METHOD AND APPARATUS FOR CANCELLATION OF DC OFFSET AND RECONSTRUCTION OF LOW FREQUENCY COMPONENTS OF BASEBAND SIGNAL

Abstract: Information loss and DC offset problems associated with AC-coupling the signal processing path of an RF modem are obviated by means of a low frequency reconstruction and DC offset cancellation signal processing arrangement which reconstructs low frequency information that has been removed by high pass-filtering of a baseband signal prior to data decision processing, and cancels slowly varying DC impairments to the baseband signal. The difference between the quantized data output of the baseband comparator and the baseband signal is integrated to derive a correction signal representative of the accumulated variation of errors. This accumulate variation is subtracted from the baseband signal, to remove slowly varying DC offset errors, so that the amplitude values of the signal constellation input to the quantization circuitry of the data recovery processor are not distorted by a DC offset level upon which the baseband modulation is riding. Low pass-filtering the output of the comparator allows an exact representation of the low frequency signal component of the baseband signal that had been removed by the AC-coupling of the baseband signal prior to its application to the signal processing comparator to be recovered, so that it can be reinjected into the baseband signal path prior to its application to the comparator.

Description: METHOD AND APPARATUS FOR CANCELLATION OF DC OFFSET AND RECONSTRUCTION OF LOW FREQUENCY COMPONENTS OF BASEBAND SIGNAL FIELD OF THE INVENTION The present invention relates in general to communication systems, and is particularly directed to a signal processing mechanism for reconstructing low freguency information that has been removed by high pass- filtering of a baseband signal prior to data decision processing, and canceling low frequency DC impairments to the baseband signal, such as may be introduced by mechanical stress in a printed circuit board containing demodulator components, AGC amplifier effets, microphonics, and DC offset introduced by a high gain amplifier through which the baseband signal is amplified upstream of data recovery. BACKGROUND OF THE INVENTION Figure 1 diagrammatically illustrates a simplified configuration of a baseband demodulator for an RF modem having a first down-converting mixer 10, to a first input 11 of which the output of a first IF local oscillator signal is applied and to a second input 12 of which a modulated RF signal of interest is applied. The output 14 of the mixer 10 is an intermediate frequency (IF) signal, which is to be down-converted to baseband for application to a data recovery signal processing circuit. For this purpose, the IF signal, which may be in the microvolt range, is coupled through an IF bandpass filter 15 to a first input 21 of a second down-converting mixer 20, which has a second input 22 coupled to receive the output of a second, baseband down-converting local oscillator, the amplitude of which typically varies in the millivolt range. Mixer 20 has an output 24 that produces a baseband signal containing the modulated data of interest. The modulated baseband signal is coupled to an AGC- driven, high gain amplifier 30, which amplifies the (microvolt) baseband modulation up to a higher range of voltage amplitude variation (e.g., the millivolt range) for application to a downstream signal processing circuit 40. The signal processing circuit 40 compares a sample of the instantaneous amplitude of the baseband signal with one or more DC guantization levels, to extract modulation/data values associated with those quantization levels. In the case of a binary modulated signal, as a non-limiting example, the signal processing circuit 40 may comprise a simple polarity-sensitive comparator, the output of which is one of two binary values (0/1) depending upon the polarity of the AC-coupled baseband signal sample of interest. Because the demodulation and amplifier components of the RF modem are very sensitive to ambient conditions, such as temperature, they are prone to introduce a substantial DC noise or offset component into the baseband signal. Such DC offset can be introduced into the baseband signal path from a number of sources including, but not limited to, mechanical stress in the printed circuit board containing the components of the demodulator circuitry, microphonics, and a DC offset introduced by the high gain amplifier. As a consequence, it is customary practice to AC- couple the down-converted baseband signal to amplifier 30 by way of a high pass-filter (capacitor) 29, to remove such unwanted DC offset components. Similarly, to remove like effects that may introduced into the amplified baseband signal by the high gain amplifier, the output 34 of the amplifier 30 is coupled to the signal processing circuit 40 through a further high pass-filter (capacitor) 39. Unfortunately, because such high pass-filtering effectively removes low frequency/DC level information prior to the data decision (quantization comparison) process carried output by signal processing circuit 40, it constitutes a source of error, sometimes referred to as low frequency 'swim'. Namely, although the intent of installing AC-coupling upstream of the data recovery circuitry is to remove what may be a very substantial DC offset level upon which the baseband modulation is riding, it has the undesirable effect of both removing a prescribed (low frequency) portion of the useful frequency spectrum of the data of interest, and of distorting the amplitude values of the signal constellation input to the quantization circuitry. SUMMARY OF THE INVENTION In accordance with the present invention, the above- described information loss and DC offset impairment problems associated with AC-coupling the signal processing path of an RF modem are effectively obviated by a low frequency reconstruction or reinsertion and DC offset cancellation signal processing methodology. Pursuant to a first aspect of this methodology, the quantized data output of a baseband slicer/comparator is differentially combined in an output combiner with the baseband signal to produce a wideband error signal. This error signal is integrated to derive a DC correction component. This DC correction component is fed back to a preprocessor combiner, which differentially combines the DC correction component with the AC-coupled baseband signal. The output of the pre-processor combiner is thus the baseband signal of interest, absent the slowly varying DC offset error component, so that the amplitude values of the signal constellation input to the quantization circuitry of the data recovery processor are not distorted by the slowly varying DC offset level upon which the baseband modulation is riding. In accordance with a second aspect of the invention, the quantized data output of the slicer/comparator is coupled to a low pass filter whose upper cut-off frequency substantially corresponds to the lower cut-off frequency of the high pass filtering (capacitive AC-coupling) of the baseband signal upstream of the comparator. The effect of the low pass filter is to recover an exact representation of the low frequency signal component of the baseband signal that had been removed by the AC-coupling of the baseband signal prior to its application to the signal processing comparator. For this purpose, the output of the low pass filter is coupled to a summing input of the preprocessor combiner, so as to be summed with the AC-coupled baseband signal (absent its removed low frequency component). Namely, by virtue of the low pass filter feedback path, the previously removed (low frequency) portion of the useful frequency spectrum of the baseband signal is reconstructed, and injected by the pre-processor combiner into the baseband signal path prior to its application to the comparator. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 diagram atically illustrates a simplified configuration of a baseband demodulator for an RF modem; and Figure 2 diagrammatically illustrates a preferred embodiment of a low freguency reinsertion and DC offset cancellation mechanism in accordance with the present invention that may be used with a baseband demodulator of the type shown in Figure 1. DETAILED DESCRIPTION A preferred, but non-limiting embodiment of the present invention will now be described with reference to Figure 2, which diagrammatically illustrates the manner in which the high pass-filter baseband signal at the output of high gain amplifier 30 is preliminarily combined, using fractional bandwidth feedback, with one or more correction signals prior to being introduced into the signal processing circuit 40. (As pointed out above, the signal processing circuit 40 comprises a comparator which is operative to compare a sample of the instantaneous amplitude of the amplified and AC-coupled baseband signal with one or more DC quantization levels, to extract modulation/data values associated with those quantization levels. ) Pursuant to a first aspect of the invention, the quantized data output 44 of slicer/comparator 40 is coupled to a first input 51 of a differential combiner or subtractor 50, a second input 52 of which is the baseband signal applied that is applied to the input 41 of comparator 40. Thus, the output 54 of differential combiner 50 is representative of the quantized/sliced data values produced by the comparator 40 and its input baseband sample values. These sampled signal amplitude differences constitute wideband error signals, which include the slowly varying DC component of the baseband signal . By slowly varying is meant a gradual rate of change approximate DC or on the order of only several Hz up to 10 Hz, as a non- limiting example, as opposed to a higher rate of change that may generally be classified as Gaussian noise. This slowly varying wideband error signal produced by differential combiner 50 is coupled via a first fractional bandwidth feedback path 56 to an integrator 60, which has a high gain in the low frequency bandwidth of interest (e.g., 10 Hz., as a non-limiting example) and a substantially reduced gain above this range. Integrator 60 integrates the errors and thereby derives a DC correction component representative of the accumulated variation of the slowly varying wideband error signal. The DC correction component is fed back to a first differential (-) input 71 of a pre-processor summing unit 70, which has a second ( +) input 72 thereof AC-coupled to the output of amplifier 30, so that the signal input 72 of summing unit 70 is the high pass-filtered baseband signal of interest, riding on a slowly varying DC offset, such as may be introduced by mechanical stress in the printed circuit board, AGC amplifier effects, or microphonics effects, as described above. The output 74 of summing unit 70 is thus the difference between the baseband signal and the DC correction component produced by integrator 60, namely the high pass-filtered baseband signal of interest, absent the slowly varying DC offset error component. Pursuant to a second aspect of the invention, the quantized data output 44 of slicer/comparator 40 is coupled via a second fractional bandwidth feedback path 58 to a low pass filter 80 having an upper cut-off frequency (e.g., 10Hz), that substantially corresponds to the lower cut-off frequency of the high pass filtering (capacitive AC- coupling) of the baseband signal upstream of the comparator 40. The effect of low pass filter 80 is to recover an exact representation of the energy content of the low frequency signal component of the baseband signal that had been removed by the AC-coupling of the baseband signal prior to its application to the signal processing circuit 40. The output of low pass filter 80 is coupled to a third ( + ) input 73 of summing unit 70, so as to be summed with the AC-coupled baseband signal (absent its removed low frequency component). Namely, by virtue of the low pass filter feedback path, the previously removed (low frequency) portion of the useful frequency spectrum of the baseband signal is reconstructed, and injected (via summing unit 70) into the baseband signal path prior to its application to comparator 40. As will be appreciated from the foregoing description, the above-described information loss and DC offset problems associated with AC-coupling the signal processing path of an RF modem are effectively obviated in accordance with the present invention by a low frequency reconstruction and DC offset cancellation signal processing methodology, which is operative to reconstruct low frequency information that has been removed by high pass-filtering of a baseband signal prior to data decision processing, and to effectively cancel slowly varying DC impairments to the baseband signal. By integrating the difference between the quantized data output of the baseband comparator and the baseband signal, a DC correction component representative of the accumulated variation of the error signal is derived. By subtracting this DC correction component from the baseband signal the slowly varying DC offset error component is effectively removed, so that the amplitude values of the signal constellation input to the quantization circuitry of the data recovery processor are not distorted by the DC offset level upon which the baseband modulation is riding. Low pass-filtering the output of the slicer/comparator allows an exact representation of the low frequency signal component of the baseband signal that had been removed by the AC-coupling of the baseband signal prior to its application to the signal processing comparator to be recovered, so that it can be reinjected into the baseband signal path prior to its application to the comparator. While I have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and I therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.

Claims: WHAT IS CLAIMED 1. For use with a communication demodulator in which a modulated communication signal is down-converted to baseband, and is AC-coupled as a baseband signal to a signal processing circuit, a method of correcting for a variation in a DC level of said baseband signal comprising the steps of: (a) coupling said baseband signal to said signal processing circuit to derive an output signal representative of modulation contained in said baseband signal; (b) processing said baseband signal and said output signal to derive a wideband error signal; (c) integrating said error signal to derive a correction signal representative of accumulated variation of said error signal; and (d) combining said correction signal with said baseband signal to remove said accumulated variation in DC level from said baseband signal. 2. A method according to claim 1, wherein step (b) comprises differentially combining said baseband signal and said output signal to derive said error signal, and step (d) comprises differentially combining said correction signal and said baseband signal. 3. A method according to claim 1, further including the step (e) of low pass-filtering the output signal derived from said signal processing circuit to recover a low freguency signal component of said baseband signal that had been removed by AC-coupling of said baseband signal to said signal processing circuit, and combining the recovered low frequency signal component of said baseband signal with said baseband signal prior to application to said signal processing circuit. 4. A method according to claim 1, wherein said signal processing circuit is operative to derive said output signal in accordance with a comparison of said baseband signal with at least one quantization threshold. 5. For use with a communication demodulator in which a modulated communication signal is down-converted to baseband, and high pass-filtered prior to being coupled as a baseband signal to a signal processing circuit for deriving modulation contained in said baseband signal, a method of correcting for removal of a low frequency signal component of said modulated communication signal as a result of high pass filtering thereof comprising the steps of: (a) coupling said high pass-filtered baseband signal to said signal processing circuit to derive an output signal representative of modulation contained in said baseband signal; (b) low pass filtering said output signal derived from said signal processing circuit to recover said low frequency signal component of said baseband signal that had been removed by high pass filtering of said baseband signal; and (c) combining the recovered low frequency signal component of said baseband signal with said baseband signal prior to application to said signal processing circuit. 6. A method according to claim 5, wherein said signal processing circuit is operative to derive said output signal in accordance with a comparison of said baseband signal with at least one quantization threshold. 7. For use with a communication demodulator in which a modulated communication signal is down-converted to baseband, amplified by an amplifier and high pass filter- coupled as a baseband signal to a signal processing circuit for recovering modulation therefrom, a method of correcting for a variation in a DC level of said baseband signal as a result of one or more of a plurality of DC level variation- introducing anomalies, and for restoring low frequency information removed from said baseband signal by said high pass filter, comprising the steps of: (a) coupling said baseband signal to said signal processing circuit to derive an output signal representative of said modulation contained in said baseband signal; (b) differentially combining said baseband signal and said output signal to derive a wideband error signal; (c) integrating said error signal to derive a correction signal representative of accumulated variation of said error signal; (d) differentially combining said correction signal with said baseband signal to remove said accumulated variation of said error signal from said baseband signal prior to application of said baseband signal to said signal processing circuit; and (e) low pass filtering the output signal derived from said signal processing circuit to recover a low freguency signal component of said baseband signal that had been removed by high pass filter-coupling of said baseband signal to said signal processing circuit, and summing the recovered low frequency signal component of said baseband signal with said baseband signal prior to application to said signal processing circuit. 8. A method according to claim 7, wherein said signal processing circuit is operative to derive said output signal in accordance with a comparison of said baseband signal with at least one quantization threshold. 9. For use with a communication demodulator in which a modulated communication signal is down-converted to baseband, and AC-coupled as a baseband signal to a signal processing circuit, said signal processing circuit deriving an output signal representative of modulation contained in said baseband signal, a signal processing arrangement for correcting for a variation in a DC level of said baseband signal comprising: a post-processing combiner which is operative to combine said baseband signal and said output signal to derive a wideband error signal; a first fractional bandwidth feedback path containing an integrator which is operative to integrate said error signal to derive a correction signal representative of accumulated variation of said error signal; and a pre-processing combiner which is operative to combine said correction signal with said baseband signal to remove said accumulated variation in said error signal from said baseband signal prior to its application to said signal processing circuit. 10. A signal processing arrangement according to claim 9, wherein sid post-processing combiner operative to differentially combine said baseband signal and said output signal to derive said error signal, and wherein said pre- processing combiner is operative to differentially combine said correction signal and said baseband signal. 11. A signal processing arrangement according to claim 9, further including a second fractional bandwidth feedback path containing a low pass filter to which the output signal derived from said signal processing circuit is applied so as to recover a low frequency signal component of said baseband signal that had been removed by AC-coupling of said baseband signal to said signal processing circuit, and wherein said pre-processing combiner is operative to combine the recovered low frequency signal component of said baseband signal with said baseband signal prior to its application to said signal processing circuit. 12. A signal processing arrangement according to claim 9, wherein said signal processing circuit is operative to derive said output signal in accordance with a comparison of said baseband signal with at least one quantization threshold. 13. For use with a communication demodulator in which a modulated communication signal is down-converted to baseband, and high pass-filtered prior to being coupled as a baseband signal to a signal processing circuit for deriving modulation contained in said baseband signal, an arrangement for correcting for removal of a low frequency signal component of said modulated communication signal as a result of high pass filtering thereof comprising: a low pass filter which is operative to low pass filter said output signal derived from said signal processing circuit to recover said low frequency signal component of said baseband signal that had been removed by high pass filtering of said baseband signal; and a pre-processing combiner which is operative to combine the recovered low frequency signal component of said baseband signal with said baseband signal prior to application to said signal processing circuit. 14. A signal processing arrangement according to claim 13, wherein said signal processing circuit is operative to derive said output signal in accordance with a comparison of said baseband signal with at least one quantization threshold. 15. For use with a communication demodulator in which a modulated communication signal is down-converted to baseband, amplified by an amplifier and high pass filter- coupled as a baseband signal to a signal processing circuit for recovering modulation therefrom, a signal processing arrangement for correcting for a variation in a DC level of said baseband signal as a result of one or more of a plurality of DC level variation-introducing anomalies, and for restoring low frequency information removed from said baseband signal by said high pass filter, comprising: a post-processing combiner which is operative to combine said baseband signal and said output signal to derive a wideband error signal; a first fractional bandwidth feedback path containing an integrator which is operative to integrate said error signal to derive a correction signal representative of accumulated variation of said error signal; a pre-processing combiner which is operative to combine said correction signal with said baseband signal to remove said accumulated variation of said error signal from said baseband signal prior to its application to said signal processing circuit; and a second fractional bandwidth feedback path containing a low pass filter to which the output signal derived from said signal processing circuit is applied so as to recover a low frequency signal component of said baseband signal that had been removed by AC-coupling of said baseband signal to said signal processing circuit, and wherein said pre-processing combiner is operative to inject the recovered low frequency signal component of said baseband signal into said baseband signal prior to its application to said signal processing circuit. 16. A signal processing arrangement according to claim 15, wherein sid post-processing combiner operative to differentially combine said baseband signal and said output signal to derive said error signal, and wherein said pre- processing combiner is operative to differentially combine said correction signal and said baseband signal . 17. A signal processing arrangement according to claim 16, wherein said signal processing circuit is operative to derive said output signal in accordance with a comparison of said baseband signal with at least one quantization threshold.

Inventors: BARBER ANDREW C, BARBER, ANDREW, C.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1997
Main classification: H04L25
IPC codes: H04L25