LOCKING SYSTEM FOR DETECTING AND TRACKING THE CENTER FREQUENCY OF ABSORPTION LINES IN GASES

• US 20140347074A1
• Filed: 05/23/2013
• Published: 11/27/2014
• Est. Priority Date: 05/23/2013
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: NADEAU, Phillip Michel
• Granted Patent: US 9,429,528 B2
• Time in Patent Office: Days
• US Class Current: 324/639
• 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: G01R 27/28 Measuring attenuation, gain, phase shift or derived characteristics of electric four pole networks, i.e. two-port networks; Measuring transient response in line transmission systems H04B3/46

Abstract

A method to detect a gas absorption line that includes alternately transmitting and sweeping two radio frequency (RF) signals through an absorption cell, wherein the two RF signals are transmitted at different frequencies separated by a range and are swept across a span of frequencies in a microwave and millimeter wave regions of a frequency spectrum. Receiving the RF signals by a receiver and analyzing the received signals by a closed loop system for relative absorption by a gas due to an absorption line of the gas in the span of the swept frequencies. Detecting the absorption line of the gas when the two RF signals straddle the gas absorption line and the relative absorption by the two RF signals is equal.

Claims

1. A method to detect a gas absorption one, comprising:

alternately transmitting and sweeping two radio frequency (RF) signals through an absorption cell, wherein the two RF signals are transmitted at different frequencies separated by a range, and are swept across a span of frequencies in a microwave and millimeter wave regions of a frequency spectrum;

receiving the RF signals by a receiver;

analyzing the received signals by a closed loop system for relative absorption by a gas due to an absorption line of the gas in the span of the swept frequencies; and

detecting the absorption line of the gas when the two RF signals straddle the gas absorption line and the relative absorption by the two RF signals is equal.

2. The method of claim 1, further comprising tracking the absorption line of the gas by the closed loop system, wherein the center frequency of the absorption line will be maintained in the middle of the range separating the two RF signals and the gas absorption line is tracked using the relative absorption between the two RF signals.

3. The method of claim 1, further comprising:

generating an offset based on the relative absorption between the two RF signals; and

applying the offset as an error signal in the closed loop system.

4. The method of claim 3, further comprising:

inverting the offset slope; and

applying the inverted offset slope as another in the closed loop system.

5. The method of claim 1, wherein detecting the absorption line comprises using frequency-shift keying as a detection scheme.

6. A system for detecting an absorption line of a gas, comprising:

a processor;

a signal generator coupled to the processor;

a transmitter coupled to the signal generator configured to generate two radio frequency (RF) signals, each at a different frequency, wherein the two RF signals are separated by a set frequency range, are alternately transmitted, and are swept across a span of frequencies in a microwave and millimeter wave region of a spectrum to detect an absorption line of the gas;

a receiver coupled to the processor;

an absorption cell filled with a gas at a pressure, wherein the transmitter transmits the two RF signals through the absorption cell and the two RF signals are detected by the receiver; and

a detector module to compare the amplitudes of the two received RF signals and to determine when an absorption line of a gas is detected based on a difference in the amplitudes between the two RF signals.

7. The system of claim 6, further comprising:

a closed-loop control module to lock-in the detection of the gas absorption line using the difference between the absorption of the two RF signals by the gas as a factor in a closed-loop control algorithm.

8. The system of claim 7, wherein the inverse of the offset slope between the absorption of the two RF signals by the gas is a factor in the closed-loop control algorithm.

9. The system of claim 6, wherein the absorption cell is filled with air at 0.1 mbar.

10. The system of claim 6, wherein a 183.31 GHz absorption line of water is the absorption line of the gas.

11. The system of claim 6, wherein the set frequency range is less than 1 MHz.

12. The system of claim 6, wherein a frequency-key shifting scheme is used by the detector module..

13. A gas absorption line detecting and tracking device, comprising:

an absorption cell filled with a gas;

a transmitter to transmit RF signals through the absorption cell;

a receiver to receive the RF signals; and

a control module coupled to the transmitter and the receiver to detect an absorption line of the gas using a frequency-shift keying (FSK) detection scheme operating in a microwave and a millimeter wave frequency region of a spectrum.

14. The device of claim 13, wherein the control module generates two RF signals separated by a set range and the two RF signals are alternately transmitted by the transmitter.

15. The device of claim 13, wherein the control module analyzes an amplitude of the received signals and compares the received amplitudes to generate an offset.

16. The device of claim 15, wherein the offset determines if the absorption line of the gas is detected, wherein the two RF signals will detect and straddle the absorption line when the offset is zero.

17. The device of claim 15, wherein the control module further comprises a closed-bop control function and the offset is an error signal that is driven to zero by the closed-loop control function.

18. The device of claim 17, wherein the inverse of the offset is a factor in the close-loop control function.

19. The device of claim 13, wherein the gas in the absorption cell is air and the air is at a pressure of 0.1 mbar.

20. The device of claim 13, wherein a water vapor absorption line at 183.31 GHz is the absorption line.

21. The device of claim 14, wherein the set range is less than 1 MHz.