System and method for measuring phase delay and amplitude of an optical signal in animal tissue
A system and method, for measuring phase delay and amplitude of a near infrared signal emanating from tissue of an animal subject in response a near infrared signal input to such tissue, operate by processing a signal from an optical detector and a corresponding signal from an optical detector emulation circuit. In some aspects, the processed signals are fed into a phase delay detection system that provides an output thereof a digital measure of the phase delay of the received optical signal.
1. A system for measuring phase delay and amplitude, of an optical signal emanating from tissue of an animal subject, in response to an optical signal input to such tissue, wherein the phase delay and amplitude are the result of scattering and absorption by material in the tissue, the system comprising:
a modulatable optical source, configured to be mounted to radiate into the tissue;
a local oscillator operative at a first frequency;
a reference oscillator, operative at a second frequency, and coupled to the modulatable optical source so that the optical output is modulated at the second frequency;
a synchronization system, coupled to the local oscillator and to the reference oscillator, configured to synchronize the reference oscillator with the local oscillator in a manner that the difference between the second frequency and the first frequency is fixed;
an optical detector, configured to be mounted to receive an optical signal emanating from the tissue as a result of radiation by the optical source into the tissue and providing an electrical signal output as a result of receiving the optical signal;
a signal processing unit having a plurality of channels, including a first channel coupled to the electrical signal output of the optical detector and a second channel coupled to an electrical signal output of an optical detector emulation circuit driven by the reference oscillator, wherein:
in the first channel a signal derived from the signal output of the optical detector is heterodyned with the local oscillator to produce a first intermediate frequency signal; and
in the second channel a signal derived from the signal output of the optical detector emulation circuit is also heterodyned with the local oscillator to produce a second intermediate frequency signal;
a phase delay detection system coupled to the first and second channels providing an output that is a measure of phase difference between the first and second intermediate frequency signals;
a first amplitude detector coupled to the first intermediate frequency signal and having a first amplitude detection output; and
a first analog-to-digital converter coupled to the first amplitude detection output so as to provide at an output thereof a digital measurement of the amplitude of the received optical signal.
2. A system according to claim 1 , wherein the synchronization system employs a phase-locked loop.
3. A system according to claim 1 , wherein the first amplitude detector includes a first peak detector.
4. A system according to claim 1 , further comprising a second amplitude detector coupled to the second intermediate frequency signal and a second analog-to-digital converter coupled to the second amplitude detection output so as to provide a digital output of the amplitude of a reference signal for calibration purposes.
5. A system according to claim 1 , further comprising a second amplitude detector with a second peak detector.
6. A system according to claim 1 , wherein the local oscillator, the reference oscillator, the synchronization system, and the signal processing unit are implemented as components in an integrated circuit.
7. A system according to claim 1 , further comprising a wireless digital data transceiver coupled to the output of the phase delay detection system and to the output of the first analog-to-digital converter, so as to communicate wirelessly the digital measure of the phase delay of the received optical signal and the digital measurement of the amplitude of the received optical signal.
8. A system according to claim 1 , wherein the system, excluding the optical source, consumes no more than 50 milliwatts of power.
9. A system according to claim 1 , wherein the system occupies a volume less than 200 cc, including the optical source and the optical detector.
10. A system according to claim 1 , wherein the system occupies a volume less than 1 cc, including the optical source and the optical detector.
11. A system according to claim 1 , further comprising an adjustable compensation circuit to compensate for phase error in measurement of the phase delay of the received optical signal.
12. A system according to claim 1 , wherein the adjustable compensation circuit includes an adjustable phase delay amplifier.
13. A system according to claim 1 , wherein the signal processing unit includes at least one additional channel coupled to a signal output of an additional optical detector, wherein a signal derived from the signal output of the additional optical detector is heterodyned with the local oscillator to produce an additional intermediate frequency signal, the system further comprising:
an additional phase delay detection system having an input coupled to the additional intermediate frequency signal and providing an additional output that is a measure of phase difference between the additional and second intermediate frequency signals;
an additional amplitude detector coupled to the additional intermediate frequency signal and having an additional amplitude detection output; and
an additional analog-to-digital converter coupled to the additional amplitude detection output so as to provide at an output a digital measurement of the amplitude of the additional received optical signal.