Method and apparatus for demodulating signals in a pulse oximetry system
First Claim
1. A method of minimizing crosstalk between two signals generated by applying a first pulse and a second pulse to measure a parameter, wherein said first pulse and said second pulse are applied periodically at a first repetition rate defining a period, and wherein said first pulse is generated during a first interval in each period and said second pulse is generated during a second interval in each period, said second interval spaced apart from said first interval, said first and second pulses producing first and second parametric signals responsive to said parameter, said first and second parametric signals being received by a single detector which outputs a composite signal responsive to said first and second parametric signals, said method comprising the steps of:
- providing said composite signal to an analog to digital converter to produce a sequence of digital values;
decimating said sequence of digital values to produce a decimated sequence of digital values according to a decimation rate that produces relatively little shaping of said first and second parametric signals and that produces a relatively larger shaping of a noise component of said composite signal according to a decimation rate that produces relatively little shaping of said first and second parametric signals and that produces a relatively larger shaping of a noise component of said composite signal, said decimation rate adjusted according to noise in said first or second parametric signals;
applying a first sequence of demodulation coefficients to said decimated sequence of digital values to generate a first demodulated output signal;
applying a second sequence of demodulation coefficients to said decimated sequence of digital values to generate a second demodulated output signal;
lowpass filtering said first demodulated output signal to generate a first recovered output signal responsive to said first parametric signal; and
lowpass filtering said second demodulated output signal to generate a second recovered output signal responsive to said second parametric signal.
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Abstract
A method and an apparatus measure blood oxygenation in a subject. A first signal source applies a first input signal during a first time interval. A second signal source applies a second input signal during a second time interval. A detector detects a first parametric signal responsive to the first input signal passing through a portion of the subject having blood therein. The detector also detects a second parametric signal responsive to the second input signal passing through the portion of the subject. The detector generates a detector output signal responsive to the first and second parametric signals. A signal processor receives the detector output signal and demodulates the detector output signal by applying a first demodulation signal to a signal responsive to the detector output signal to generate a first output signal responsive to the first parametric signal. The signal processor applies a second demodulation signal to the signal responsive to the detector output signal to generate a second output signal responsive to the second parametric signal. The first demodulation signal and the second demodulation signal both include at least a first component having a first frequency and a first amplitude and a second component having a second frequency and a second amplitude. The second frequency is a harmonic of the first frequency. The second amplitude is related to the first amplitude to minimize crosstalk from the first parametric signal to the second output signal and to minimize crosstalk from the second parametric signal to the first output signal.
943 Citations
9 Claims
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1. A method of minimizing crosstalk between two signals generated by applying a first pulse and a second pulse to measure a parameter, wherein said first pulse and said second pulse are applied periodically at a first repetition rate defining a period, and wherein said first pulse is generated during a first interval in each period and said second pulse is generated during a second interval in each period, said second interval spaced apart from said first interval, said first and second pulses producing first and second parametric signals responsive to said parameter, said first and second parametric signals being received by a single detector which outputs a composite signal responsive to said first and second parametric signals, said method comprising the steps of:
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providing said composite signal to an analog to digital converter to produce a sequence of digital values;
decimating said sequence of digital values to produce a decimated sequence of digital values according to a decimation rate that produces relatively little shaping of said first and second parametric signals and that produces a relatively larger shaping of a noise component of said composite signal according to a decimation rate that produces relatively little shaping of said first and second parametric signals and that produces a relatively larger shaping of a noise component of said composite signal, said decimation rate adjusted according to noise in said first or second parametric signals;
applying a first sequence of demodulation coefficients to said decimated sequence of digital values to generate a first demodulated output signal;
applying a second sequence of demodulation coefficients to said decimated sequence of digital values to generate a second demodulated output signal;
lowpass filtering said first demodulated output signal to generate a first recovered output signal responsive to said first parametric signal; and
lowpass filtering said second demodulated output signal to generate a second recovered output signal responsive to said second parametric signal.
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2. A method of minimizing crosstalk between two signals generated by applying a first pulse and a second pulse to measure a parameter, wherein said first pulse and said second pulse are applied periodically at a first repetition rate defining a period, and wherein said first pulse is generated during a first interval in each period and said second pulse is generated during a second interval in each period, said second interval spaced apart from said first interval, said first and second pulses producing first and second parametric signals responsive to said parameter, said first and second parametric signals being received by a single detector which outputs a composite signal responsive to said first and second parametric signals, said method comprising the steps of:
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providing said composite signal to an analog to digital converter to produce a sequence of digital values;
decimating said sequence of digital values to produce a decimated sequence of digital values;
applying a first sequence of demodulation coefficients to said decimated sequence of digital values to generate a first demodulated output signal;
applying a second sequence of demodulation coefficients to said decimated sequence of digital values to generate a second demodulated output signal;
lowpass filtering said first demodulated output signal to generate a first recovered output signal responsive to said first parametric signal;
lowpass filtering said second demodulated output signal to generate a second recovered output signal responsive to said second parametric signal applying a third sequence of demodulation coefficients to said decimated sequence of digital values to generate a third demodulated output signal; and
lowpass filtering said third demodulated output signal to generate a recovered output signal responsive to noise detected by said detector.
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3. A pulse oximetry system, comprising:
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a modulation signal generator, said modulation signal generator generating a first modulation signal comprising a first pulse which repeats at a first repetition frequency, said first pulse having a duty cycle of less than 50%, said modulation signal generator generating a second modulation signal comprising a second pulse which also repeats at said first repetition frequency, said second pulse having said duty cycle of less than 50%, said second pulse occurring at non-overlapping times with respect to said first pulse;
a first transmitter which emits electromagnetic energy at a first wavelength in response to said first pulse;
a second transmitter which emits electromagnetic energy at a second wavelength in response to said second pulse;
a detector which receives electromagnetic energy at said first and second wavelengths after passing through a portion of a subject and which generates a detector output signal responsive to the received electromagnetic energy, said detector output signal including a signal component responsive to attenuation of said electromagnetic energy at said first wavelength and a signal component responsive to attenuation of said electromagnetic energy at said second wavelength;
a sampling analog to digital converter which converts said detector output into a sequence of digital values;
a decimator which decimates said sequence of digital values at a first decimation rate to produce a decimated sequence;
a first demodulator which multiplies said decimated sequence by a demodulation sequence and generates a first demodulated output signal; and
a processor that selects said first decimation rate to improve said first demodulated output signal;
a second demodulator which multiplies said decimated sequence by a second demodulation sequence and generates a second demodulated output signal. - View Dependent Claims (4, 5, 6, 7, 8, 9)
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Specification