CALIBRATION SCHEME FOR GAS ABSORPTION SPECTRA DETECTION

• US 20140368376A1
• Filed: 06/12/2013
• Published: 12/18/2014
• Est. Priority Date: 05/23/2013
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: NADEAU, Phillip Michel, HAROUN, Baher S., RAMASWAMY, Srinath M.
• Granted Patent: US 9,128,023 B2
• Time in Patent Office: Days
• US Class Current: 342/174
• CPC Class Codes: G01J 3/28 Investigating the spectrum using colour filters G01J3/51
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 22/00 Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more G01N3/00 - G01N17/00, G01N24/00 take precedence
• CPC Class Codes: G01S 7/40 Means for monitoring or calibrating
• CPC Class Codes: G01S 7/41 using analysis of echo signal for target characterisation; Target signature; Target cross-section

Abstract

A technique for removing the background from a transmission spectrum including determining performance characteristics of a detector, measuring a transmission spectrum that includes an absorption line, determining performance characteristics of a gas cell, and removing a background spectrum from the transmission spectrum by combining the transmission spectrum with the performance characteristics of the detector and the performance characteristics of the gas cell.

Claims

1. A method for removing the background from a transmission spectrum, comprising:

determining performance characteristics of a detector;

measuring a transmission spectrum that includes an absorption line;

determining performance characteristics of a gas cell; and

removing a background spectrum from the transmission spectrum by combining the transmission spectrum with the performance characteristics of the detector and the performance characteristics of the gas cell.

2. The method of claim 1, wherein determining the performance characteristics of the detector comprises estimating a responsivity and a DC-offset of the detector.

3. The method of claim 1, wherein determining the performance characteristics of the gas cell comprises fitting the transmission spectrum in the vicinity of the absorption line with a polynomial, the performance characteristics of the gas cell are represented by the slope and y-intercept of the polynomial.

4. The method of claim 2, wherein combining comprises subtracting the DC-offset from the transmission spectrum.

5. The method of claim 4, wherein combining further comprises:

at a frequency, computing a quotient of the transmission spectrum without the DC-offset to the product of the responsivity, the characteristics of the gas cell, and an input power value, wherein the result of the quotient is the transmission spectrum without the background spectrum at the frequency.

6. The method of claim 1 further comprising determining edge frequencies of the absorption line, wherein the edge frequencies correspond to a point of maximum slope of the transmission spectrum of the absorption line.

7. The method of claim 6, further comprising computing the quotient for the edge frequencies of the absorption line.

8. The method of claim 7, further comprising computing the difference between the quotients for the two edge frequencies wherein the difference between the quotients for the two edge frequencies is an error signal.

9. The method of claim 8, wherein the error signal is used to track the absorption line.

10. A system for removing a background spectrum from a measured spectrum, comprising:

an absorption cell containing a gas;

a radio frequency (RF) transmitter to transmit a RF signal through the absorption cell;

a detector to receive the RF signal; and

a control module coupled to the transmitter and the detector to;

estimate performance characteristics of the detector and performance characteristics of the gas cell;

measure a transmission spectrum of the gas in the absorption cell; and

combine the transmission spectrum with the performance characteristics of the gas cell and the detector to remove a background spectrum from the transmission spectrum.

11. The system of claim 10, wherein the control module locks-in on an absorption line of the transmission spectrum.

12. The system of claim 10, wherein the control module estimates the performance characteristics of the gas cell by fitting the transmission spectrum in the vicinity of the absorption line with a polynomial, wherein a slope and a y-intercept of the polynomial represent the performance characteristics of the gas cell.

13. The system of claim 12, wherein the control module determines a responsivity and a DC-offset of the detector to estimate the performance characteristics of the detector.

14. The system of claim 13, wherein the control module subtracts the DC-offset from the transmission spectrum then divides the result of the subtraction by a product of the responsivity, the performance characteristics of the gas cell, a frequency, and an input power to remove the background spectrum from the transmission spectrum at that frequency.

15. The system of claim 14, wherein the control module determines edge frequencies of the transmission line spectrum of the absorption line, and the edge frequencies correspond to a point of maximum slope of the transmission spectrum of the absorption line.

16. The system of claim 15, wherein the control module removes the background spectrum from the transmission spectrum at the edge frequencies of the absorption line.

17. The system of claim 16, wherein the control module generates an error signal by taking the difference of the transmission spectrums without the background spectrum for the edge frequencies.

18. A spectrometer, comprising:

an absorption cell containing a gas;

a radio frequency (RF) transmitter to transmit RF signals into the absorption cell;

a RF receiver to receive the RF signals; and

a RF signal generation and detection module coupled to the transmitter and the receiver to;

generate the RF signals;

detect the RF signals received by the receiver;

measure a transmission spectrum based on the received RF signals, wherein the measured transmission spectrum includes an absorption line of the gas and a background spectrum;

remove the background spectrum from the transmission spectrum leaving an absorption line spectrum, wherein the removal of the background spectrum calibrates the spectrometer;

generate an error signal based on the absorption line spectrum; and

adjust the transmitted RF signals to lock-in on the absorption line spectrum.

19. The device of claim 18, wherein the RF signals are two frequency-shift keying (FSK) modulation tones.

20. The device of claim 18, wherein the background spectrum is a combination of performance characteristics of the absorption cell and of the RF signal generation and detection module.

21. The device of claim 20, wherein the RF signal generation and detection module fits the absorption line spectrum with a polynomial and the slope and y-intercept of the polynomial represent the performance characteristics of the absorption cell.

22. The device of claim 20, wherein the RF signal generation and detection module performance characteristics are estimates of a responsivity and a DC-offset.

23. The device of claim 19, wherein the error signal is a difference between an absorption spectrum value at the two FSK modulation tones calculated after the background spectrum has been removed.