Bandpass delta-sigma analog-to-digital converters

1. An apparatus comprising:

a delta-sigma analog-to-digital converter for converting an analog input signal, comprising:

an analog band-pass loop filter configured to filter an analog signal derived from the analog input signal, the loop filter having a center band-pass frequency; and

a quantizer configured to produce a series of digital signals by sampling the filtered analog signal from the loop filter at a sampling frequency, wherein the series of digital signals has a data-carrying frequency spectrum that is a mirror image of a data-carrying frequency spectrum of the analog input signal, the data-carrying spectrum of the series being located between the center band-pass frequency and zero; and

a digital-to-analog converter configured to generate a series of the analog feedback signals at the sampling frequency such that each analog feedback signal has a duty cycle of less than ½ and corresponds to one of the digital signals.

2. The apparatus of claim 1 , wherein the sampling frequency is f s , the center band-pass frequency of the loop filter is f c , and f s =(4/3)f c ±10%.

3. The apparatus of claim 1 , wherein the loop filter has an order of four or higher.

4. The apparatus of claim 1 , further comprising a digital demodulator being coupled to receive the digital signals and configured to remove frequencies above a lower edge of the loop filter's band-pass frequency.

5. The apparatus of claim 1 ,

wherein the analog band-pass loop filter is configured to filter the analog signal derived from the analog input signal by sequentially combining the analog feedback signals with the analog input signal.

6. The apparatus of claim 5 , wherein the loop filter has an order that is higher than two.

7. The apparatus of claim 1 , wherein the digital-to-analog converter is configured to produce the analog feedback signals in a return-to-zero format.

8. The apparatus of claim 1 , wherein the digital-to-analog converter is configured to produce the analog feedback signals with duty cycles of less than about ⅓.

9. A method, comprising:

transmitting an analog input signal having a data-carrying band to a delta-sigma analog-to-digital converter to convert the analog input signal into a series of digital signals having a data-carrying band;

filtering a modified analog signal with a loop band-pass filter having a center band-pass frequency; and

wherein the series of digital signals has a data-carrying frequency spectrum that is a mirror image of a data-carrying frequency spectrum of the analog input signal, the data-carrying spectrum of the series being located between the center band-pass frequency and zero;

performing digital-to-analog conversions of the digital signals to sequentially produce analog feedback signals with duty cycles of less than ½; and

producing the modified analog signal filter by sequentially combining the analog feedback signals with the analog input signal.

10. The method of claim 9 , further comprising;

sampling the filtered modified analog signal to produce the digital signals at a sampling frequency.

11. The method of claim 10 , wherein the sampling frequency is f s , a center band-pass frequency of the loop filter is f c , and f c =(4/3)f c ±10%.

12. The method of claim 10 , wherein the loop filter has an order of four or higher.

13. The method of claim 9 , further comprising:

filtering the digital signals with a digital demodulator whose band pass is configured to remove frequencies higher than a lower edge of the loop filter's band pass.

14. The method of claim 9 , wherein the act of performing produces the analog feedback signals with duty cycles of less than ⅓.

15. The method of claim 9 , wherein the act of performing produces the analog feedback signals in a return-to-zero format.