Demodulator system and method
View Patent ↗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.
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 by the CORDIC mixer, wherein the filter path is coupled to at least one 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.
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 at least one decimator performs 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, wherein the filter path is coupled to at least one 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.
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; 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 or 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 20 , 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 an audio signal substantially to baseband with a Coordinate Rotation Digital Computer (CORDIC) mixer having multiple pipelined mixing stages;
decimating an output of the CORDIC mixer with a decimator having an adjustable decimation rate;
demodulating a filtered output of the decimator with a CORDIC demodulator; and
receiving a phase signal from the CORDIC demodulator at a symbol decoder, 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.
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.