Tunable tracking filter

1. An integrated tuner circuit, comprising:

a tuned LC band-pass filter having a variable capacitance (C t ) and fixed inductance (L);

an external load capacitor having a variable capacitance (C t ); and

a fixed-frequency control loop for producing a voltage (V TUN ) for adjusting the variable capacitances of the band-pass filter and external load capacitor to achieve tracking of the band-pass filter with an arbitrary oscillator frequency ω LO .

2. The integrated tuner circuit according to claim 1 , wherein the fixed-frequency control loop further comprises a fixed-frequency oscillator and a circuit for receiving a programmable value N for setting the value of ω LO , wherein the fixed-frequency control loop adjusts the variable capacitances C t such that C t :: (ω LO ±ω IF ) −2 :: N −2 , wherein ω IF is an intermediate frequency.

3. The integrated tuner circuit according to claim 2 , wherein the band-pass filter is tuned to each of a plurality of different IF distances from ω LO by adjusting the programmable value N.

4. The integrated tuner circuit according to claim 2 , wherein the fixed-frequency oscillator outputs a signal having a frequency ω xtal , and wherein the tuned LC band-pass filter is tuned to a virtual oscillator frequency ω LO given by Nω xtal .

5. The integrated tuner circuit according to claim 1 , wherein the fixed-frequency control loop provides compensation for parasitic capacitance (C p ).

6. The integrated tuner circuit according to claim 5 , further comprising a capacitor corresponding to the parasitic capacitance C p in parallel with the external load capacitor.

7. The integrated tuner circuit according to claim 1 , wherein the fixed-frequency control loop operates to produce a signal:

1−(αω xtal 2 R 2 C )N 2 C t →0

where α is a variable gain, ω xtal is a frequency of a fixed-frequency oscillator, R is a resistance, C is a capacitance, and N is a programmable value for setting the value of ω LO .

8. The integrated tuner circuit according to claim 7 , wherein N and C t are the only oscillator frequency dependent variables.

9. A method for tracking a LC tuned band-pass filter with an arbitrary oscillator ωLO, wherein the band-pass filter includes a variable capacitance C t and a fixed inductance (L), comprising:

providing a fixed-frequency control loop for producing a voltage (V TUN ) for adjusting the variable capacitance C t of the tuned band-pass filter and for adjusting a variable capacitance C t of a load capacitor; and

inputting a programmable value N into the fixed-frequency control loop for setting the value of ω LO , wherein the fixed-frequency control loop adjusts the variable capacitances C t such that C t :: (ω LO ±ω IF ) −2 :: N −2 , wherein ω IF is an intermediate frequency.

10. The method according to claim 9 , further comprising:

tuning the band-pass filter to each of a plurality of different IF distances from ω LO by adjusting the programmable value N.

11. The method according to claim 9 , wherein the fixed-frequency control loop includes a fixed-frequency oscillator that outputs a signal having a frequency ω xtal , further comprising:

tuning the band-pass filter to a virtual oscillator frequency ω LO given by Nω xtal .

12. The method according to claim 9 , wherein the fixed-frequency control loop provides compensation for parasitic capacitance (C p ).

13. The method according to claim 12 , further comprising:

providing a capacitor corresponding to the parasitic capacitance C p in parallel with the load capacitor.

14. The method according to claim 9 , wherein the fixed-frequency control loop operates to produce a signal:

1−(αω xtal 2 R 2 C ) N 2 C t →0

where a is a variable gain,ω xtal is a frequency of a fixed-frequency oscillator, R is a resistance, and C is a capacitance.

15. The method according to claim 14 , wherein N and C t are the only oscillator frequency dependent variables.