of at least one filtered signal; and

at least one multiplexer, each for receiving a previous output signal of a previous multiplexer at a first input terminal and the one of the least one filtered signal at a second input terminal, and outputting one of at least one output signal according to a level of the one of the at least one narrow band interference signal and the second threshold.

12. The narrow band interference detector of claim 11 , wherein the each of the at least one multiplexer outputs the one of at least one filtered signal as the one of at least one output signal when the level of the one of the at least one narrow band interference signal is higher than the second threshold.

13. The narrow band interference detector of claim 9 , wherein the narrow band interference canceller does not cancel a frequency component among an operating frequency of the communication system from the input signal.

14. The narrow band interference detector of claim 1 , wherein the receiver further comprises a second switch, for controlling connections between the narrow band interference detector and the input signal and the filtered input signal to output the filtered input signal to the narrow band interference detector according to whether the filtered input signal is generated.

15. The narrow band interference detector of claim 14 , wherein the second switch conducts the connection between the narrow band interference detector and the filtered input signal, to output the filtered input signal to the narrow band interference detector if the filtered input signal is generated.

16. The narrow band interference detector of claim 15 , wherein the narrow band interference detector determines at least one remained narrow band interference signal according to the filtered input signal.

17. The narrow band interference detector of claim 1 , wherein the communication system is a GPS communication system.