Systems and methods for suppressing even harmonics in a molecular clock

1. A molecular clock comprising:

a waveguide gas cell containing gas molecules having a rotational spectral line with a first frequency;

a voltage-controlled oscillator (VCO) to generate a clock signal;

a transmitter referenced to the clock signal to generate a probing signal for transmission through the waveguide gas cell; and

a receiver to receive the probing signal transmitted through the waveguide gas cell and interacting with gas molecules, the receiver comprising:

a filter circuit configured to filter out even harmonic components from the received signal; and

a lock-in detector to generate an error signal indicating an offset between the first frequency and the second frequency, wherein the error signal is fed back to control generation of the VCO clock signal.

2. The molecular clock of claim 1 , wherein the filter circuit comprises a N-path notch filter.

3. The molecular clock of claim 2 , wherein the N-path notch filter comprises an input buffer and an output buffer.

4. The molecular clock of claim 3 , wherein the input buffer and output buffer each comprise a transconductance amplifier.

5. The molecular clock of claim 3 , wherein the input buffer and output buffer each comprise a high impedance buffer.

6. The molecular clock of claim 2 , wherein the N-path notch filter comprises multiple switches, each switch configured to oscillate at a frequency equal to twice a modulation frequency of the spectrometer.

7. The molecular clock of claim 1 , wherein the gaseous molecules comprise gaseous molecules having a linear structure.

8. The molecular clock of claim 7 , wherein the gaseous molecules having a linear structure are provided as carbonyl sulfide compound (OCS).

9. A molecular clock comprising:

a waveguide gas cell having a radio frequency (RF) input port, a RF output port and a gas inlet and outlet;

a spectrometer configured to wavelength modulate a spectral signal from gaseous molecules contained within the waveguide gas cell;

a filter circuit configured to filter out even harmonic components from the wavelength modulated spectral signal and

a lock-in detector to generate a signal for controlling the molecular clock, wherein the filter circuit is coupled between the spectrometer and the lock-in detector.

10. The molecular clock of claim 9 , wherein the gaseous molecules comprise gaseous molecules having a linear structure.

11. The molecular clock of claim 10 , wherein the gaseous molecules having a linear structure are provided as carbonyl sulfide compound (OCS).

12. The molecular clock of claim 11 , wherein the carbonyl sulfide compound (OCS) is provided having a rotational spectrum in the frequency range of about 12 GHz to about 1 THz.

13. The molecular clock of claim 11 , wherein the carbonyl sulfide compound (OCS) is provided having a rotational spectrum in the frequency range of about 200 GHz to about 300 GHz.

14. The molecular clock of claim 9 , wherein the spectral signal from the gaseous molecules comprises a rotational spectral line.

15. The molecular clock of claim 10 , wherein the filter circuit comprises a N-path notch filter.

16. The molecular clock of claim 15 , wherein the N-path notch filter comprises an input buffer and an output buffer.

17. The molecular clock of claim 16 , wherein the input buffer and output buffer each comprise a transconductance amplifier.

18. The molecular clock of claim 16 , wherein the input buffer and output buffer each comprise a high impedance buffer.

19. The molecular clock of claim 15 , wherein the N-path notch filter comprises multiple switches, each switch configured to oscillate at a frequency equal to twice a modulation frequency of the spectrometer.

20. The molecular clock of claim 9 , comprising a low-pass filter configured to remove up-converted frequency components from the spectral signal.