atomic clock system

Abstract

The atomic clock system includes a sealed waveguide cavity containing a gas enclosed therein. The waveguide cavity has a length approximately equal to an integral multiple of half wavelengths of the resonant frequency of the gas between two states. An oscillator system generates an RF signal through the waveguide cavity. The RF signal has a signal frequency approximately equal to the resonant frequency of the gas. A detection system measures a characteristic of the RF signal passing through the waveguide cavity to detect a maximum transition between the two states of the gas, and based on detecting the maximum transition, provides a feedback signal to the oscillator system to lock the signal frequency of the RF signal to the resonant frequency of the gas. The detection system provides a frequency reference output signal based on the signal frequency of the RF signal.

Description

[Technical Field]

[0001] The present disclosure relates generally to time reference systems, and more particularly to atomic clock systems. [Background technology]

[0002] Atomic clocks can be implemented as extremely accurate and stable frequency standards, such as for use in aerospace applications. As an example, atomic clocks can be used in bistatic radar systems, Global Navigation Satellite systems (GNSS), and other navigation and positioning systems, such as satellite systems. Atomic clocks can also be used in communication systems, such as cellular telephone systems. Atomic clocks can typically be realized by providing a signal (e.g., an optical signal or an RF signal) to atomic material to excite it into different excited states based on the very precise frequency of the signal required to do so. Examples of atomic clocks include coherent population trapping (CPT) atomic clocks, thermal beam atomic clocks, alkali vapor cell atomic clocks, and various other atomic clocks. Summary of the Invention

[0003] One embodiment includes an atomic clock system including a sealed waveguide cavity containing a gas enclosed therein. The waveguide cavity has a length approximately equal to an integer multiple of half wavelengths of the resonant frequency of the gas between two states. An oscillator system generates an RF signal through the waveguide cavity. The RF signal has a signal frequency approximately equal to the resonant frequency of the gas. A detection system measures a characteristic of the RF signal passing through the waveguide cavity to detect a maximum transition between the two states of the gas, and based on detecting the maximum transition, provides a feedback signal to the oscillator system to lock the signal frequency of the RF signal to the resonant frequency of the gas. The detection system provides a frequency reference output signal based on the signal frequency of the RF signal.

[0004] Another embodiment includes a method for providing a stable frequency reference output signal. The method includes generating an RF signal having a signal frequency approximately equal to the resonant frequency of ammonia gas and radiating the RF signal via a transmit antenna through a sealed waveguide cavity containing ammonia gas trapped therein. The waveguide cavity can have a length approximately equal to an integer multiple of half wavelengths of the resonant frequency of the ammonia gas between two states. The method also includes receiving the RF signal through the waveguide cavity at a receive antenna opposite the transmit antenna and measuring a characteristic of the RF signal at the receive antenna to detect a maximum transition between the two states of the ammonia gas. The method also includes generating a feedback signal related to the difference between the signal frequency and the resonant frequency of the gas based on detecting the maximum transition and adjusting the signal frequency of the RF signal to approximately equal the resonant frequency in response to the feedback signal. The method further includes providing a stable frequency reference output signal based on the signal frequency of the RF signal.

[0005] Another embodiment includes an atomic clock system. The system includes a sealed waveguide cavity containing ammonia gas sealed therein. The waveguide cavity can have a length approximately equal to an integer multiple of half a wavelength of the resonant frequency of the ammonia gas between two states. The system also includes an oscillator system configured to generate an RF dither signal between a first signal frequency and a second signal frequency and provide the RF dither signal through the waveguide cavity via a transmit antenna. The RF dither signal can have a center frequency approximately equal to the resonant frequency of the ammonia gas. The system further includes a detection system configured to measure characteristics of the RF dither signal received at a receive antenna opposite the transmit antenna through the waveguide cavity at each of the first and second signal frequencies to detect a maximum transition between the two states of the ammonia gas, and to provide a feedback signal to the oscillator system based on the detection of the maximum transition to lock the center frequency of the RF signal to the resonant frequency of the ammonia gas. The detection system can be configured to provide a frequency reference output signal based on the center frequency of the RF signal. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a diagram illustrating an example of an atomic clock system. [Figure 2] FIG. 2 is a diagram showing an example of a waveguide cavity. [Figure 3] FIG. 3 is a diagram showing an example of a power absorption frequency spectrum. [Figure 4] FIG. 4 is a diagram showing another example of a power absorption frequency spectrum. [Figure 5] FIG. 5 is a diagram showing another example of a power absorption frequency spectrum. [Figure 6] FIG. 6 is a diagram showing another example of a power absorption frequency spectrum. [Figure 7] FIG. 7 is a diagram showing another example of a power absorption frequency spectrum. [Figure 8] FIG. 8 is a diagram showing another example of a power absorption frequency spectrum. [Figure 9] FIG. 9 is a diagram illustrating an example of an integrated atomic clock system. [Figure 10] FIG. 10 illustrates an example method for providing a stable frequency reference output signal. DETAILED DESCRIPTION OF THE INVENTION

[0007] The present disclosure relates generally to time reference systems, and more particularly to atomic clock systems. Atomic clock systems can be implemented to adjust the frequency of a local oscillator, such as a quartz crystal oscillator, to provide a stable frequency reference, thereby improving the stability and accuracy of the local oscillator. The atomic clock systems described herein can implement a radio frequency (RF) signal having a signal frequency approximately equal to the resonant frequency of a gas (e.g., ammonia gas, NH3) to provide a state transition for a maximum population of gas molecules. Because the resonant frequency of a gas is highly accurate under a defined set of conditions, a stable frequency reference can be generated from the signal frequency of the RF signal by locking the signal frequency to be equal to the resonant frequency of the gas.

[0008] As an example, an atomic clock system may include a waveguide cavity filled with a gas and selected to have a length approximately equal to an integer multiple (e.g., half the wavelength) of half the wavelength of the resonant frequency of the gas. The waveguide cavity includes a transmitting antenna disposed at one end of the waveguide cavity and a receiving antenna disposed at the opposite end of the waveguide cavity, providing an RF signal through the waveguide cavity. As an example, the RF signal may be generated by an oscillator system including a frequency controller. The frequency controller may provide the RF signal as a dither signal oscillating between a first frequency and a second frequency, such that the center frequency of the dither signal corresponds to a signal frequency locked to the resonant frequency of the gas. The received RF signal may be monitored by a detection system to determine the difference between the signal frequency and the resonant frequency, such as based on the power of the received RF signal. Thus, the detection system may generate a feedback signal provided to the oscillator system to adjust the signal frequency of the RF signal to be equal to the resonant frequency of the gas, thereby locking the signal frequency to the resonant frequency.

[0009] As described herein, the atomic clock system can be implemented to accommodate frequency instabilities caused by temperature changes, such as those resulting from cavity pulling, and can accommodate fluctuations in the pressure of a gas trapped in a waveguide cavity. For example, a frequency controller in an oscillator system can periodically determine the resonant frequency of the waveguide cavity. Accordingly, the detection system can include a stub tuner configured to adjust the electrical length of the waveguide cavity in response to changes in the physical length of the waveguide cavity to match the electrical length of the waveguide cavity to an appropriate fraction of the resonant frequency of the gas. As another example, the detection system can include a detection processor configured to measure the pressure of a gas trapped in the waveguide cavity based on varying the frequency offset from the center frequency of a dither signal. Thus, in response to changes in the gas pressure, and therefore the resonant frequency of the gas, the detection processor can adjust the stable frequency reference to accommodate changes in the resonant frequency of the gas.

[0010] 1 illustrates an example atomic clock system 100. The atomic clock system 100 may be implemented in any of a variety of applications requiring a highly stable frequency reference, such as an inertial navigation system (INS) for an aerospace vehicle. As described in more detail herein, the atomic clock system 100 adjusts the frequency of an oscillator system 102, such as a local oscillator, operating therein to generate a stable frequency reference output signal f OUT As an example, atomic clock system 100 may be fabricated as an integrated atomic clock system, such as based on any of a variety of integrated circuit (IC) fabrication technologies.

[0011] The atomic clock system 100 includes a waveguide cavity 104 containing a gas 106 enclosed therein. By way of example, but not limitation, the gas 106 may be ammonia gas (NH3). In the example of FIG. 1, the oscillator system 102 receives a radio frequency (RF) signal f CTL to a transmit antenna (“TX antenna”) 108 to receive an RF signal f CTL is configured to propagate through the waveguide cavity 104 and thus through the gas 106. Thus, the RF signal f CTL can be received via a receive antenna 110 at the opposite end of the waveguide cavity 104 relative to the transmit antenna 108. The oscillator system 102 generates an RF signal f CTL As described herein, the atomic clock system 100 includes a frequency controller 112 configured to control the signal frequency of the RF signal f CTL The RF signal f may be operated to lock the signal frequency to be approximately equal to the resonant frequency of the gas, and therefore the frequency at which the maximum population of molecules of the gas exhibits a transition between two energy states. As described in more detail herein, the RF signal f CTL The signal frequency of the dither signal can refer to the center frequency of the dither signal including the offset frequency, and the RF signal f CTL The signal frequency of the RF signal f may be a time-averaged frequency approximately equal to the resonant frequency of the gas 106. CTL By locking the signal frequency of RF signal f to the resonant frequency, the atomic clock system 100 synchronizes the RF signal f CTL A stable frequency reference f based on the signal frequency OUT can be provided.

[0012] As an example, the waveguide cavity 104 can have a length that is an integer N times half the wavelength of the resonant frequency of the gas 106. For example, the integer N can be equal to 1, such that the length of the waveguide cavity 104 is half the wavelength of the resonant frequency of the gas 106. In an example where the gas 106 is ammonia gas, the integer N can be equal to 10 -3 Torr (approx. 133.322 x 10 -3The resonant frequency under defined conditions, such as a vapor pressure of 100 Pa, is approximately 22.8 GHz. Thus, the waveguide cavity 104 may have a length of approximately 34.8 millimeters. However, the integer multiple N is not limited to 1; other multiples may be implemented instead. Thus, the waveguide cavity 104 may exhibit a resonant frequency related to the resonant frequency of the gas 106.

[0013] 2 shows an example diagram 200 of a waveguide cavity 202. The waveguide cavity 202 may correspond to the waveguide cavity 104 in the example of FIG. 1. The waveguide cavity 202 is shown as including a transmit antenna 204 at a first end of the waveguide cavity 202 and a receive antenna 206 at a second end of the waveguide cavity 202 opposite the first end. Thus, an RF signal f is shown at 208, which in the example of FIG. 1 may be a CTL An RF signal corresponding to may be provided to a transmit antenna 204 (e.g., from oscillator system 102), propagate through waveguide cavity 202, and be received by a receive antenna 206. As described above in the example of Figure 1, waveguide cavity 202 may be filled with a gas, shown in the example of Figure 2 as ammonia gas. Molecules of ammonia gas are shown in a first state at 210 in the example of Figure 2. Thus, an RF signal 208 is provided through the volume of ammonia gas.

[0014] The waveguide cavity 202 is shown as having a length of N*λ / 2, where N is an integer and equal to 1 (N=1) in the example of FIG. 2 , and λ is the wavelength of the resonant frequency of ammonia gas. Therefore, the RF signal 208 is shown as having an antinode at each of the transmit antenna 204 and the receive antenna 206 to transmit the RF signal 208 from the transmit antenna 204 to the receive antenna 206 to provide a maximum signal-to-noise ratio (SNR). Therefore, an RF signal 208 having a signal frequency approximately equal to the resonant frequency of ammonia gas, shown as approximately 22.8 GHz in the example of FIG. 2 , can result in a maximum population of ammonia gas undergoing an inversion transition between a first state at 210 and a second state at 212 in which the nitrogen atoms are inverted relative to the plane of the hydrogen atoms. The inversion transition of the ammonia gas atoms between state 210 and state 212 consumes power from the RF signal 208. Thus, as an example, the measured power of the RF signal 208 received at the receive antenna 206 may indicate a population of ammonia gas exhibiting an inversion transition, and therefore, the signal frequency of the RF signal 208.

[0015] FIG. 3 shows an exemplary diagram 300 of a power absorption frequency spectrum. The power absorption frequency spectrum is a plot of the power of the RF signal 208 passing through the waveguide cavity 202, as measured at the receiving antenna 206, as a function of the signal frequency of the RF signal 208. As shown in the example of FIG. 3, the diagram 300 shows a pair of low absorption peaks 302 where the measured power of the RF signal 208 is maximized, thus exciting the inversion transition of the ammonia gas for the smallest population of ammonia gas. The diagram 300 also shows an inversion peak 304 located at approximately 22.8 GHz. The inversion peak 304 is therefore located at the resonant frequency of the ammonia gas, indicating maximum absorption of the power of the RF signal 208 and providing the inversion transition of the ammonia gas between the two states 210 and 212 for the largest population of ammonia gas.

[0016] The low absorption peak 302 and the inverted peak 304 may be symmetrical about the inverted peak 304, and therefore about the resonant frequency of the ammonia gas. The power absorption frequency spectrum can be known a priori from experiments, and the symmetry of the power absorption frequency spectrum and the RF signal f CTL A relationship between the measured power and frequency of the RF signal f can be identified by the detection processor 116. Thus, as described in more detail herein, the atomic clock system 100 detects the RF signal f CTL to the resonant frequency of the ammonia gas, and generates a stable frequency reference f based on the signal frequency of the RF signal 208. OUT to provide.

[0017] Referring back to the example of FIG. 1, the atomic clock system 100 receives an RF signal f at a receive antenna 110 that propagates through a gas 106 and is shown in the example of FIG. 1 as signal PWR. CTL The detection system 114 includes a detection system 114 configured to measure the RF signal f CTL and monitors the RF signal f CTL The detection processor 116 includes a detection processor 116 configured to compare the signal frequency of the RF signal f based on the resonant transition of the ammonia gas, as described above in the example of FIG. CTL For example, to detect the power absorption of the RF signal f CTL In the example of FIG. 1, the detection processor 116 may be configured to monitor the power of the RF signal f CTL and a resonant frequency of the gas 106. The feedback signal FDBK is provided to the oscillator system 102, which causes the frequency controller 112 to generate an RF signal f corresponding to the difference. CTL Therefore, in this way, the RF signal f CTL The signal frequency can be locked to the very stable resonant frequency of the gas 106.

[0018] The above example shows that the detection processor 116 detects the RF signal f CTL The power of the RF signal f CTL This example illustrates comparing the signal frequency of the RF signal f with the resonant frequency of the gas 106. CTL Other characteristics of the RF signal f may also be monitored for detection of the inversion transition of the ammonia gas. For example, the detection processor 116 may CTL and, similarly to above, based on detecting the maximum inversion transition, determine the phase of the RF signal f CTL The signal frequency may be configured to lock to the resonant frequency of the ammonia gas.

[0019] In the example of FIG. 1, the detection processor 116 generates a stable frequency reference output signal f OUT In this way, the RF signal f CTL Based on the signal frequency of OUT The RF signal f CTL The signal frequency of f is locked to the very stable resonant frequency of the gas 106, resulting in a stable frequency output signal f OUT can likewise be provided as having a very stable and precise frequency. As an example, the detection processor 116 may detect the RF signal f CTL down-sample the signal frequency to produce a stable frequency output signal f having a more manageable frequency, such as in the megahertz range (e.g., 5 MHz) OUT As described in more detail herein, the atomic clock system 100 may be configured to provide a stable frequency output signal f, such as those resulting from variations in the temperature of the waveguide cavity 104 and / or variations in the pressure of the gas 106. OUT Therefore, atomic clock system 100 can exhibit superior accuracy and stability relative to conventional atomic clock systems that also provide frequency references based on gas state transitions.

[0020] As mentioned above, the RF signal f CTLcan be provided from oscillator system 102 as a dither signal including an offset frequency. Figure 4 shows another exemplary diagram 400 of a power absorption frequency spectrum. Diagram 400 shows the same power absorption frequency spectrum as shown in diagram 300 in the example of Figure 3. Therefore, in the following description of the example of Figure 4, reference is made to the examples of Figures 1 and 3.

[0021] In the example of FIG. 4, the RF signal f generated by the oscillator system 102 CTL is shown as a dither signal. For example, the frequency controller 112 generates an RF signal f having an offset frequency δ about a center frequency. CTL The center frequency may be configured to provide an RF signal f locked to the resonant frequency of the gas 106. CTL It can handle RF signals f CTL The dither signal corresponding to may have a time-averaged frequency equal to the center frequency. During the first time interval, the frequency controller 112 generates an RF signal f having a first frequency +δ. CTL whereby an offset frequency δ is added to the center frequency locked to the resonant frequency of the gas 106. For a second time interval approximately equal to the first time interval, the frequency controller 112 provides an RF signal f having a second frequency −δ. CTL whereby the offset frequency δ is subtracted from a center frequency locked to the resonant frequency of the gas 106. The offset frequency δ may be selected as a small offset to identify the slope on each side of the inversion peak 304 of the power absorption frequency spectrum. Thus, in the example of FIG. 4, the first frequency +δ and the second frequency −δ may each be associated with a portion of the power absorption frequency spectrum that is approximately half the height of the inversion peak 304.

[0022] Figure 5 shows another exemplary diagram 500 of a power absorption frequency spectrum. Diagram 500 shows an expanded portion of the same power absorption frequency spectrum shown in diagrams 300 and 400 in the examples of Figures 3 and 4, respectively. Accordingly, in the following description of the example of Figure 5, reference is made to the examples of Figures 1, 3, and 4.

[0023] The power absorption frequency spectrum is scaled such that only peak 302 and inverted peak 304 are shown in the example of Figure 5. As mentioned above, the detection system 114 detects the RF signal f propagating through the gas 106 in the waveguide cavity 104. CTL , whereby the detection processor 116 detects the power of the RF signal f relative to the resonant frequency of the gas 106. CTL Thus, the detection processor 116 can determine the signal frequency of the RF signal f for the resonant frequency of the gas 106. CTL A feedback signal FDBK indicative of the difference in the signal frequencies can be generated to adjust the signal frequency by the difference, thereby locking the signal frequency to the resonant frequency of the gas 106.

[0024] The example in Figure 5 shows an RF signal f shown as frequency cf. CTL 5 shows an example in which the center frequency of RF signal f is shifted relative to the resonant frequency of the gas, shown as frequency rf. In the example of FIG. 5, the center frequency cf is shown as being slightly less than the resonant frequency rf by a frequency difference Δf. Similar to the example of FIG. 4, the frequency controller 112 varies the RF signal f as a dither signal between frequencies +δ and −δ in each of two respective equal time intervals. CTL During the first time interval, when the dither signal has a frequency +δ and is therefore greater than the center frequency cf by an offset frequency δ, the detection system 114 detects the received RF signal f CTL During the second time interval, when the dither signal has a frequency −δ and is therefore less than the center frequency cf by an offset frequency δ, the detection system 114 detects the power amplitude PWR1 of the received RF signal fCTL The power amplitude PWR2 of the signal is detected.

[0025] Therefore, based on the symmetry of the power absorption frequency spectrum, the difference between the measured power amplitudes PWR1 and PWR2 indicates the frequency difference Δf between the center frequency cf and the resonant frequency rf. Thus, the detection processor 116 can determine the frequency difference Δf based on the difference between the measured power amplitudes PWR1 and PWR2. This enables the detection processor 116 to generate a feedback signal FDBK indicative of the frequency difference Δf. Thus, in response to the feedback signal FDBK, the frequency controller 112 adjusts the RF signal f CTL 5, the frequency controller 112 can adjust the center frequency cf of the RF signal f by the frequency difference Δf. CTL The center frequency cf of the RF signal f is increased by the frequency difference Δf. CTL Therefore, the center frequency cf of the RF signal f received by the receiving antenna 110 can be made approximately equal to the resonant frequency rf. CTL By continuously measuring the power of the RF signal f, the power difference between the measured powers PWR1 and PWR2 should be approximately zero. CTL The signal frequency (eg, center frequency cf) can be locked to the resonant frequency rf of the ammonia gas 106.

[0026] 1, as discussed above, the waveguide cavity 104 can be fabricated to have a length approximately equal to an integer multiple of the wavelength (e.g., half the wavelength) of the resonant frequency of the gas 106. Such dimensions of the waveguide cavity 104 can be used to maximize the propagation of low-order cavity modes TE 102Such low-order cavity modes offer several advantages over the use of higher-order modes as implemented in conventional atomic clock systems. Advantages of using low-order cavity modes can include Dicke confinement, reduced mode-pulling, and improved temperature compensation fidelity. Because the gas 106 is confined within the waveguide cavity 104 at a distance comparable to the transition wavelength of the resonant frequency of the gas 106, Dicke confinement can result in Doppler narrowing of the transition of the gas 106. Additionally, adjacent TE 101 Mode and TE 103 The relatively large frequency separation between modes can suppress line-pulling of transitions in the gas 106 by adjacent modes. Furthermore, as described in more detail below, the increased free spectral range (FSR) of the low-order modes of the waveguide cavity 104 provides a passive temperature compensation scheme that accommodates cavity-pulling more easily than in higher-order cavities.

[0027] While the length of the waveguide cavity 104 provides the above-mentioned advantages, temperature fluctuations can change the physical length of the waveguide cavity 104. Thus, the physical length of the waveguide cavity 104 can be greater than or less than an integer multiple of the wavelength of the resonant frequency of the gas 106. However, as described below, the atomic clock system 100 can be configured to accommodate variations in the physical length of the waveguide cavity 104 to adjust the RF signal f to the resonant frequency of the gas 106. CTL The stability of the signal frequency lock and therefore the frequency reference output signal f OUT The stability of the

[0028] As a first example, in the example of FIG. 1 , the waveguide cavity 104 includes a temperature sensor 118 configured to monitor the temperature of the waveguide cavity 104, and the detection system 114 includes a stub tuner 120. The temperature sensor 118 is shown providing an indication of the temperature of the waveguide cavity 104 to the stub tuner 120, depicted as a signal TMP. As an example, a relationship between the temperature of the waveguide cavity 104 and the physical length of the waveguide cavity 104 may be modeled and / or tested. Accordingly, the detection processor 116 and / or the stub tuner 120 may be configured to identify variations in the physical length of the waveguide cavity 104 based on the measured temperature TMP. In response to changes in the temperature of the waveguide cavity 104, as depicted by the signal TMP, the stub tuner 120 may provide a signal T N to provide a reactive load to the transmit antenna 108. As an example, the reactive load may provide an adjustment to the capacitance and / or inductance of the transmit antenna 108 to reduce the RF signal f transmitted by the transmit antenna 108. CTL The reactive load provided to the transmit antenna 108 can induce a phase shift in the waveguide cavity 104. As a result, the reactive load provided to the transmit antenna 108 provides an adjustment to the electrical length of the waveguide cavity 104. Thus, the change in the electrical length of the waveguide cavity 104 can compensate for the change in the physical length of the waveguide cavity 104. Thus, the waveguide cavity 104 can compensate for temperature-induced cavity pulling based on the temperature sensor 118 and the stub tuner 120.

[0029] As a second example, the temperature sensor 118 can be omitted from the waveguide cavity 104, and the RF signal f CTL A passive temperature compensation scheme can be implemented based on a periodic frequency sweep of . A passive temperature compensation scheme is shown in the example of Figure 6.

[0030] Figure 6 shows another example diagram 600 of power absorption frequency spectra. Diagram 600 shows a first power absorption frequency spectrum 602 and a second power absorption frequency spectrum 604. First power absorption frequency spectrum 602 is shown as the same power absorption frequency spectrum shown in diagram 300 in the example of Figure 3. Therefore, in the following description of the example of Figure 6, reference is made to the examples of Figures 1 and 3.

[0031] The first power absorption frequency spectrum 602 can correspond to steady-state operation of the atomic clock system 100, where the resonant frequency of the waveguide cavity 104 is approximately equal to the resonant frequency of the gas 106. In the example of FIG. 6, the resonant frequency of the waveguide cavity 104 is indicated by a virtual peak 606 that is centered between the low absorption peaks 302. The virtual peak 606 is therefore approximately co-located with the inverted peak 304, indicating that the resonant frequencies of the waveguide cavity 104 and the gas 106 are approximately equal. The first power absorption frequency spectrum 602 is therefore shown to be symmetrical about the inverted peak 304, as discussed above. By approximately equalizing the resonant frequencies of the waveguide cavity 104 and the gas 106, the RF signal f CTL This realizes the low-order mode of propagation of the frequency reference output signal f OUT can be generated accurately.

[0032] As described above, the frequency controller 112 controls the RF signal f to determine the resonant frequency of the waveguide cavity 104. CTL The frequency of the RF signal f can be periodically swept. The periodic sweep of the frequency is illustrated as sweeping the frequency between a first frequency −Δ and a second frequency +Δ. In one example, the frequencies −Δ and +Δ are CTLand / or may correspond to a large offset frequency relative to the resonant frequency of the gas 106. The first frequency −Δ corresponds to approximately the middle of an ascending portion of the power absorption frequency spectrum below the first low absorption peak 302, and the second frequency +Δ corresponds to approximately the middle of a descending portion of the power absorption frequency spectrum above the second low absorption peak 302. Thus, the periodic sweep of frequencies between the first frequency −Δ and the second frequency +Δ is a wide frequency band across the power absorption frequency spectrum, having frequency boundaries at portions of approximately equal power across the first power absorption frequency spectrum 602.

[0033] As discussed above, the resonant frequency of the waveguide cavity 104 can change based on temperature fluctuations. Therefore, by determining the resonant frequency of the waveguide cavity 104, temperature can be easily compensated for by adjusting the resonant frequency of the waveguide cavity 104 to be approximately equal to the resonant frequency of the gas 106. The second power absorption frequency spectrum 604 illustrates the shift in the resonant frequency of the waveguide cavity 104, shown as frequency fc, relative to the resonant frequency of the gas 106, shown as frequency fg. In the example of FIG. 6, the resonant frequency of the gas 106, fg, is shown as being slightly less than the resonant frequency of the waveguide cavity 104, fc, by a frequency difference Δf. Therefore, the virtual peak 606 is shown as being shifted relative to the inverted peak 304, resulting in an asymmetry in the power absorption frequency spectrum.

[0034] As the frequency controller 112 performs a frequency sweep, the detection system 114 detects the received RF signal f at the first frequency −Δ. CTL and a power amplitude PWR1 of a received RF signal f at a second frequency +Δ CTLThe detection processor 116 can detect the power amplitude PWR2 of the second power absorption frequency spectrum 604. Therefore, based on the asymmetry of the second power absorption frequency spectrum 604, the difference between the measured power amplitudes PWR1 and PWR2 indicates the frequency difference Δf between the resonant frequency fg of the gas 106 and the resonant frequency fc of the waveguide cavity 104. Therefore, the detection processor 116 can determine the frequency difference Δf based on the difference between the measured power amplitudes PWR1 and PWR2. Therefore, the detection processor 116 can provide instructions to the stub tuner 120 to change the electrical length of the waveguide cavity 104, similar to the first example of temperature compensation described above.

[0035] For example, the stub tuner 120 provides a signal T N to provide a reactive load to the transmit antenna 108 to generate an RF signal f transmitted by the transmit antenna 108. CTL , which can induce a phase shift. As a result, the reactive load provided to the transmit antenna 108 provides an adjustment to the electrical length of the waveguide cavity 104. Therefore, changes in the electrical length of the waveguide cavity 104 can compensate for changes in the physical length of the waveguide cavity 104 that result in an offset between the resonant frequency fg of the gas 106 and the resonant frequency fc of the waveguide cavity 104. Therefore, by changing the electrical length of the waveguide cavity 104, the resonant frequency fc of the waveguide cavity 104 can be made to approximately match the resonant frequency fg of the gas 106. Therefore, the waveguide cavity 104 can compensate for temperature-induced cavity pulling based on performing a periodic frequency sweep between a first frequency −Δ and a second frequency +Δ and implementing a stub tuner 120 to change the electrical length of the waveguide cavity 104.

[0036] In addition to accommodating frequency changes, the atomic clock system 100 can also accommodate changes in the pressure of the gas 106 within the waveguide cavity 104. For example, temperature fluctuations can adversely affect not only the resonant frequency of the waveguide cavity 104, but also the pressure of the gas 106 enclosed therein. The pressure of the gas 106 can affect the resonant frequency of the gas 106, and as a result, fluctuations in the pressure of the gas 106 can cause the signal frequency to lock to an inaccurate frequency value. As a result, a stable frequency reference output signal f OUT may contain errors if there is no pressure compensation.

[0037] Figure 7 shows another example diagram 700 of power absorption frequency spectra. Diagram 700 shows a first power absorption frequency spectrum 702 and a second power absorption frequency spectrum 704. First power absorption frequency spectrum 702 is shown as the same power absorption frequency spectrum shown in diagram 300 in the example of Figure 3. Therefore, in the following description of the example of Figure 7, reference is made to the examples of Figures 1 and 3.

[0038] The first power absorption frequency spectrum 702 can correspond to steady-state operation of the atomic clock system 100 at a first pressure of ammonia gas in the waveguide cavity 104. A change in the pressure of the ammonia gas can result in a change in the separation of the low absorption peaks 302 of the power absorption frequency spectrum. Such a change in the peak separation can result in a change in the slope of the inversion peak 304. In the example of FIG. 7 , the second power absorption frequency spectrum 704 can correspond to steady-state operation of the atomic clock system 100 at a second pressure of ammonia gas in the waveguide cavity 104, the second pressure being different from the first pressure.

[0039] The pressure of the ammonia gas has a direct correlation with the resonant frequency of the ammonia gas enclosed within the volume of the waveguide cavity 104. Accordingly, the inversion peaks of the first power absorption frequency spectrum 702 and the second power absorption frequency spectrum 704 are shown at the resonant frequency fr corresponding to the maximum population of ammonia gas molecules exhibiting an inversion transition between the first state 210 and the second state 212. As mentioned above, approximately 10 -3 Torr (approx. 133.322 x 10 -3 For a pressure of 100 psi (22.8 Pa), the resonant frequency fr is approximately 22.8 GHz. However, in response to changes in pressure, the resonant frequency fr may drift. Therefore, if the detection processor 116 is unaware of a change in the resonant frequency fr, the detection processor 116 may generate an RF signal f CTL signal frequency to the changed resonant frequency, resulting in a stable frequency reference output signal f OUT However, as explained in the example of Figure 8, the detection processor 116 can detect and compensate for variations in the pressure of the gas 106 within the waveguide cavity 104.

[0040] Figure 8 shows another example diagram 800 of power absorption frequency spectra. Diagram 800 includes a first power absorption frequency spectrum 802 that corresponds to the enlarged region 706 of first power absorption frequency spectrum 702 in the example of Figure 7. Diagram 800 also includes a second power absorption frequency spectrum 804 that corresponds to the enlarged region 708 of second power absorption frequency spectrum 704 in the example of Figure 7. Accordingly, in the following description of the example of Figure 8, reference is made to the examples of Figures 1, 3, and 7.

[0041] As described in the examples of FIGS. 4 and 5, the RF signal f generated by the oscillator system 102 CTLmay be provided as a dither signal with an offset frequency δ added to and subtracted from a center frequency. To detect the pressure of the gas 106 in the waveguide cavity 104, the detection processor 116 may measure the slope of the inversion peak 304 based on the dither signal. As an example, the frequency controller 112 may provide the dither signal by adding and subtracting a first offset frequency δ1 to and from the center frequency for a first duration, and by adding and subtracting a second offset frequency δ2 to and from the center frequency for a second duration, where the second offset frequency δ2 is greater than the first offset frequency δ1.

[0042] As described above in the examples of FIGS. 4 and 5, the detection processor 116 detects the received RF signal f during each of the time intervals of the two dither frequencies +δ and −δ. CTL The center frequency can be locked to the resonant frequency based on measuring the power PWR. During steady-state operation, each iteration of the measurement can produce a time-averaged difference of the power measurements near zero, thus indicating that the center frequency is approximately equal to the resonant frequency of the gas 106. The power measurements PWR1 and PWR2 are based on the power absorption on each side of the inversion peak 304 at each of the dither frequencies +δ and −δ. By adjusting the dither frequency from +δ1 and −δ1 to +δ2 and −δ2, the power measurements PWR1 and PWR2 differ based on the portion of the inversion peak aligned with the dither frequencies +δ2 and −δ2.

[0043] 8, in each of the first power absorption frequency spectrum 802 and the second power absorption frequency spectrum 804, the center frequency is the same for the dither frequencies from +δ1 and −δ1 and +δ2 and −δ2, and is approximately equal to the resonant frequency of the gas 106. In the first power absorption frequency spectrum 802, the dither frequencies +δ1 and −δ1 provided for a first duration each correspond to a portion of the power absorption frequency spectrum having a power Pδ1 (e.g., PWR1 = PWR2 = Pδ1). Thus, during the first duration, the dither frequencies are provided as +δ1 and −δ1 to measure the respective powers PWR1 and PWR2 for the power Pδ1. During the second duration, the dither frequencies +δ2 and −δ2 each correspond to a portion of the power absorption frequency spectrum having a power Pδ2 (e.g., PWR1 = PWR2 = Pδ2). Thus, during the second time duration, the dither frequencies are provided as +δ2 and −δ2 to measure the respective powers PWR1 and PWR2 with respect to the power Pδ2.

[0044] Based on the slope of the inversion peak 304 of the first power absorption frequency spectrum 802, the power Pδ2 is greater than the power Pδ1. The difference between the powers Pδ1 and Pδ2 and the difference between the relative offset frequencies δ1 and δ2 can determine the slope of the inversion peak 304. Thus, the detection processor 116 can calculate the pressure of the gas 106 in the waveguide cavity 104 based on the known relationship between the pressure of the gas 106 and the power absorption frequency spectrum of the gas 106. Thus, the pressure of the gas 106 can indicate the resonant frequency of the gas 106 based on the known relationship between the pressure of the gas 106 and the resonant frequency of the gas 106.

[0045] As an example, in response to a change in the pressure of the gas 106, such as due to a change in the temperature of the waveguide cavity 104, the gas 106 may change from exhibiting power absorption frequency spectrum 702 to exhibiting power absorption frequency spectrum 704. Thus, the detection processor 116 may calculate a change in slope, and thus a change in pressure, and may identify a change in the resonant frequency of the gas 106.

[0046] In the second power absorption frequency spectrum 804, the dither frequencies +δ1 and −δ1 provided during the first duration each correspond to a portion of the power absorption frequency spectrum having power Pδ1 (e.g., PWR1 = PWR2 = Pδ1). Thus, during the first duration, the dither frequencies are provided as +δ1 and −δ1 to measure the respective powers PWR1 and PWR2 for power Pδ1. During the second duration, the dither frequencies +δ2 and −δ2 each correspond to a portion of the power absorption frequency spectrum having power Pδ2 (e.g., PWR1 = PWR2 = Pδ2). Thus, during the second duration, the dither frequencies are provided as +δ2 and −δ2 to measure the respective powers PWR1 and PWR2 for power Pδ2.

[0047] Similar to the above with respect to the first power absorption frequency spectrum 802, the detection processor 116 can calculate the slope of the inversion peak 304. For example, the difference between the powers Pδ1 and Pδ2 and the difference between the relative offset frequencies δ1 and δ2 can determine the slope of the inversion peak 304 in the second power absorption frequency spectrum 804. However, the detection processor 116 can identify a change in slope between the first power absorption frequency spectrum 802 and the second power absorption frequency spectrum 804. In particular, the difference between the powers Pδ1 and Pδ2 in the first power absorption frequency spectrum 802 is greater than the difference between the powers Pδ1 and Pδ2 in the second power absorption frequency spectrum 804. Thus, the detection processor 116 can monitor the change in the slope of the inversion peak during each of the two time durations to calculate the change in the pressure of the gas 106.

[0048] As described above, the detection processor 116 provides the feedback signal FDBK to lock the center frequency to the resonant frequency of the gas 106. This continues to occur even as the resonant frequency of the gas 106 shifts based on changes in the pressure of the gas 106. However, based on identifying the change in the pressure of the gas 106, and therefore the amount of drift in the resonant frequency resulting from the pressure change, the detection processor 116 generates a stable frequency reference output signal f OUT For example, the detection processor 116 may be configured to mathematically compensate for changes in the resonant frequency when providing a downsampled frequency reference output signal f OUT The denominator in the calculation of can be changed to proportionally compensate for changes in the numerator of the resonant frequency. Thus, the frequency reference output signal f OUT can serve as a stable frequency reference regardless of changes in the resonant frequency of the gas 106.

[0049] Figure 9 shows an example of an integrated atomic clock system 900. The integrated atomic clock system 900 may correspond to the atomic clock system 100 in the example of Figure 1. Therefore, in the following description of the example of Figure 9, reference will be made to the example of Figure 1.

[0050] The integrated atomic clock system 900 can be fabricated in any of a variety of ways using integrated circuit fabrication techniques. The integrated atomic clock system 900 includes a waveguide cavity 902 and a transmitting antenna 904 and a receiving antenna 906 disposed at opposite ends of the waveguide cavity 902. The integrated atomic clock system 900 includes a circuit portion 908 including the oscillator system 102 and the detection system 114. Microstrip lines or other RF signal transmission conductors can be routed between the circuit portion 908 and the antennas 904 and 906. Thus, the integrated atomic clock system 900 can be formed into an integrated package with a compact form factor to provide a stable frequency reference such as that implemented in an INS or other precision device.

[0051] With the above structural and functional features in mind, methods according to various embodiments of the present invention will be better understood with reference to Figure 10. For purposes of simplicity of explanation, the method of Figure 10 is shown and described as being performed sequentially; however, it should be understood and appreciated that the present invention is not limited by the illustrated order, as some aspects may occur in a different order and / or concurrently with other aspects in accordance with the present invention than as shown and described herein. Furthermore, not all illustrated features may be required to practice a method in accordance with an embodiment of the present invention.

[0052] FIG. 10 illustrates a stable frequency reference output signal (e.g., a stable frequency reference output signal f OUT 1 illustrates an example of a method 1000 for providing an RF signal (e.g., RF signal f ) having a signal frequency approximately equal to the resonant frequency of ammonia gas. CTL ) is generated. At 1004, the RF signal is radiated via a transmit antenna (e.g., transmit antenna 108) through a waveguide cavity (e.g., waveguide cavity 104) that is sealed and contains ammonia gas trapped therein. The waveguide cavity can have a length that is approximately an integer multiple of half wavelengths of the resonant frequency of the ammonia gas between the two states. At 1006, the RF signal is received through the waveguide cavity at a receive antenna (e.g., receive antenna 110) opposite the transmit antenna. At 1008, a characteristic of the RF signal at the receive antenna is measured to detect a maximum transition between the two states of the ammonia gas. At 1010, a feedback signal (e.g., feedback signal FDBK) related to the difference between the signal frequency and the resonant frequency of the gas is generated based on detecting the maximum transition. At 1012, the signal frequency of the RF signal is adjusted in response to the feedback signal to be approximately equal to the resonant frequency. At 1014, a stable frequency reference output signal is generated based on the signal frequency of the RF signal.

[0053] The foregoing has been described as an example of the present invention. Of course, it is not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to encompass all such changes, modifications, and variations that fall within the scope of this application, including the appended claims.

Claims

1. 1. An atomic clock system comprising: a sealed waveguide cavity containing a gas enclosed therein, the waveguide cavity having a length approximately an integral multiple of half wavelengths of a resonant frequency of the gas between two states; an oscillator system configured to generate an RF signal through the waveguide cavity, the RF signal having a signal frequency approximately equal to the resonant frequency of the gas; a detection system configured to measure a characteristic of the RF signal passing through the waveguide cavity to detect a maximum transition between the two states of the gas, and based on detecting the maximum transition, provide a feedback signal to the oscillator system to lock the signal frequency of the RF signal to the resonant frequency of the gas; and wherein the detection system is configured to provide a frequency reference output signal based on the signal frequency of the RF signal.

2. 2. The atomic clock system according to claim 1, wherein the gas is ammonia gas.

3. 2. The atomic clock system of claim 1, wherein the oscillator system includes a frequency controller configured to provide the RF signal as a dither signal between a first signal frequency and a second signal frequency, and the detection system is configured to provide the feedback signal to lock a center frequency of the dither signal to the resonant frequency of the gas.

4. 4. The atomic clock system of claim 3, wherein the frequency controller is configured to provide the dither signal by adding and subtracting a first offset frequency to and from the center frequency of the RF signal at approximately equal time intervals to determine a difference between the center frequency of the RF signal and the resonant frequency of the gas, and the detection system includes a detection processor configured to provide the feedback signal and adjust the center frequency to be approximately equal to the resonant frequency of the gas based on the measured difference in the characteristic between adding and subtracting the first offset frequency from the center frequency.

5. 5. The atomic clock system of claim 4, wherein the frequency controller is configured to provide the dither signal by adding and subtracting the first offset frequency to the center frequency for a first duration and by adding and subtracting a second offset frequency to the center frequency for a second duration, the second offset frequency being greater than the first offset frequency, and the detection processor is configured to determine the resonant frequency of the gas based on a time-averaged difference in the measured characteristic of the RF signal during each of the first duration and the second duration, and to adjust the frequency reference output signal based on the determined resonant frequency.

6. 6. The atomic clock system of claim 5, wherein the detection processor is configured to determine the resonant frequency of the gas based on identifying a pressure of the gas in the waveguide cavity in response to the time-averaged difference in the measured characteristic of the RF signal at each of the first duration and the second duration.

7. 2. The atomic clock system of claim 1, wherein the waveguide cavity includes a transmitting antenna at a first end of the waveguide cavity and a receiving antenna at a second end of the waveguide cavity opposite the first end, the transmitting antenna configured to radiate the RF signal through the waveguide cavity to be received by the receiving antenna, and the detection system includes a stub tuner configured to provide a reactive load to the transmitting antenna to adjust the electrical length of the waveguide cavity in response to changes in the physical length of the waveguide cavity.

8. 8. The atomic clock system of claim 7, wherein the waveguide cavity further includes a temperature sensor configured to measure a temperature of the waveguide cavity, and wherein the stub tuner is configured to adjust the electrical length of the waveguide cavity in response to the change in the physical length of the waveguide cavity based on the temperature.

9. 8. The atomic clock system of claim 7, wherein the oscillator system includes a frequency controller configured to periodically sweep a frequency range between a first frequency and a second frequency on either side of a power transmission peak of a power absorption frequency spectrum associated with the gas, and wherein the stub tuner is configured to adjust the electrical length of the waveguide cavity in response to detecting the change in the physical length of the waveguide cavity in response to the periodic sweep of the frequency range.

10. An integrated circuit (IC) comprising the atomic clock system of claim 1.

11. 1. A method for providing a stable frequency reference output signal, comprising: generating an RF signal having a signal frequency approximately equal to the resonant frequency of ammonia gas; radiating the RF signal via a transmitting antenna through a waveguide cavity that is sealed and contains the ammonia gas enclosed therein, the waveguide cavity having a length that is approximately an integral multiple of half wavelengths of the resonant frequency of the ammonia gas between two states; receiving the RF signal at a receive antenna opposite the transmit antenna through the waveguide cavity; measuring a characteristic of the RF signal at the receiving antenna to detect a maximum transition between the two states of the ammonia gas; generating a feedback signal related to a difference between the signal frequency and the resonant frequency of the ammonia gas based on detecting the maximum transition; adjusting the signal frequency of the RF signal to be approximately equal to the resonant frequency in response to the feedback signal; providing the stable frequency reference output signal based on the signal frequency of the RF signal; A method comprising:

12. generating the RF signal adding a first offset frequency to a center frequency of the RF signal during a first time interval; subtracting the first offset frequency from the center frequency for a second time interval approximately equal to the first time interval. Including, 12. The method of claim 11, wherein generating the feedback signal comprises generating the feedback signal associated with a difference between the center frequency of the first and second offset frequencies and the resonant frequency of the ammonia gas based on detecting the maximum transition.

13. Adding and subtracting the first offset frequency from the center frequency includes adding and subtracting the first offset frequency from the center frequency for a first duration, and the method further comprises: adding a second offset frequency greater than the first offset frequency to the center frequency of the RF signal for a first time interval of a second duration; subtracting the second offset frequency from the center frequency for a second time interval approximately equal to the first time interval during the second duration; determining the resonant frequency of the ammonia gas based on a time-averaged difference of the measured characteristic of the RF signal during each of the first duration and the second duration; adjusting the stable frequency reference output signal based on the determined resonant frequency; The method of claim 12 further comprising:

14. measuring the temperature of the waveguide cavity; detecting a change in the physical length of the waveguide cavity based on the temperature; Varying the reactive load of the transmitting antenna to adjust the electrical length of the waveguide cavity to be approximately equal to the physical length of the waveguide cavity. The method of claim 11 further comprising:

15. periodically sweeping the signal frequency of the RF signal through a frequency range between a first frequency and a second frequency on either side of a power transmission peak on a power absorption frequency spectrum associated with the ammonia gas; detecting a change in the physical length of the waveguide cavity in response to the periodic sweep of the frequency range; Varying the reactive load of the transmitting antenna to adjust the electrical length of the waveguide cavity to be approximately equal to the physical length of the waveguide cavity. The method of claim 11 further comprising:

16. 1. An integrated atomic clock system, comprising: a sealed waveguide cavity containing ammonia gas enclosed therein, the waveguide cavity having a length approximately an integral multiple of half wavelengths of a resonant frequency of the ammonia gas between two states; an oscillator system configured to generate an RF dither signal between a first signal frequency and a second signal frequency and provide the RF dither signal through the waveguide cavity via a transmit antenna, the RF dither signal having a center frequency approximately equal to the resonant frequency of the ammonia gas; a detection system configured to measure a characteristic of the RF dither signal received at a receiving antenna opposite the transmitting antenna through the waveguide cavity at each of the first signal frequency and the second signal frequency to detect a maximum transition between the two states of the ammonia gas, and based on detecting the maximum transition, provide a feedback signal to the oscillator system to lock the center frequency of the RF signal to the resonant frequency of the ammonia gas; and wherein the detection system is configured to provide a frequency reference output signal based on the center frequency of the RF signal.

17. 17. The integrated atomic clock system of claim 16, wherein the oscillator system includes a frequency controller configured to provide the RF dither signal by adding and subtracting a first offset frequency to and from the center frequency of the RF dither signal at approximately equal time intervals to determine a difference between the center frequency of the RF dither signal and the resonant frequency of the ammonia gas, and the detection system includes a detection processor configured to provide the feedback signal and adjust the center frequency to be approximately equal to the resonant frequency of the ammonia gas based on the measured difference in characteristic between adding and subtracting the first offset frequency from the center frequency.

18. 18. The integrated atomic clock system of claim 17, wherein the frequency controller is configured to provide the RF dither signal by adding and subtracting the first offset frequency to the center frequency for a first duration and by adding and subtracting a second offset frequency to the center frequency for a second duration, the second offset frequency being greater than the first offset frequency, and the detection processor is configured to determine the resonant frequency of the ammonia gas based on a time-averaged difference in the measured characteristic of the RF dither signal during each of the first duration and the second duration, and to adjust the frequency reference output signal based on the determined resonant frequency.

19. 17. The integrated atomic clock system of claim 16, wherein the waveguide cavity further includes a temperature sensor configured to measure a temperature of the waveguide cavity, and wherein the detection system further includes a stub tuner configured to adjust the electrical length of the waveguide cavity in response to a change in the physical length of the waveguide cavity based on the temperature.

20. 17. The integrated atomic clock system of claim 16, wherein the oscillator system includes a frequency controller configured to periodically sweep a frequency range between a first frequency and a second frequency on either side of a power transmission peak on a power absorption frequency spectrum associated with the ammonia gas, and the detection system further includes a stub tuner configured to adjust the electrical length of the waveguide cavity in response to detecting a change in the physical length of the waveguide cavity in response to the periodic sweeping of the frequency range.

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