APPARATUS AND METHOD FOR SYSTEM IDENTIFICATION
Methods and apparatus for system identification operate by computing phase and amplitude using linear filters. By digitally processing the linearly filtered signals or data, the phase and amplitude based on measurements of the input and output of a system, are determined.
1 . A down-conversion method for analyzing light emissions from a luminescent sample comprising the following steps:
a. generating an analog excitation signal with a frequency greater than the Nyquist frequency of an excitation ADC;
b. reducing the frequency of the analog excitation signal below the Nyquist frequency;
c. converting the reduced analog excitation signal to a reduced digital excitation signal using the excitation ADC;
d. processing the reduced digital excitation signal to determine a first phase shift,
e. generating an analog detection signal in a system for creating and measuring with luminescent light, the signal having a frequency greater than the detection Nyquist frequency of a detection ADC;
f. reducing the frequency of the analog detection signal below the detection Nyquist frequency;
g. converting the reduced analog detection signal to a reduced digital detection signal using the detection ADC;
h. processing the digital detection signal to determine a second phase shift,
i. subtracting the first phase shift from the second phase shift to compute a phase difference caused by the luminescent sample.
2 . The down-conversion method of claim 1 wherein the Nyquist frequency of the excitation and detection ADC are less than 48 kHz.
3 . The down-conversion method of claim 2 wherein the excitation and detection ADC are implemented with a stereo Sigma-Delta converter.
4 . The down-conversion method of claim 1 where by the excitation signal frequency is greater than 8 kHz but less than 100 MHz.
5 . A down-converting apparatus for analyzing light emissions from a luminescent sample, comprising:
a. A pulse width modulation unit configured to provide a digital excitation signal to a system for determining phase shift,
b. an excitation mixer connected to the excitation ADC and configured to provide the excitation ADC with a mixer signal having a frequency that is below the Nyquist limit;
c. an excitation signal source connected to the excitation mixer and having a frequency greater than the Nyquist frequency;
d. a detector configured to generate a detector signal based on the output of a luminescent probe and having a frequency above a detector Nyquist limit of a detector ADC that is configured to provide a digital detector signal to a system for determining phase shift,
e. a detector mixer connected to the detector and configured to provide the detector ADC with a mixer signal having a frequency that is below the Nyquist limit.
6 . The down-converting apparatus of claim 5 wherein, the detector is a photodiode.
7 . The down-converting apparatus of claim 5 wherein, the Nyquist limit is less than 48 kHz.