Subsampling RF receiver architecture

• US 20020181614A1
• Filed: 03/19/2002
• Published: 12/05/2002
• Est. Priority Date: 04/09/2001
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Mostafa, Mohamed A.I., Embabi, Sherif, Fernando, Moderage C., Chan, Wing Kan, Gore, Charles Jr.
• Granted Patent: US 7,110,732 B2
• Time in Patent Office: Days
• US Class Current: 375/316
• CPC Class Codes: H04B 1/0003 Software-defined radio [SDR] systems, i.e. systems wherein components typically implemented in hardware, e.g. filters or modulators/demodulators, are implented using software, e.g. by involving an AD or DA conversion stage such that at least part of the signal processing is performed in the digital domain digital baseband systems H04L25/00; digital modulation/demodulation H04L27/00; CDMA H04B1/707; TDMA H04B7/2643; image transmission H04N5/00
• CPC Class Codes: H04B 1/0025 using a sampling rate lower than twice the highest frequency component of the sampled signal for demodulation of angle-modulated signals H03D3/006
• CPC Class Codes: H04B 1/28 the receiver comprising at least one semiconductor device having three or more electrodes
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

A subsampling receiver (50, 50′, 50″) for converting an RF signal to baseband is disclosed. The subsampling receiver (50, 50′, 50″) may be implemented into a wireless communications device (40), such as a wireless telephone handset. In one disclosed embodiment, the receiver (50) includes a sample and hold circuit (80) that samples a bandpass filtered input modulated signal at the subsampling frequency (f_s) that is well below the RF carrier frequency but twice the bandwidth (BW) of the payload; the sampled signal is digitized, and applied to two digital mixers (85I, 85Q) to produce in-phase and quadrature components (I,Q) of the payload. In another embodiment, the receiver (50′) includes two sample and hold circuits (96I, 96Q) to sample the filtered signal at different phases of the sampling frequency, to produce the in-phase and quadrature digital components. In a third embodiment, the receiver (50″) includes an analog mixer (116) to downconvert the RF input to an intermediate frequency, prior to digitization and digital mixing at quadrature phase.

Claims

1. A subsampling receiver, comprising:

a first bandpass filter coupled to receive a modulated signal, the modulated signal including at least one carrier signal at radio frequencies, modulated by a payload signal having frequencies significantly lower than the carrier signal and within a first bandwidth, the first bandpass filter passing frequencies within a frequency band at least as wide as the first bandwidth in a range including the at least one carrier signal;

a first sample and hold circuit, having an input coupled to the first bandpass filter, for sampling the modulated signal at a sampling frequency substantially lower than the frequency of the at least one carrier signal; and

a first analog to digital converter, coupled to the first sample and hold circuit, for digitizing the sampled modulated signal to produce a digital signal at a baseband frequency and corresponding to the payload signal.

2. The receiver of claim 1, wherein the sampling frequency is at least twice the first bandwidth.

3. The receiver of claim 1, further comprising:

first and second digital mixers, each coupled to the output of the first analog to digital converter, for applying a sequence of varying amplitude values to the digitized sampled modulated signal at a frequency lower than the sampling frequency;

wherein the sequence applied by the second digital mixer is phase-shifted from the sequence applied by the first digital mixer;

and wherein the first and second digital mixers present digital sequences corresponding to first and second components of the payload signal.

4. The receiver of claim 3, wherein the first and second digital mixers are for applying the sequences of varying amplitude values at a frequency that is one-fourth of the sampling frequency;

and wherein the sequence applied by the second digital mixer is phase-shifted by 90°

from the sequence applied by the first digital mixer so that the first and second components correspond to in-phase and quadrature components of the payload signal.

5. The receiver of claim 3, further comprising:

first and second digital low pass filters coupled to the outputs of the first and second digital mixers, respectively.

6. The receiver of claim 3, wherein the sampled modulated signal includes a plurality of aliases of the modulated signal, the plurality of aliases including a near baseband alias.

7. The receiver of claim 6, wherein the output of the first and second digital mixers correspond to first and second components of the modulated signal at the baseband frequency.

8. The receiver of claim 3, further comprising:

a first low noise amplifier, coupled to the output of the first bandpass filter; and

a second bandpass filter, coupled to the output of the first low noise amplifier.

9. The receiver of claim 1, further comprising:

a second sample and hold circuit, having an input coupled to the first bandpass filter, for sampling the modulated signal at the sampling frequency and phase-shifted from the sampling by the first sample and hold circuit; and

a second analog to digital converter, coupled to the second sample and hold circuit, for digitizing the sampled modulated signal to produce a digital signal at a baseband frequency and corresponding to the payload signal;

wherein the digital signals produced by the first and second analog to digital converters correspond to first and second components of the payload signal.

10. The receiver of claim 9, wherein the sampling by the first and second sample and hold circuits are phase-shifted from one another by 90°

;

and wherein the first and second components of the payload signal correspond to in-phase and quadrature components of the payload signal.

11. The receiver of claim 9, further comprising:

first and second digital low pass filters, coupled to the output of the first and second sample and hold circuits, respectively.

12. The receiver of claim 9, further comprising:

a first low noise amplifier, coupled to the input of the first bandpass filter; and

a second low noise amplifier, coupled to the output of the first bandpass filter.

13. The receiver of claim 12, wherein the second low noise amplifier is integrated into the same integrated circuit as the first and second sample and hold circuits and the first and second analog to digital converters.

14. The receiver of claim 1, further comprising:

an analog mixer, coupled to the output of the first bandpass filter, for downconverting the modulated signal to an intermediate frequency;

first and second digital mixers, each coupled to the output of the first analog to digital converter, for applying a sequence of varying amplitude values to the digitized sampled modulated signal at a frequency lower than the sampling frequency;

wherein the sequence applied by the second digital mixer is phase-shifted from the sequence applied by the first digital mixer;

and wherein the first and second digital mixers present digital sequences corresponding to first and second components of the payload signal.

15. The receiver of claim 14, wherein the first and second digital mixers are for applying the sequences of varying amplitude values at a frequency that is one-fourth of the sampling frequency;

and wherein the sequence applied by the second digital mixer is phase-shifted by 90°

from the sequence applied by the first digital mixer so that the first and second components correspond to in-phase and quadrature components of the payload signal.

16. The receiver of claim 14, wherein the analog-to-digital converter comprises:

a comparator, for generating an error signal corresponding to a difference between the sampled modulated signal and a feedback signal;

a bandpass filter coupled to the comparator, for filtering the error signal;

a quantizer, for quantizing the filtered error signal; and

a digital low pass filter, for filtering the quantized error signal;

wherein the feedback signal corresponds to the quantized error signal.

18. The communications unit of claim 17, wherein the sampling frequency is at least twice the first bandwidth.

19. The communications unit of claim 17, wherein the subsampling receiver further comprises:

first and second digital mixers, each coupled to the output of the first analog to digital converter, for applying a sequence of varying amplitude values to the digitized sampled modulated signal at a frequency that is one-fourth of the sampling frequency;

wherein the sequence applied by the second digital mixer is phase-shifted by 90°

from the sequence applied by the first digital mixer;

and wherein the first and second digital mixers present digital sequences corresponding to in-phase and quadrature components of the payload signal.

20. The communications unit of claim 19, wherein the sampled modulated signal includes a plurality of aliases of the modulated signal, the plurality of aliases including a near baseband alias;

and wherein the output of the first and second digital mixers correspond to first and second components of the modulated signal at the baseband frequency.

21. The communications unit of claim 17, wherein the subsampling receiver further comprises:

a second sample and hold circuit, having an input coupled to the first bandpass filter, for sampling the modulated signal at the sampling frequency and phase-shifted by 90°

from the sampling by the first sample and hold circuit; and

a second analog to digital converter, coupled to the second sample and hold circuit, for digitizing the sampled modulated signal to produce a digital signal at a baseband frequency and corresponding to the payload signal;

wherein the digital signals produced by the first and second analog to digital converters correspond to in-phase and quadrature components of the payload signal.

22. The communications unit of claim 21, further comprising:

a first low noise amplifier, coupled to the input of the first bandpass filter; and

a second low noise amplifier, coupled to the output of the first bandpass filter;

wherein the second low noise amplifier is integrated into the same integrated circuit as the first and second sample and hold circuits and the first and second analog to digital converters.

23. The communications unit of claim 17, wherein the subsampling receiver further comprises:

an analog mixer, coupled to the output of the first bandpass filter, for downconverting the modulated signal to an intermediate frequency;

first and second digital mixers, each coupled to the output of the first analog to digital converter, for applying a sequence of varying amplitude values to the digitized sampled modulated signal at a frequency values at a frequency that is one-fourth of the sampling frequency;

wherein the sequence applied by the second digital mixer is phase- shifted by 90°

from the sequence applied by the first digital mixer;

and wherein the first and second digital mixers present digital sequences corresponding to in-phase and quadrature components of the payload of the payload signal.

24. The communications unit of claim 17, wherein the analog-to-digital converter comprises:

a comparator, for generating an error signal corresponding to a difference between the sampled modulated signal and a feedback signal;

a bandpass filter coupled to the comparator, for filtering the error signal;

a quantizer, for quantizing the filtered error signal; and

a digital low pass filter, for filtering the quantized error signal;

wherein the feedback signal corresponds to the quantized error signal.

26. The method of claim 25, wherein the sampling frequency is at least twice the first bandwidth.

27. The method of claim 25, further comprising:

applying first and second sequences of varying amplitude values to the digitized sampled modulated signal at a frequency lower than the sampling frequency;

wherein the second sequence is phase-shifted from the first sequence, to produce first and second digital sequences corresponding to first and second components of the payload signal.

28. The method of claim 27, wherein the applying step applies the first and second sequences of varying amplitude values at a frequency that is one-fourth of the sampling frequency;

and wherein the second sequence is phase-shifted by 90°

from the first sequence so that the first and second components correspond to in-phase and quadrature components of the payload signal.

29. The method of claim 27, wherein the sampled modulated signal includes a plurality of aliases of the modulated signal, the plurality of aliases including a near baseband alias.

30. The method of claim 29, wherein the first and second components of the modulated signal are at the baseband frequency.

31. The method of claim 25, wherein the sampling step comprises:

sampling the modulated signal at the sampling frequency to produce a first sampled sequence of the modulated signal; and

also sampling the modulated signal at a phase-shifted version of the sampling frequency to produce a second sampled sequence of the modulated signal, phase-shifted from the first sampled sequence;

and wherein the digitizing step digitizes the first and second sampled sequences corresponding to first and second components of the payload signal.

32. The method of claim 31, wherein first and second components of the payload signal correspond to in-phase and quadrature components of the payload signal.

33. The method of claim 25, further comprising:

downconverting the modulated signal to an intermediate frequency; and

after the digitizing step, applying first and second sequences of varying amplitude values to the digitized sampled modulated signal at a frequency lower than the sampling frequency, the second sequence phase-shifted from the first sequence, to produce first and second components of the payload signal.

34. The method of claim 33, wherein the applying step is applies the first and second sequences at a frequency that is one-fourth of the sampling frequency;

and wherein the second sequence is phase-shifted by 90°

from the first sequence, so that the first and second components correspond to in-phase and quadrature components of the payload signal.

35. The method of claim 33, wherein the digitizing step comprises:

comparing the sampled modulated signal to a feedback signal to produce an error signal;

bandpass filtering the error signal;

quantizing the filtered error signal; and

low pass filtering the quantized error signal;

wherein the feedback signal corresponds to the quantized error signal.

17. A wireless communications unit, comprising:

an antenna;

a speaker;

a digital signal processor, for performing digital signal processing on signals received at the antenna;

an audio interface, for communicating processed received signals from the digital signal processor to the speaker; and

a subsampling receiver, comprising;

a first bandpass filter coupled to receive a modulated signal from the antenna, the modulated signal including at least one carrier signal at radio frequencies, modulated by a payload signal having frequencies significantly lower than the carrier signal and within a first bandwidth, the first bandpass filter passing frequencies within a frequency band at least as wide as the first bandwidth in a range including the at least one carrier signal;

a first sample and hold circuit, having an input coupled to the first bandpass filter, for sampling the modulated signal at a sampling frequency substantially lower than the frequency of the at least one carrier signal; and

a first analog to digital converter, coupled to the first sample and hold circuit, for digitizing the sampled modulated signal to produce, at an output coupled to the digital signal processor, a digital signal at a baseband frequency and corresponding to the payload signal.

25. A method of converting a received modulated signal to a baseband payload signal, the modulated signal including at least one carrier signal at radio frequencies, modulated by a payload signal having frequencies significantly lower than the carrier signal and within a first bandwidth:

, the method comprising the steps of;

bandpass filtering the modulated signal, to pass frequencies within a frequency band at least as wide as the first bandwidth in a range including the at least one carrier signal;

sampling the modulated signal at a sampling frequency substantially lower than the frequency of the at least one carrier signal; and

digitizing the sampled modulated signal to produce a digital signal at a baseband frequency and corresponding to the payload signal.