Dual-mode modulation systems and methods including oversampling of narrow bandwidth signals and DC offset compensation

• US 6,535,561 B2
• Filed: 09/11/1998
• Issued: 03/18/2003
• Est. Priority Date: 11/17/1997
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Arpaia, Domenico, Boesch, Ronald D.
• Primary Examiner(s): Le, Amanda T.
• Application Number:
  US09/151,996
• Publication Number:
  US 20020131389A1
• Time in Patent Office: 1,649 Days
• Field of Search: 375/216, 375/146, 375/296, 375/298, 375/302, 455/552, 455/553, 455/103, 455/102, 341/118, 332/103, 332/119
• US Class Current: 375/296
• CPC Class Codes: H03C 3/40 using two signal paths the outputs of which have a predetermined phase difference and at least one output being amplitude-modulated
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7101 with estimation filters
• CPC Class Codes: H04L 27/2071 in which the data are represented by the carrier phase, e.g. systems with differential coding

Abstract

A narrow bandwidth signal, such as a narrowband FM signal, is modulated in a modulator that modulates a wide bandwidth signal, such as a CDMA signal, by oversampling the narrow bandwidth signal and applying the oversampled narrow bandwidth signal to the modulator. By oversampling the narrow bandwidth signal, the same fixed low pass filter can be used for both the wide bandwidth signal and the oversampled narrow bandwidth signal. Accordingly, different low pass filters or switched low pass filters are not needed. The DC offset that is introduced by the digital-to-analog converter and/or the low pass filter of the modulator is compensated, preferably in the digital domain, to thereby reduce DC offset within acceptable limits for the modulation that is being used. More preferably, compensation is provided by subtracting from the sampled signal, a digital value representing the DC offset in the filtered analog signal that is introduced by the digital-to-analog converter and/or the low pass filter. A sensor senses the DC offset in the filtered analog signal. An analog-to-digital converter is responsive to the sensor to convert the sensed DC offset into a digital offset signal. A subtractor is responsive to the analog-to-digital converter to subtract the digital DC offset signal from the sampled signal and to apply the sampled signal minus the DC offset signal, to the digital-to-analog converter.

Claims

1. A dual-mode modulation system comprising:

means for sampling an applied signal;

means for converting the sampled signal to an analog signal;

means for low pass filtering the analog signal to produce a filtered analog signal, at least one of the means for converting and the means for low pass filtering introducing DC offset into the filtered analog signal;

means for modulating the low pass filtered analog signal onto a carrier;

means for compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering;

means for applying a first signal to the sampling means to thereby modulate the first signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying a second signal of narrower bandwidth than the first signal to the sampling means to thereby sample the second signal and modulate the second signal on a carrier using the sampling means, the converting means and the low pass filtering means; and

wherein the means for compensating further comprises;

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal;

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal; and

means for scaling the digital DC offset signal into a scaled digital DC offset signal;

the means for subtracting being responsive to the means for scaling, to subtract the scaled digital DC offset signal from the sampled signal.

2. A dual-mode modulation system according to claim 1 wherein the low pass filtering means is a fixed low pass filter having a passband that encompasses the sampled first signal and the sampled second narrower bandwidth signal, such that the fixed low pass filter is used to filter both the first signal and the second narrower bandwidth signal.

3. A dual-mode modulation system according to claim 1 wherein the first signal is a CDMA signal and wherein the second signal is an FM signal.

4. A dual-mode modulation system according to claim 3 wherein the CDMA signal is a direct sequence spread spectrum signal and wherein the FM signal is an analog cellular telephone signal.

5. A dual-mode modulation system according to claim 1

wherein the modulating means comprises an IQ modulator having I and Q inputs; and

wherein the sampling means comprises first and second samplers, the first sampler being responsive to the I input and the second sampler being responsive to the Q input.

6. A dual-mode modulation system according to claim 1 wherein the means for compensating comprises means for subtracting from the sampled signal, a digital value representing the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering.

7. A dual-mode modulation system according to claim 1 wherein the means for compensating comprises:

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal; and

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal.

8. A dual-mode modulation system according to claim 1 wherein the means for sensing comprises means for low pass filtering.

9. A dual-mode modulation system according to claim 1 wherein the means for converting the sensed DC offset into a digital DC offset signal comprises a one-bit delta-sigma analog-to-digital converter.

10. A dual-mode modulation system comprising:

means for sampling an applied signal;

means for converting the sampled signal to an analog signal;

means for low pass filtering the analog signal to produce a filtered analog signal, at least one of the means for converting and the means for low pass filtering introducing DC offset into the filtered analog signal;

means for modulating the low pass filtered analog signal onto a carrier;

means for compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering;

means for applying a first signal to the sampling means to thereby modulate the first signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying a second signal of narrower bandwidth than the first signal to the sampling means to thereby sample the second signal and modulate the second signal on a carrier using the sampling means, the converting means and the low pass filtering means; and

wherein the means for compensating further comprises;

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal;

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal; and

means for intermittently latching the digital DC offset signal, and for applying the latched digital DC offset signal to the means for subtracting, such that the latched digital DC offset signal is subtracted from the sampled signal.

11. A dual-mode modulation system according to claim 10 wherein the means for converting the sensed DC offset into a digital DC offset signal is clocked at a first clock rate and wherein the means for intermittently latching is clocked at a second clock rate that is lower than the first clock rate.

12. A dual-mode modulation system according to claim 10 wherein the

means for compensating further comprises:

means for scaling the digital DC offset signal into a scaled digital DC offset signal;

the means for intermittently latching being responsive to the means for scaling to periodically latch the scaled digital DC offset signal such that the latched scaled digital DC offset signal is subtracted from the sampled signal.

13. A dual-mode modulation system comprising:

means for sampling an applied signal;

means for converting the sampled signal to an analog signal;

means for low pass filtering the analog signal to produce a filtered analog signal, at least one of the means for converting and the means for low pass filtering introducing DC offset into the filtered analog signal;

means for modulating the low pass filtered analog signal onto a carrier;

means for compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering;

means for applying a first signal to the sampling means to thereby modulate the first signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying a second signal of narrower bandwidth than the first signal to the sampling means to thereby sample the second signal and modulate the second signal on a carrier using the sampling means, the converting means and the low pass filtering means; and

wherein the means for compensating further comprises;

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal;

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal; and

means for periodically inverting the polarity of the sensed DC offset and for applying the polarity inverted sensed DC offset to the means for converting the sensed DC offset in a digital DC offset signal.

14. A dual-mode radiotelephone comprising:

means for sampling an applied signal;

means for converting the sampled signal to an analog signal;

means for low pass filtering the analog signal to produce a filtered analog signal, at least one of the means for converting and the means for low pass filtering introducing DC offset into the filtered analog signal;

means for modulating the low pass filtered analog signal onto a radiotelephone carrier;

means for compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering;

means for generating a first radiotelephone signal and a second radiotelephone signal of narrower bandwidth than the first radiotelephone signal;

means for applying the first radiotelephone signal to the sampling means to thereby modulate the first radiotelephone signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying the second radiotelephone signal of narrower bandwidth than the first radiotelephone signal to the sampling means to thereby sample the second radiotelephone signal and modulate the second radiotelephone signal on a carrier using the sampling means, the converting means and the low pass filtering means; and

wherein the means for compensating further comprises;

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal;

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal; and

means for scaling the digital DC offset signal into a scaled digital DC offset signal;

the means for subtracting being responsive to the means for scaling, to subtract the scaled digital DC offset signal from the sampled signal.

15. A dual-mode radiotelephone according to claim 14 wherein the low pass filtering means is a fixed low pass filter having a fixed low pass filter having a passband that encompasses the sampled first signal and the sampled second narrower bandwidth signal, such that the fixed low pass filter is used to filter both the first radiotelephone signal and the second narrower bandwidth radiotelephone signal.

16. A dual-mode radiotelephone according to claim 14 wherein the first radiotelephone signal is a CDMA signal and wherein the second radiotelephone signal is an FM signal.

17. A dual-mode radiotelephone according to claim 16 wherein the CDMA signal is a direct sequence spread spectrum signal and wherein the FM signal is an analog cellular telephone signal.

18. A dual-mode radiotelephone according to claim 14:

wherein the modulating means comprises an IQ modulator having I and Q inputs; and

wherein the sampling means comprises first and second samplers, the first sampler being responsive to the I input and the second sampler being responsive to the Q input.

19. A dual-mode radiotelephone according to claim 14 wherein the means for compensating comprises means for subtracting from the sampled signal, a digital value representing the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering.

20. A dual-mode radiotelephone according to claim 14 wherein the means for compensating comprises:

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal; and

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal.

21. A dual-mode radiotelephone comprising:

means for sampling an applied signal;

means for converting the sampled signal to an analog signal;

means for low pass filtering the analog signal to produce a filtered analog signal, at least one of the means for converting and the means for low pass filtering introducing DC offset into the filtered analog signal;

means for modulating the low pass filtered analog signal onto a radiotelephone carrier;

means for compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering;

means for generating a first radiotelephone signal and a second radiotelephone signal of narrower bandwidth than the first radiotelephone signal;

means for applying the first radiotelephone signal to the sampling means to thereby modulate the first radiotelephone signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying the second radiotelephone signal of narrower bandwidth than the first radiotelephone signal to the sampling means to thereby sample the second radiotelephone signal and modulate the second radiotelephone signal on a carrier using the sampling means, the converting means and the low pass filtering means; and

wherein the means for compensating further comprises;

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal;

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal; and

means for intermittently latching the digital DC offset signal, and for applying the latched digital DC offset signal to the means for subtracting, such that the latched digital DC offset signal is subtracted from the sampled signal.

22. A dual-mode radiotelephone according to claim 21 wherein the means for converting the sensed DC offset into a digital DC offset signal is clocked at a first clock rate and wherein the means for intermittently latching is clocked at a second clock rate that is lower than the first clock rate.

23. A dual-mode radiotelephone according to claim 21 wherein the means for compensating further comprises:

means for scaling the digital DC offset signal into a scaled digital DC offset signal;

the means for intermittently latching being responsive to the means for scaling to periodically latch the scaled digital DC offset signal such that the latched scaled digital DC offset signal is subtracted from the sampled signal.

24. A dual-mode radiotelephone comprising:

means for sampling an applied signal;

means for converting the sampled signal to an analog signal;

means for low pass filtering the analog signal to produce a filtered analog signal, at least one of the means for converting and the means for low pass filtering introducing DC offset into the filtered analog signal;

means for modulating the low pass filtered analog signal onto a radiotelephone carrier;

means for compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the means for converting and the means for low pass filtering;

means for generating a first radiotelephone signal and a second radiotelephone signal of narrower bandwidth than the first radiotelephone signal;

means for applying the first radiotelephone signal to the sampling means to thereby modulate the first radiotelephone signal on a carrier using the sampling means, the converting means and the low pass filtering means, and for applying the second radiotelephone signal of narrower bandwidth than the first radiotelephone signal to the sampling means to thereby sample the second radiotelephone signal and modulate the second radiotelephone signal on a carrier using the sampling means, the converting means and the low pass filtering means; and

wherein the compensating means further comprises;

means for sensing the DC offset in the filtered analog signal;

means for converting the sensed DC offset into a digital DC offset signal;

means for subtracting the digital DC offset signal from the sampled signal, and for applying the sampled signal less the DC offset signal to the means for converting the digital input signal into an analog signal; and

means for periodically inverting the polarity of the sensed DC offset and for applying the polarity inverted sensed DC offset to the means for converting the sensed DC offset in a digital DC offset signal.

25. A dual-mode radiotelephone according to claim 24 wherein the means for converting the sensed DC offset into a digital DC offset signal comprises a one-bit delta-sigma analog-to-digital converter.

26. A dual-mode radiotelephone comprising:

a sampler that samples an applied signal;

a digital-to-analog converter that converts the sampled signal into an analog signal;

a low pass filter that filters the analog signal to produce a filtered analog signal, at least one of the digital-to-analog converter and the low pass filter introducing DC offset into the filtered analog signal;

a modulator that modulates the filtered analog signal onto a radiotelephone carrier;

a DC offset compensator that compensates for the DC offset in the filtered analog signal that is introduced by the at least one of the digital-to-analog converter and the low pass filter;

a circuit that generates a first radiotelephone signal and a second radiotelephone signal of narrower bandwidth than the first radiotelephone signal, that applies the first radiotelephone signal to the sampler to thereby modulate the first radiotelephone signal on a carrier using the sampler, the digital-to-analog converter and the low pass filter, and that applies the second radiotelephone signal of narrower bandwidth than the first radiotelephone signal to the sampler, to thereby sample the second radiotelephone signal and modulate the second radiotelephone signal on a carrier using the sampler, the digital-to-analog converter and the low pass filter;

wherein the DC offset compensator comprises;

a sensor that senses the DC offset in the filtered analog signal;

an analog-to-digital converter that is responsive to the sensor to convert the sensed DC offset into a digital DC offset signal;

a subtractor that is responsive to the analog-to-digital converter to subtract the digital DC offset signal from the sampled signal, and to apply the sampled signal less the DC offset signal, to the digital-to-analog converter; and

a scaler that is responsive to the analog-to-digital converter to scale the digital DC offset signal into a scaled digital DC offset signal;

the subtractor being responsive to the scaler to subtract the scaled digital DC offset signal from the sampled signal.

27. A dual-mode radiotelephone according to claim 26 wherein the first radiotelephone signal is a CDMA signal and wherein the second radiotelephone signal is an FM signal.

28. A dual-mode radiotelephone according to claim 27 wherein the CDMA signal is a direct sequence spread spectrum signal and wherein the FM signal is an analog cellular telephone signal.

29. A dual-mode radiotelephone according to claim 26 wherein the modulator is an IQ modulator that modulates in-phase and quadrature filtered analog signals onto a carrier.

30. A dual-mode radio telephone according to claim 26 wherein the low pass filter is a fixed low pass filter having a passband that encompasses the sampled first signal and the sampled second narrower bandwidth signal, such that the fixed low pass filter is used to filter both the first radiotelephone signal and the second narrower bandwidth radiotelephone signal.

31. A dual-mode radiotelephone comprising:

a sampler that samples an applied signal;

a digital-to-analog converter that converts the sampled signal into an analog signal;

a low pass filter that filters the analog signal to produce a filtered analog signal, at least one of the digital-to-analog converter and the low pass filter introducing DC offset into the filtered analog signal;

a modulator that modulates the filtered analog signal onto a radiotelephone carrier;

a DC offset compensator that compensates for the DC offset in the filtered analog signal that is introduced by the at least one of the digital-to-analog converter and the low pass filter;

a circuit that generates a first radiotelephone signal and a second radiotelephone signal of narrower bandwidth than the first radiotelephone signal, that applies the first radiotelephone signal to the sampler to thereby modulate the first radiotelephone signal on a carrier using the sampler, the digital-to-analog converter and the low pass filter, and that applies the second radiotelephone signal of narrower bandwidth than the first radiotelephone signal to the sampler, to thereby sample the second radiotelephone signal and modulate the second radiotelephone signal on a carrier using the sampler, the digital-to-analog converter and the low pass filter;

wherein the DC offset compensator comprises;

a sensor that senses the DC offset in the filtered analog signal;

an analog-to-digital converter that is responsive to the sensor to convert the sensed DC offset into a digital DC offset signal;

a subtractor that is responsive to the analog-to-digital converter to subtract the digital DC offset signal from the sampled signal, and to apply the sampled signal less the DC offset signal, to the digital-to-analog converter; and

a latch that is responsive to the analog-to-digital converter to intermittently latch the digital DC offset signal, and to apply the latched digital DC offset signal to the subtractor, such that the latched digital DC offset signal is subtracted from the sampled signal.

32. A dual-mode radiotelephone according to claim 31 wherein the analog-to-digital converter is clocked at a first clock rate and wherein the latch is clocked at a second clock rate that is lower than the first clock rate.

33. A dual-mode radiotelephone according to claim 31 wherein the DC offset compensator further comprises:

a scaler that is responsive to the analog-to-digital converter to scale the digital DC offset signal into a scaled digital DC offset signal;

the latch being responsive to the scaler to periodically latch the scaled digital DC offset signal such that the latched scaled digital DC offset signal is subtracted from the sampled signal.

34. A dual-mode radiotelephone comprising:

a sampler that samples an applied signal;

a digital-to-analog converter that converts the sampled signal into an analog signal;

a low pass filter that filters the analog signal to produce a filtered analog signal, at least one of the digital-to-analog converter and the low pass filter introducing DC offset into the filtered analog signal;

a modulator that modulates the filtered analog signal onto a radiotelephone carrier;

a DC offset compensator that compensates for the DC offset in the filtered analog signal that is introduced by the at least one of the digital-to-analog converter and the low pass filter;

a circuit that generates a first radiotelephone signal and a second radiotelephone signal of narrower bandwidth than the first radiotelephone signal, that applies the first radiotelephone signal to the sampler to thereby modulate the first radiotelephone signal on a carrier using the sampler, the digital-to-analog converter and the low pass filter, and that applies the second radiotelephone signal of narrower bandwidth than the first radiotelephone signal to the sampler, to thereby sample the second radiotelephone signal and modulate the second radiotelephone signal on a carrier using the sampler, the digital-to-analog converter and the low pass filter;

wherein the DC offset compensator comprises;

a sensor that senses the DC offset in the filtered analog signal;

an analog-to-digital converter that is responsive to the sensor to convert the sensed DC offset into a digital DC offset signal;

a subtractor that is responsive to the analog-to-digital converter to subtract the digital DC offset signal from the sampled signal, and to apply the sampled signal less the DC offset signal, to the digital-to-analog converter; and

a polarity inverter that is responsive to the sensor to periodically invert the polarity of the sensed DC offset and to apply the polarity inverted sensed DC offset to the analog-to-digital converter, the analog-to-digital converter converting the periodically polarity inverted sensed DC offset signal into the digital offset signal.

35. A dual-mode radiotelephone according to claim 34 wherein the analog-to-digital converter is a one-bit delta-sigma analog-to-digital converter.

36. A dual-mode modulation method for a first digital input signal and a second digital input signal having narrower bandwidth than the first digital input signal, comprising the steps of:

oversampling the second digital input signal;

converting the first digital input signal and the oversampled second digital input signal into an analog signal;

low pass filtering the analog signal to produce a filtered analog signal, at least one of the converting and low pass filtering steps introducing DC offset into the filtered analog signal;

modulating the filtered analog signal onto a carrier;

compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the converting and low pass filtering steps;

wherein the compensating step comprises the steps of;

sensing the DC offset in the filtered analog signal;

converting the sensed DC offset into a digital DC offset signal;

subtracting the digital DC offset signal from the first and second oversampled digital input signals; and

scaling the digital DC offset signal into a scaled digital DC offset signal.

37. A dual-mode modulation method according to claim 36 wherein the first digital input signal is a CDMA signal and wherein the second digital input signal is an FM signal.

38. A dual-mode modulation method according to claim 37 wherein the CDMA signal is a direct sequence spread spectrum signal and wherein the FM signal is an analog cellular telephone signal.

39. A dual-mode modulation method according to claim 36 wherein the step of modulating comprises the step of IQ modulating in-phase and quadrature filtered analog signals onto a carrier.

40. A dual-mode method according to claim 36 wherein the low pass filtering step comprises the step of employing a fixed low pass filter having a passband that encompasses the first digital input signal and the oversampled second digital input signal, such that the fixed low pass filter is used to filter both the first digital input signal and the oversampled second digital input signal.

41. A dual-mode modulation method for a first digital input signal and a second digital input signal having narrower bandwidth than the first digital input signal, comprising the steps of:

oversampling the second digital input signal;

converting the first digital input signal and the oversampled second digital input signal into an analog signal;

low pass filtering the analog signal to produce a filtered analog signal, at least one of the converting and low pass filtering steps introducing DC offset into the filtered analog signal;

modulating the filtered analog signal onto a carrier;

compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the converting and low pass filtering steps;

wherein the compensating step comprises the steps of;

sensing the DC offset in the filtered analog signal;

converting the sensed DC offset into a digital DC offset signal;

subtracting the digital DC offset signal from the first and second oversampled digital input signals; and

intermittently latching the digital DC offset signal.

42. A dual-mode modulation method according to claim 41 wherein the step of converting the sensed DC offset into a digital DC offset signal is clocked at a first clock rate and wherein the step of intermittently latching is clocked at a second clock rate that is lower than the first clock rate.

43. A dual-mode modulation method according to claim 41 wherein the step of compensating further comprises the step of:

scaling the digital DC offset signal into a scaled digital DC offset signal.

44. A dual-mode modulation method for a first digital input signal and a second digital input signal having narrower bandwidth than the first digital input signal, comprising the steps of:

oversampling the second digital input signal;

converting the first digital input signal and the oversampled second digital input signal into an analog signal;

low pass filtering the analog signal to produce a filtered analog signal, at least one of the converting and low pass filtering steps introducing DC offset into the filtered analog signal;

modulating the filtered analog signal onto a carrier;

compensating for the DC offset in the filtered analog signal that is introduced by the at least one of the converting and low pass filtering steps;

wherein the compensating step comprises the steps of;

sensing the DC offset in the filtered analog signal;

converting the sensed DC offset into a digital DC offset signal;

subtracting the digital DC offset signal from the first and second oversampled digital input signals; and

periodically inverting the polarity of the sensed DC offset.

45. A dual-mode modulation method according to claim 44 wherein the step of converting the sensed DC offset into a digital DC offset signal comprises the step of delta-sigma analog-to-digital converting the sensed DC offset into a digital DC offset signal.