Noise cancellation system

1. A system comprising:

an I-channel mixer configured to receive a radio frequency signal from an antenna and an intermediate frequency signal from a local oscillator;

a Q-channel mixer configured to receive the radio frequency signal from the antenna and a phase-shifted intermediate frequency signal from the local oscillator;

a first noise cancellation apparatus connected with the I-channel mixer, wherein the first noise cancellation apparatus includes:

two adjustable resistors connected in series between two output terminals of the first noise cancellation apparatus, wherein a common node of the two adjustable resistors is connected to an output of a buffer, and wherein an input of the buffer is connected to a reference voltage, and wherein an I-channel differential mode second-order intermodulation component and an I-channel common mode second-order intermodulation component cancel each other in the first noise cancellation apparatus based on adjustable settings of the two adjustable resistors; and

a second noise cancellation apparatus connected with the Q-channel mixer, wherein:

a Q-channel differential mode second-order intermodulation component and a Q-channel common mode second-order intermodulation component cancel each other in the second noise cancellation apparatus.

2. The system of claim 1 , further comprising:

a first amplifier coupled to the first noise cancellation apparatus, wherein the first amplifier comprises a first adjustable resistor; and

a second amplifier coupled to the second noise cancellation apparatus, wherein the second amplifier comprises a second adjustable resistor, and wherein:

the first adjustable resistor and the second adjustable resistor are configured to adjust I-channel and Q-channel signal amplitudes in response to an amplitude mismatch.

3. The system of claim 1 , wherein the first noise cancellation apparatus comprises:

a trans-impedance stage coupled to an output of the I-channel mixer;

a common mode feedback loop stage connected with the trans-impedance stage; and

a noise cancellation stage connected in parallel with the trans-impedance stage.

4. The system of claim 3 , wherein:

the common mode feedback loop stage is configured to regulate an output voltage of the trans-impedance stage in response to the reference voltage.

5. The system of claim 4 , wherein the noise cancellation stage comprises the two adjustable resistors including:

a first adjustable resistor;

a second adjustable resistor connected in series with the first adjustable resistor, and wherein:

the first adjustable resistor has a first resistance variation portion and the second adjustable resistor has a second resistance variation portion of a resistance value equal to that of the first resistance variation portion in an opposite direction; and

the I-channel differential mode second-order intermodulation component and the I-channel common mode second-order intermodulation component cancel each other based on adjustable settings of the first resistance variation portion and the second resistance variation portion.

6. The system of claim 5 , wherein:

the buffer is configured to regulate a voltage at the common node approximately equal to the reference voltage.

7. A system comprising:

an I-channel mixer configured to receive a radio frequency signal from an antenna and an intermediate frequency signal from a local oscillator;

a Q-channel mixer configured to receive the radio frequency signal from the antenna and a phase-shifted intermediate frequency signal from the local oscillator;

a first noise cancellation apparatus connected to the I-channel mixer, wherein the first noise cancellation apparatus comprises a trans-impedance amplifier stage coupled to an output of the I-channel mixer, a noise reduction stage coupled to the trans-impedance amplifier stage and a feedback loop stage coupled to the noise reduction stage, and wherein the noise reduction stage comprises a first adjustable resistor and a second adjustable resistor connected in series with the first adjustable resistor, and wherein the first adjustable resistor and the second adjustable resistor are configured such that an I-channel differential mode second-order intermodulation component and an I-channel common mode second-order intermodulation component cancel each other in the first noise cancellation apparatus, and wherein a common node of the first adjustable resistor and the second adjustable resistor is directly connected to both an input and an output of a buffer; and

a second noise cancellation apparatus connected to the Q-channel mixer, wherein the second noise cancellation apparatus and the first noise cancellation apparatus have a similar structure.

8. The system of claim 7 , wherein:

the first adjustable resistor and the second adjustable resistor are configured such that the first adjustable resistor has a first resistance variation and the second adjustable resistor has a second resistance variation equal to the first resistance variation in an opposite direction.

9. The system of claim 7 , wherein:

the second noise cancellation apparatus is configured such that a Q-channel differential mode second-order intermodulation component and a Q-channel common mode second-order intermodulation component cancel each other.

10. The system of claim 7 , wherein:

the trans-impedance amplifier stage comprises a common-gate trans-impedance amplifier, and wherein the common-gate trans-impedance amplifier comprises two common gate transistors; and

the feedback loop stage is a common mode feedback loop stage configured to regulate an output voltage of the trans-impedance amplifier stage in response to a reference voltage.

11. The system of claim 10 , wherein:

the buffer is coupled between the reference voltage and the common node of the first adjustable resistor and the second adjustable resistor.

12. The system of claim 11 , wherein:

the buffer provides a low impedance output; and

the buffer is configured to regulate a voltage at the common node approximately equal to the reference voltage.

13. The system of claim 7 , wherein:

the noise reduction stage and the feedback loop stage are connected in parallel between two terminals of the trans-impedance amplifier stage.

14. A system comprising:

an I-channel mixer configured to receive a radio frequency signal from an antenna and an intermediate frequency signal from a local oscillator;

a Q-channel mixer configured to receive the radio frequency signal from the antenna and a phase-shifted intermediate frequency signal from the local oscillator;

a first noise cancellation apparatus having an input directly connected to an output of the I-channel mixer, wherein the first noise cancellation apparatus comprises:

a first trans-impedance amplifier stage coupled to an output of the I-channel mixer;

a first noise reduction stage coupled to the first trans-impedance amplifier stage; and

a first feedback loop stage coupled to the first noise reduction stage, and wherein the first noise reduction stage comprises a first adjustable resistor and a second adjustable resistor connected in series with the first adjustable resistor, and wherein the first adjustable resistor and the second adjustable resistor are configured such that an I-channel differential mode second-order intermodulation component and an I-channel common mode second-order intermodulation component cancel each other, and wherein a common node of the first adjustable resistor and the second adjustable resistor is connected to a reference voltage through a buffer, and wherein an output of the buffer is directly connected to the common node of the first adjustable resistor and the second adjustable resistor; and

a second noise cancellation apparatus having an input directly connected to an output of the Q-channel mixer, wherein the second noise cancellation apparatus comprises:

a second trans-impedance amplifier stage coupled to an output of the Q-channel mixer;

a second noise reduction stage coupled to the second trans-impedance amplifier stage; and

a second feedback loop stage coupled to the second noise reduction stage, and wherein the second noise reduction stage comprises a third adjustable resistor and a fourth adjustable resistor connected in series with the third adjustable resistor, and wherein the third adjustable resistor and the fourth adjustable resistor are configured such that a Q-channel differential mode second-order intermodulation component and a Q-channel common mode second-order intermodulation component cancel each other.

15. The system of claim 14 , wherein:

the first adjustable resistor and the second adjustable resistor connected in series and coupled to the output of the I-channel mixer through a first trans-impedance amplifier; and

the third adjustable resistor and the fourth adjustable resistor connected in series and coupled to the output of the Q-channel mixer through the second trans-impedance amplifier stage.

16. The system of claim 14 , wherein:

the common node of the first adjustable resistor and the second adjustable resistor is regulated to a reference voltage level through the buffer coupled between the common node and a reference voltage.

17. The system of claim 14 , wherein:

the first feedback loop stage comprises:

a detector circuit coupled to an output of the first trans-impedance amplifier stage; and

an amplifier configured to regulate a dc output of the first trans-impedance amplifier stage.

18. The system of claim 17 , wherein:

a first input of the amplifier is coupled to a reference voltage;

a second input of the amplifier is coupled to the detector circuit; and

an output of the amplifier is coupled to the output of the first trans-impedance amplifier stage through a voltage adjustment apparatus.

19. The system of claim 14 , wherein:

the I-channel mixer, the Q-channel mixer, the first noise cancellation apparatus and the second noise cancellation apparatus are in a receiver coupled between the antenna and a baseband processor.

20. The system of claim 19 , further comprising:

an I-channel filter, an I-channel gain stage and an I-channel A/D converter connected in cascade between the first noise cancellation apparatus and the baseband processor; and

a Q-channel filter, a Q-channel gain stage and a Q-channel A/D converter connected in cascade between the second noise cancellation apparatus and the baseband processor.