Demodulator system and method

• US 20080144743A1
• Filed: 12/19/2006
• Published: 06/19/2008
• Est. Priority Date: 12/19/2006
• Status: Active Grant

• Current Assignee: North Star Innovations Inc. (Wi-LAN Inc.)
• Original Assignee: SigmaTel Incorporated (NXP Semiconductors NV)
• Inventors: Ifesinachukwu, K. Gozie, Tinker, Darrell, Alderson, Jeffrey Donald
• Granted Patent: US 7,792,220 B2
• Time in Patent Office: Days
• US Class Current: 375/324
• CPC Class Codes: H04L 27/2331 wherein the received signal is demodulated using one or more delayed versions of itself
• CPC Class Codes: H04L 27/3845 using non - coherent demodulation, i.e. not using a phase synchronous carrier

Abstract

A demodulator system and method is disclosed. In an embodiment, the demodulator system can include a Coordinate Rotation Digital Computer (CORDIC) mixer to mix a first signal substantially to baseband using a first input frequency and to mix a second signal substantially to baseband using a second input frequency. In another embodiment, the demodulator system can include a phase detector to receive a pilot signal and to generate a control signal to adjust a decimation rate based on the pilot signal. In another embodiment, the demodulator system can include a symbol decoder to determine a symbol from a phase signal.

Claims

1. A demodulator system comprising:

an analog-to-digital converter (ADC) configured to sample a modulated signal and to output a digital signal;

a Coordinate Rotation Digital Computer (CORDIC) mixer coupled to an output of the ADC, the CORDIC mixer to generate an Inphase (I) signal and a Quadrature (Q) signal based on the digital signal output by the ADC and further based on an input frequency;

a filter path to process the I signal and the Q signal generated at the CORDIC mixer; and

a demodulation stage to demodulate an output of the filter path, wherein the demodulation stage includes a CORDIC demodulator.

2. The demodulator system of claim 1, wherein the CORDIC mixer includes a plurality of pipelined mixing stages.

3. The demodulator system of claim 2, wherein each mixing stage of the plurality of pipelined mixing stages performs at least four CORDIC iterations per sample of the digital signal received at the CORDIC mixer.

4. The demodulator system of claim 3, wherein the CORDIC mixer is configured to perform eight CORDIC iterations per sample of the digital signal received at the CORDIC mixer.

5. The demodulator system of claim 4, wherein the filter path includes at least one decimator to perform decimation of the I signal and the Q signal at an adjustable decimation rate.

6. A demodulator system comprising:

a Coordinate Rotation Digital Computer (CORDIC) mixer to receive a first input signal modulated using a first modulation mode and to generate a first Inphase (I) signal and a first Quadrature (Q) signal by mixing the first input signal substantially to baseband using a first input frequency, the CORDIC mixer further to receive a second input signal modulated using a second modulation mode and to generate a second I signal and a second Q signal by mixing the second input signal substantially to baseband using a second input frequency;

a filter path to process an output of the CORDIC mixer; and

a demodulation stage to demodulate an output of the filter path, wherein the demodulation stage includes a CORDIC demodulator.

7. The demodulator system of claim 6, wherein the first modulation mode is a first one of amplitude modulation (AM), frequency modulation (FM), and differential quadrature phase shift key (DQPSK) modulation, and wherein the second modulation mode is a different one of AM, FM, and DQPSK modulation.

8. A demodulator system comprising:

a Coordinate Rotation Digital Computer (CORDIC) mixer to generate an Inphase (I) signal and a Quadrature (Q) signal based on a modulated input signal and an input frequency;

a decimator to perform decimation of the I signal and the Q signal at an adjustable decimation rate;

a phase detector to receive a pilot signal and including decimation rate logic to generate a control signal to adjust the decimation rate based on the pilot signal, wherein an oversample rate of the pilot signal is not less than approximately two and not more than approximately sixty-four; and

a demodulation stage to demodulate a filtered output of the decimator, wherein the demodulation stage includes a CORDIC demodulator.

9. The demodulator system of claim 8, wherein the phase detector further includes logic to sample the pilot signal at a sample rate approximately equal to an integer multiple of a frequency of the pilot signal, and wherein the integer multiple is one or two.

10. The demodulator system of claim 8, wherein the oversample rate is four or thirty-two.

11. The demodulator system of claim 8, further comprising:

a channel filter path to filter an output of the decimator; and

a pilot filter path to selectively recover the pilot signal from an output of the channel filter path and an output of the demodulator stage, the pilot filter path further to provide the pilot signal to the phase detector.

12. The demodulator system of claim 11, wherein the CORDIC mixer includes multiple pipelined mixing stages.

13. The demodulator system of claim 8, wherein the pilot signal has a pilot signal frequency approximately equaling a Near Instantaneous Companded Audio Multiplex (NICAM) symbol rate.

14. The demodulator system of claim 8, wherein the decimation rate logic includes logic to generate a control signal to increase the decimation rate when a sample of the pilot signal is negative.

15. The demodulator system of claim 8, wherein the decimation rate logic includes logic to generate a control signal to decrease the decimation rate when a sample of the pilot signal is positive.

16. The demodulator system of claim 8, wherein the phase detector is a second order phase detector.

17. A demodulator system comprising:

a Coordinate Rotation Digital Computer (CORDIC) mixer to generate an Inphase (I) signal and a Quadrature (Q) signal;

a decimator coupled to the CORDIC mixer to adjust a sample rate of at least one of the I signal and the Q signal;

a filter path coupled to the decimator to process an output of the decimator;

a demodulation stage to demodulate an output of the filter path, wherein the demodulation stage includes a CORDIC demodulator, and

a symbol decoder to receive a phase signal from the CORDIC demodulator, the symbol decoder including symbol recognition logic to adjust a sample of the phase signal by an offset value, to map the adjusted sample to a nearest predetermined phase value of a plurality of predetermined phase values, and to determine a symbol using a difference between the nearest predetermined phase value and a prior nearest predetermined phase value, wherein the offset value at least partially compensates for a phase drift generated at the CORDIC mixer.

18. The demodulator system of claim 17, wherein the plurality of predetermined phase values includes 0 degrees, 90 degrees, 180 degrees, and 270 degrees.

19. The demodulator system of claim 17, wherein the symbol is a Near Instantaneous Companded Audio Multiplex (NICAM) symbol.

20. The demodulator system of claim 19, wherein the symbol decoder includes a phase accumulator to provide the offset value.

21. The demodulator system of claim 17, wherein the symbol recognition logic includes logic to modify a value of the phase accumulator based on a difference between an adjusted sample and the nearest predetermined phase value.

22. A demodulation method comprising:

receiving a modulated signal;

mixing the audio signal substantially to baseband at a Coordinate Rotation Digital Computer (CORDIC) mixer having multiple pipelined mixing stages;

decimating an output of the CORDIC mixer at a decimator having an adjustable decimation rate; and

demodulating a filtered output of the decimator with a CORDIC demodulator.

23. The method of claim 22, wherein a signal output by the decimator has a sampling frequency approximately equal to a pilot frequency multiplied by an oversample factor, wherein the oversample factor is an integer not less than two and not more than sixty-four.

24. The method of claim 23, further comprising:

sampling a pilot signal at a phase detector;

comparing a sample of the pilot signal to zero; and

adjusting the decimation rate based on the comparison of the sample to zero.

25. The method of claim 24, wherein the pilot frequency is approximately 364 kHz, and wherein the pilot signal is recovered from the filtered output of the decimator.

26. The method of claim 24, wherein the pilot frequency is approximately 15.734 kHz, and wherein the pilot signal is recovered from an output of the CORDIC demodulator.

27. The method of claim 24, wherein the sample of the pilot signal is compared to zero every N samples of the pilot signal, wherein N is the oversample factor.

28. The method of claim 24, further comprising decreasing the decimation rate based on the sample of the pilot signal having a positive value or having a greater value than a prior sample of the pilot signal.

29. The method of claim 24, further comprising increasing the decimation rate based on the sample of the pilot signal having a negative value or having a lesser value than a prior sample of the pilot signal.

30. The method of claim 22, further comprising:

adjusting a phase value by an error measurement corresponding to a nearly constant phase difference value;

mapping the adjusted phase value to a nearest predetermined phase value of a plurality of predetermined phase values;

subtracting the adjusted phase value from the previous nearest predetermined phase value; and

determining an audio symbol corresponding to a result of the subtraction.