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Method and apparatus for demodulating signals in a pulse oximetry system
| Details |
Inventors: Diab, Mohamed K.; Weber, Walter M.; Al-Ali, Ammar;
Assignee: Masimo Corporation (Irvine, CA)
Primary Examiner: Winakur; Eric F.
Assistant Examiner:
Attorney, Agent or Firm: Knobbe, Martens, Olson & Bear LLP
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. |
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DETAILED DESCRIPTION The present invention avoids the problems associated with conventional demodulation and separation of TDM signals.
In particular, the present invention avoids the problem of aliasing of the ambient noise into the passband of the system by selectively demodulating certain harmonics of the TDM signal.
For example, in one embodiment, only two harmonics (e.
g.
, the fundamental and the first harmonic) are demodulated.
In other embodiments, more harmonics are demodulated.
The present invention specifically addresses solutions to problems caused by crosstalk resulting from filtering and also resulting from demodulating with only certain harmonics instead of demodulating with all harmonics as is done using conventional square wave demodulation.
In a digital implementation of the present invention, the output of the photodetector is initially sampled at a very high frequency (e.
g.
, 46,875 Hz), and the signals are decimated (where decimation is lowpass filtering followed by sample rate compression) such that the final output signals are generated at a relatively low sampling rate (e.
g.
, 62.
5 Hz) which provides increased resolution at the output.
Thus, bandwidth is traded for resolution in the output signal, thus increasing the signal to noise ratio.
One aspect of the present invention is an apparatus for measuring blood oxygenation in a subject.
The apparatus comprises a first signal source which 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.
The signal processor 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 and by applying 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
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Communications 
