Adaptive FSK demodulator and threshold detector
First Claim
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1. Apparatus for demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components, said apparatus includingfirst means for providing a delayed version of said FSK signal, each frequency component in said delayed version being phase shifted in an amount which is linearly related to the frequency of said component,second means for shifting the phase of said FSK signal by a preselected constant value such that a center frequency which is halfway between said upper and lower frequency components is 90 degrees out of phase with the phase of the center frequency of said delayed version, andthird means responsive to said delayed version and the output of said second means for providing an output indicative of the relative phase difference therebetween.
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Abstract
An FSK demodulator of the differential type is arranged to provide two versions of an input FSK signal that, relative each other, (1) have a linear phase shift versus frequency characteristic and (2) are phase shifted by 90 degrees at a center frequency halfway between the upper and lower FSK frequencies. The linear phase shift is provided by an N sample delay element, while a constant phase shift circuit which includes a K factor gain element provides the 90 degree phase shift. By virtue of this arrangement, the demodulator characteristics can be adaptively varied and the demodulator tuned simply by changing the value of K. A similar adaptive technique is applied to a threshold detector arranged to position a threshold midway between the extremes of an input signal.
144 Citations
22 Claims
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1. Apparatus for demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components, said apparatus including
first means for providing a delayed version of said FSK signal, each frequency component in said delayed version being phase shifted in an amount which is linearly related to the frequency of said component, second means for shifting the phase of said FSK signal by a preselected constant value such that a center frequency which is halfway between said upper and lower frequency components is 90 degrees out of phase with the phase of the center frequency of said delayed version, and third means responsive to said delayed version and the output of said second means for providing an output indicative of the relative phase difference therebetween.
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10. Apparatus for demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components, said apparatus including
first means for providing two versions of said FSK signal such that (1) said two versions have a linear phase shift vs. frequency characteristic relative to each other, and (2) said two versions are shifted by 90 degrees in phase relative to each other at a center frequency halfway between said upper and lower frequencies, and second means responsive to said two versions for providing an output indicative of the relative phase difference between said two versions, wherein said first means includes a constant phase shift circuit for providing said 90 degree shift in phase at said center frequency.
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13. Apparatus for demodulating a sampled frequency shift keyed (FSK) signal having upper and lower frequency components F1 and F2 and a center frequency Fc=F1+F2/2, said apparatus including
means for forming a first version of said FSK signal having a linear phase shift vs. frequency characteristic, means for forming a second version of said FSK signal having a preselected constant phase shift at said center frequency, and means for forming the product of said first and second versions, such that said upper and lower frequencies produce signals with a positive and negative d.c. component, wherein said first version forming means includes means for delaying said samples by N sample intervals.
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14. Apparatus for demodulating a sampled frequency shift keyed (FSK) signal having upper and lower frequency components F1 and F2 and a center frequency Fc=F1+F2/2, said apparatus including
means for forming a first version of said FSK signal having a linear phase shift vs. frequency characteristic, means for forming a second version of said FSK signal having a preselected constant phase shift at said center frequency, and means for forming the product of said first and second versions, such that said upper and lower frequencies produce signals with a positive and negative d.c. component, wherein said second version forming means includes a combining circuit arranged to combine a level shifted version of said FSK signal with a phase shifted version of said FSK signal formed in a Hilbert transformation circuit.
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17. Apparatus for demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components, said apparatus including
first means for providing a delayed version of said FSK signal, each frequency component in said delayed version being phase shifted in an amount which is linearly related to the frequency of said component, second means for shifting the phase of said delayed version by a preselected constant value such that a center frequency which is halfway between said upper and lower frequency components is 90 degrees out of phase with the phase of the center frequency of said FSK signal, and third means for forming the product of said FSK signal with the output of said second means.
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18. Apparatus for demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components, including
means responsive to said FSK signal for generating a slowly varying signal which moves towards a first signal level while said upper frequency component is present in said FSK signal and which moves towards a second signal level while said lower frequency component is present in said FSK signal, means for comparing said slowly varying signal to a threshold T to provide an indication of which of said upper or lower frequency components is present in said FSK signal, and means for adaptively varying T to lie midway between said first and second signal levels, wherein said last mentioned means includes means for measuring the value of said slowly varying signal at a preselected time interval after said slowly varying signal crosses the currently set value of said threshold, said time interval being a small fraction of the smallest expected time interval between crossings of said threshold, and means for adjusting said threshold as a function of said measured values and the current value of said threshold.
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19. A method of demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components comprising the steps of
providing a delayed version of said FSK signal, each frequency component in said delayed version being phase shifted in an amount which is linearly related to the frequency of said component, shifting the phase of said FSK signal by a preselected constant value such that a center frequency which is halfway between said upper and lower frequency components is 90 degrees out of phase with the phase of the center frequency of said delayed version, and providing an output in response to said delayed version and said phase shifted signal indicative of the relative phase difference therebetween.
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20. A method of demodulating a sampled frequency shift keyed (FSK) signal having upper and lower frequency components F1 and F2 and a center frequency Fc=F1+F2/2 including the steps of
forming a first version of said FSK signal having a linear phase shift vs. frequency characteristic, forming a second version of said FSK signal having a preselected constant phase shift at said center frequency, and forming the product of said first and second versions, such that said upper and lower frequencies produce signals with positive and negative d.c. components, respectively, wherein said first version forming step includes delaying said samples by N sample intervals.
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21. A method of demodulating a sampled frequency shift keyed (FSK) signal having upper and lower frequency components F1 and F2 and a center frequency Fc=F1+F2/2 including the steps of
forming a first version of said FSK signal having a linear phase shift vs. frequency characteristic, forming a second version of said FSK signal having a preselected constant phase shift at said center frequency, and forming the product of said first and second versions, such that said upper and lower frequencies produce positive and negative d.c. signals, respectively, wherein said second version forming step includes the step of combining a level shifted version of said FSK signal with a phase shifted version of said FSK signal formed in a Hilbert transformation circuit.
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22. A method of demodulating a frequency shift keyed (FSK) signal having upper and lower frequency components including the steps of
generating in response to said FSK signal a slowly varying signal which moves towards a first signal level while said upper frequency component is present in said FSK signal and which moves towards a second signal level while said lower frequency component is present in said FSK signal, comparing said slowly varying signal to a threshold T to provide an indication of which of said upper or lower frequency components is present in said FSK signal, and adaptively varying T to lie midway between said first and second signal levels, wherein said varying step includes measuring the value of said slowly varying signal at a preselected time interval after said slowly varying signal crosses the currently set value of said threshold, said time interval being a small fraction of the smallest expected time interval between crossings of said threshold, and adjusting said threshold as a function of said measured values and the current value of said threshold.
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Current AssigneeAT&T Information Systems Inc. (AT&T, Inc.)
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Original AssigneeAT&T Information Systems Inc. (AT&T, Inc.)
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InventorsDaudelin, Douglas S.
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Primary Examiner(s)LaRoche, Eugene R.
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Assistant Examiner(s)Lee, James C.
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Application NumberUS06/697,001Time in Patent Office1,062 DaysField of Search329/104, 329/107, 329/110, 329/126, 329/137, 329/145, 375/80, 375/88, 375/94, 375/76, 375/100US Class Current329/300CPC Class CodesH04L 25/069 by detecting edges or zero ...H04L 27/1563 using transition or level d...
