Receiver with analog and digital channel selectivity

• US 5,926,513 A
• Filed: 01/27/1997
• Issued: 07/20/1999
• Est. Priority Date: 01/27/1997
• Status: Expired due to Term

• Current Assignee: Alcatel SA (Nokia Corporation)
• Original Assignee: Alcatel Alsthom Compagnie Generale d'Electricite (Nokia Corporation)
• Inventors: Voboril, Charles J., Suominen, Edwin A.
• Primary Examiner(s): Bocure, Tesfaldet
• Application Number:
  US08/788,241
• Time in Patent Office: 904 Days
• Field of Search: 375/316, 375/350, 375/229, 375/285, 455/130
• US Class Current: 375/346
• CPC Class Codes: H03J 3/08 by varying a second parameter simultaneously with the tuning, e.g. coupling bandpass filter
• 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 1/001 Channel filtering, i.e. selecting a frequency channel within the SDR system multiplexing of multicarrier modulation signals being represented by different frequencies H04L5/06; multiplexing of multicarrier modulation signals H04L5/023
• CPC Class Codes: H04B 1/0017 Digital filtering H04B1/001 takes precedence; digital filters per se H03H17/00
• CPC Class Codes: H04B 1/26 for superheterodyne receivers multiple frequency-changing H03D7/16

Abstract

A radio receiver for receiving radio frequency (RF) signals includes an analog down-converting and filtering section for down-converting and selecting the RF signals to an intermediate passband and a stopband which substantially determine the widest bandwidth of operation of the radio receiver. An analog-to-digital converter converts the intermediate frequency signal to a sampled digital signal. A digital filtering section, responsive to the sampled digital signal, includes wideband and narrowband digital filters for providing wide bandwidth and narrow bandwidth baseband quadrature output signals for the widest bandwidth of operation and a narrow bandwidth of operation at a frequency within a passband. The wideband digital filtering is designed to only provide a modest amount of additional stopband attenuation with a significantly sharper transition band rolloff from passband to stopband. When this limited digital filtering is applied to the signal that has already been subjected to the analog filtering, the selectivity of the overall receiver frequency response is improved. The receiver may be used for multi-bandwidth operation by simply adding digital filtering to define any narrower passband of interest. This allows bandwidth switching to take place without any changes to the analog circuitry. Simultaneous processing of both wide and narrow modes of operation may be done in the digital signal processor (DSP) performing the digital filtering.

Claims

1. A radio receiver for receiving radio frequency signals, the radio receiver having a wideband channel selectivity and a narrowband channel selectivity, corresponding respectively to a wide bandwidth mode of operation and a narrow bandwidth mode of operation, comprising:

analog down-converting and filtering means, having a passband and a stopband which in combination substantially determine the wide bandwidth mode of operation of the radio receiver, said analog down-converting and filtering means responsive to the received radio frequency signals, for selecting and down-converting the received radio frequency signals to an intermediate frequency signal within the passband;

an analog-to-digital converter, responsive to said intermediate frequency signal, for converting said intermediate frequency signal to a sampled digital signal;

wideband digital filtering means, responsive to said sampled digital signal, for providing wide bandwidth baseband quadrature output signals for the wide bandwidth of operation;

narrowband digital filtering means, responsive to said wide bandwidth baseband quadrature output signals, for providing narrow bandwidth baseband quadrature output signals for the narrow bandwidth mode of operation; and

wherein said analog down-converting and filtering means provides the majority of the radio receiver wideband channel selectivity, thereby providing more than half of the signal attenuation for received radio frequency signals outside of the passband.

2. A radio receiver as claimed in claim 1, wherein said analog down-converting and filtering means includes:

a first analog filtering stage having a bandwidth equal to the wide bandwidth mode of operation of the receiver; and

a second analog filtering stage following said first analog filtering stage, said second analog filtering stage having a bandwidth and frequency response such that the combined frequency response of said first and second analog filtering stages substantially define said passband and said stopband for the wide bandwidth mode of operation.

3. A radio receiver as claimed in claim 2, wherein a center frequency of said second analog filtering stage is 455 kHz.

4. A radio receiver as claimed in claim 2, wherein a center frequency of said second analog filtering stage is 450 kHz.

5. A radio receiver as claimed in claim 2, wherein said wideband digital filtering means comprises:

a first digital filtering stage, responsive to said sampled digital signal, for providing quadrature-related output signals;

a numerically controlled oscillator and modulator, responsive to said quadrature-related output signals, for shifting a frequency of said quadrature-related output signals to baseband, thereby providing baseband quadrature signals; and

a pair of wide channel low pass filters, each responsive to a respective one of said baseband quadrature signals, for providing said wide bandwidth baseband quadrature output signals.

6. A radio receiver as claimed in claim 5, wherein said first digital filtering stage implements:

a moving average having unity weighting coefficients which have been modulated by a complex exponential to thereby shift a center frequency of said first digital filtering stage from baseband to a center frequency of a signal of interest; and

rejection of the image of a real-valued input signal to said first digital filtering stage.

7. A radio receiver as claimed in claim 6, wherein said narrowband digital filtering means includes a pair of narrow channel low pass filters, each responsive to a respective one of said wide bandwidth baseband quadrature output signals, for providing said narrow bandwidth baseband quadrature output signals.

8. A radio receiver as claimed in claim 7, wherein said narrow and wide channel low pass filters are selectively replaceable for modifying said narrow bandwidth mode of operation and said wide bandwidth mode of operation, respectively.

9. A radio receiver as claimed in claim 5, wherein said narrowband digital filtering means includes a pair of narrow channel low pass filters, each responsive to a respective one of said wide bandwidth baseband quadrature output signals, for providing said narrow bandwidth baseband quadrature output signals.

10. A radio receiver as claimed in claim 9, wherein said narrow and wide channel low pass filters are selectively replaceable for modifying said narrow bandwidth mode of operation and said wide bandwidth mode of operation, respectively.

11. A radio receiver as claimed in claim 1, wherein the closest alias of said sampled digital signal is separated from said frequency range of interest by a plurality of analog filter bandwidths.

12. A radio receiver for receiving radio frequency signals, the radio receiver having a wideband channel selectivity and a narrowband channel selectivity, corresponding to a wide bandwidth mode of operation and a narrow bandwidth mode of operation, comprising:

analog down-converting and filtering means, responsive to the received radio frequency signals, having a passband and a stopband which in combination substantially determine the wide bandwidth mode of operation of the radio receiver, said analog down-converting and filtering means for selecting and down-converting the received radio frequency signals to an intermediate frequency signal within the passband;

an analog-to-digital converter, responsive to said intermediate frequency signal, for converting said intermediate frequency signal to a sampled digital signal;

wideband digital filtering means, responsive to said sampled digital signal, for providing wide bandwidth baseband quadrature output signals for the wide bandwidth mode of operation, said wideband digital filtering means comprising;

a first digital filtering stage, responsive to said sampled digital signal, for providing quadrature-related output signals;

a numerically controlled oscillator and modulator, responsive to said quadrature-related output signals, for shifting a frequency of said quadrature-related output signals to baseband, thereby providing baseband quadrature signals; and

a pair of wide channel low pass filters, each responsive to a respective one of said baseband quadrature signals, for providing said wide bandwidth baseband quadrature output signals; and

narrowband digital filtering means, responsive to said wide bandwidth baseband quadrature output signals, for providing narrow bandwidth baseband quadrature output signals for a narrow bandwidth mode of operation.

13. A radio receiver as claimed in claim 12, wherein said first digital filtering stage implements:

a moving average having unity weighting coefficients which have been modulated by a complex exponential to thereby shift a center frequency of said first digital filtering stage from baseband to a center frequency of a signal of interest; and

rejection of the image of a real-valued input signal to said first digital filtering stage.

14. A radio receiver as claimed in claim 13, wherein said narrowband digital filtering means includes a pair of narrow channel low pass filters, each responsive to a respective one of said wide bandwidth baseband quadrature output signals, for providing said narrow bandwidth baseband quadrature output signals.

15. A radio receiver as claimed in claim 14, wherein said narrow and wide channel low pass filters are selectively replaceable for modifying said narrow bandwidth mode of operation and said wide bandwidth mode of operation, respectively.

16. A radio receiver as claimed in claim 12, wherein said narrowband digital filtering means includes a pair of narrow channel low pass filters, each responsive to a respective one of said wideband baseband quadrature output signals, for providing said narrow bandwidth baseband quadrature output signals.

17. A radio receiver as claimed in claim 16, wherein said narrow and wide channel low pass filters are selectively replaceable for modifying said narrow bandwidth mode of operation and said wide bandwidth mode of operation, respectively.

25. A radio receiver as claimed in claim 12, wherein the closest alias of said sampled digital signal is separated from said frequency range of interest by a plurality of analog filter bandwidths.

18. A radio receiver for receiving radio frequency signals, the radio receiver having a wideband channel selectivity corresponding to a wide bandwidth mode of operation, comprising:

analog down-converting and filtering means, having a passband and a stopband which in combination substantially determine the wide bandwidth mode of operation of the radio receiver, said analog down-converting and filtering means responsive to the received radio frequency signals, for selecting and down-converting the received radio frequency signals to an intermediate frequency signal within the passband, said analog down-converting and filtering means providing the majority of the radio receiver wideband channel selectivity thereby providing more than half of the signal attenuation for received radio frequency signals outside of the passband;

an analog-to-digital converter, responsive to said intermediate frequency signal, for converting said intermediate frequency signal to a sampled digital signal;

a selectively replaceable digital filter, responsive to said sampled digital signal, for providing output signals in a selected channel bandwidth, said selected channel bandwidth being determined by a selection of a selectively replaceable digital filter having said selected channel bandwidth.

19. A radio receiver as claimed in claim 18, wherein each said selectively replaceable digital filter comprises:

a first digital filtering stage, responsive to said sampled digital signal, for providing quadrature-related output signals;

a numerically controlled oscillator and modulator, responsive to said quadrature-related output signals, for shifting a frequency of said quadrature-related output signals to baseband, thereby providing baseband quadrature signals; and

a pair of selectively replaceable low pass filters, each responsive to a respective one of said baseband quadrature signals, for providing said output signals as quadrature output signals.

20. A radio receiver as claimed in claim 18, wherein said selectively replaceable digital filter comprises:

a wideband digital filter, responsive to said sampled digital signal, for providing wide bandwidth output signals for a wide bandwidth mode of operation; and

a narrowband digital filter, responsive to said wide bandwidth output signals, for providing narrow bandwidth output signals for a narrow bandwidth mode of operation.

21. A receiver that performs both analog and digital filtering of received radio frequency signals for determining a channel selectivity of the receiver, the receiver having a wideband channel selectivity and a narrowband channel selectivity, corresponding to a wide bandwidth mode of operation and a narrow bandwidth mode of operation, the receiver comprising:

analog filtering means having a passband and a stopband which in combination substantially determine the wide bandwidth mode of operation of the receiver, said analog filtering means responsive to the received radio frequency signals, and for providing the majority of the receiver wideband channel selectivity, thereby providing more than half of the signal attenuation for received signals outside of the passband, for substantially determining the channel selectivity of the wide bandwidth mode of operation of the receiver; and

digital filtering means which compensates for imperfections in the analog filtering means and limits a noise amplitude of noise generated after analog filtering by the analog filtering means which would alias into the passband. for selecting and down-converting the received radio frequency signals to intermediate frequency signal within the passband.

22. A receiver as claimed in claim 21, wherein said analog filtering means includes a plurality of analog filtering stages, and wherein a center frequency of a last analog filtering stage is 455 kHz.

23. A receiver as claimed in claim 21, wherein said analog filtering means includes a plurality of analog filtering stages, and wherein a center frequency of a last analog filtering stage is 450 kHz.

24. A receiver as claimed in claim 21, wherein said digital filtering means includes a plurality of digital filtering stages, and wherein a first digital filtering stage implements a moving average whose unity weighting coefficients have been modulated by a complex exponential so as to shift a center frequency of said digital filtering means from baseband to a center frequency of a signal of interest.