Programmable filter circuits and methods

1. A programmable filter for filtering an input signal comprising:

a storage element for storing a plurality of digital values representing a discrete time window function; and

a plurality of filter channels, each channel comprising

a multiplying digital-to-analog converter having a plurality of digital inputs coupled to the storage element and a continuous time analog input for receiving said input signal to be filtered, wherein said input signal is a continuous time analog signal;

at least one capacitor having at least one terminal coupled to an output of the multiplying digital-to-analog converter; and

a sampling device coupled between the at least one terminal of the at least one capacitor and an output of the filter.

2. The filter of claim 1 further comprising an auxiliary filter for pre-filtering the analog input.

3. The filter of claim 1 wherein the multiplying digital-to-analog converter is oversampled.

4. The filter of claim 3 wherein the plurality of digital values representing a discrete time window function are processed with sigma delta filter.

5. The filter of claim 1 wherein the multiplying digital-to-analog converter receives digital values at non-uniform time intervals.

6. The filter of claim 1 further comprising a plurality of transconductance circuits for receiving the analog input as a voltage signal and converting the analog input into a current signal.

7. The filter of claim 1 further comprising a decoder coupled between the storage element and the multiplying digital-to-analog converter.

8. The filter of claim 1 wherein the storage element is a memory.

9. The filter of claim 1 wherein the storage element is a shift memory.

10. The filter of claim 1 wherein the sampling device comprises one or more MOS transistors.

11. The filter of claim 1 wherein said programmable filter is used in a software defined radio.

12. A method of filtering a signal comprising:

storing a discrete time window function;

receiving said signal at an input of a multiplying digital-to-analog converter, wherein said signal is a continuous time analog signal;

multiplying said continuous time analog signal by the discrete time window function in the multiplying digital-to-analog converter, and in accordance therewith, generating a multiplied signal;

integrating the multiplied signal in a first capacitor; and

sampling a voltage on the first capacitor.

13. The method of claim 12 further comprising, before multiplying said signal, filtering said signal with a filter having a continuous time input and output.

14. The method of claim 12 wherein the multiplying digital-to-analog converter is oversampled, and wherein the plurality of digital values representing a discrete time window function are processed with sigma delta filter.

15. The method of claim 12 wherein the multiplying digital-to-analog converter receives digital values at non-uniform time intervals.

16. The method of claim 12 wherein the signal is a voltage signal, the method further comprising converting the voltage signal into a plurality of current signals.

17. The method of claim 12 further comprising shifting the discrete time window function to successively provide discrete time window function values to the multiplying digital-to-analog converter.

18. The method of claim 12 further comprising decoding the discrete time window function.