Method and apparatus for demodulating signals in a pulse oximetry system
View Patent ↗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.
1. In a system that includes: a sampling frequency generator generating a sampling frequency f s ; a detector; a modulation signal generator, said modulation signal generator generating a sequence of pulses having a pulse repetition frequency, said pulses having a duty cycle of Q sample periods of said sampling frequency f s ; a transmitter which emits electromagnetic energy at a wavelength in response to said pulse, said detector receiving said electromagnetic energy to generate a detector output signal responsive to the received electromagnetic energy, said detector output signal including noise caused by ambient electromagnetic energy detected by said detector; a digital to analog converter which produces digital samples of said detector output signal at said sampling frequency; and a demodulator which demodulates said digital samples to produce a desired output signal, a method for minimizing said noise due to ambient electromagnetic energy in said desired output signal comprising the steps of:
identifying all undesired frequency components of said ambient electromagnetic energy detected by said detector;
using said undesired frequency components to compute a set of acceptable modulation cycle times T; and
using said acceptable modulation cycle times to select said f s and said Q using an equation T=4Q/f s .