Method and apparatus for demodulating signals in a pulse oximetry system

An apparatus for measuring blood oxygenation in a subject, said apparatus comprising:

• a first signal source which applies a first input signal during a first time interval;
• a second signal source which applies a second input signal during a second time interval;
• a detector which detects a first parametric signal responsive to said first input signal passing through a portion of said subject having blood therein and which detects a second parametric signal responsive to said second input signal passing through said portion of said subject, said detector generating a detector output signal responsive to said first and second parametric signals;
• and
• a signal processor which receives said detector output signal, said signal processor demodulating said detector output signal by applying a first demodulation signal to a signal responsive to said detector output signal to generate a first output signal responsive to said first parametric signal and applying a second demodulation signal to said signal responsive to said detector output signal to generate a second output signal responsive to said second parametric signal, each of said first demodulation signal and said second demodulation signal comprising 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, said second frequency being a harmonic of said first frequency, said second amplitude selected to be related to said first amplitude to minimize crosstalk from said first parametric signal to said second output signal and to minimize crosstalk from said second parametric signal to said first output signal, wherein the second amplitude is determined by turning off one of the first and second signal sources and measuring the crosstalk between one of the parametric signals and the non-corresponding output signal while varying the second amplitude and selecting a second amplitude which minimizes the measured crosstalk.

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:

• applying a first demodulation signal to said composite signal to generate a first demodulated output signal, said first demodulation signal comprising at least a first component having a first frequency corresponding to said first repetition rate and having a first amplitude, said first demodulation signal further comprising a second component having a second frequency which is a harmonic of said first frequency and having a second amplitude which has a selected proportional relationship to said first amplitude;
• applying a second demodulation signal to said composite signal to generate a second demodulated output signal, said second demodulation signal comprising said first component at said first frequency and said first amplitude and comprising said second component at said second frequency and said second amplitude, at least one of said first and second components of said second demodulation signal having a selected phase difference with respect to the corresponding one of said first and second components of said first demodulation signal;
• and
• 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.

A pulse oximetry system, comprising:

• 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 a duty cycle of less than 50%, said second pulse occurring at non-overlapping times with respect to said first pulse, said first and second pulses comprising a plurality of components wherein a first component has a frequency corresponding to said repetition frequency and a second component has a second frequency corresponding to twice said first frequency, said second component having an amplitude which has a first predetermined relationship to an amplitude of said first component;
• 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 first demodulator which multiplies said detector signal by a first demodulation signal and generates a first demodulated output signal, said first demodulation signal comprising a plurality of components, said plurality of components including a first component having said first frequency and having a first amplitude and said plurality of components including a second component having said second frequency and having a second amplitude, said second amplitude having a second predetermined relationship to said first amplitude which second predetermined relationship is inversely proportional to said first predetermined relationship;
• and
• a second demodulator which multiplies said detector signal by a second demodulation signal and generates a second demodulated output signal, said second demodulation signal comprising a first component having said first frequency and having said first amplitude and comprising a second component having said second frequency and having said second amplitude, at least one component of said second demodulation signal having a selected phase relationship with at least one of said plurality of components of said first demodulation signal.