Abstract:
A rubidium frequency standard is compensated for frequency variations over temperature by allowing the rubidium frequency standard to vary while holding the output frequency constant. A voltage controlled crystal oscillator, locked to a physics package, provides the output signal. A temperature sensor senses temperature and proves a temperature signal to a microcontroller. A frequency synthesizer receives the output signal from the voltage controlled crystal oscillator as a reference and provides an RF signal to the physics package. The microcontroller looks up a frequency error in a memory in accordance with the temperature signal, generates an offset control word for the frequency synthesizer to compensate for the temperature and adjusts the VCXO with an error signal to compensate for temperature.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to frequency standards, rubidium frequency standards, and more specifically to temperature compensation of a rubidium frequency standard. 
     As hopping rates increase in spread spectrum radios such as JTIDS, LINK 16, HAVE QUICK, SINCGARS, and SATURN, time of day clocks are required to become more accurate to insure quick synchronization and longer mission times without synchronization. These radios require very accurate frequency standards that are pressing the limits of oven controlled crystal oscillators (OCXO) and temperature compensated crystal oscillators (TCXO). 
     Rubidium frequency standards have been known in the art for many years. Rubidium frequency standards provide greater accuracy than OCXO and TCXO frequency standards. Rubidium frequency standards operate by locking a crystal oscillator to a hyperfine transition at 6.834,682,612 GHz in rubidium. The amount of light from a rubidium discharge lamp that reaches a photo detector through a rubidium gas resonance cell is reduced when rubidium vapor in the resonance cell is excited by a microwave signal near the transition frequency. The crystal oscillator is locked to the rubidium transition by detecting the light output drop when sweeping an RF frequency synthesizer containing the crystal through the transition frequency. 
     Rubidium frequency standards are useful in spread spectrum radios and such systems as GPS to provide high frequency accuracy. However, rubidium frequency standards are large and sensitive to changes in temperature that cause changes in frequency. Rubidium frequency standards also consume large amounts of power. 
     In a rubidium frequency standard a gas cell, light source, and photo detector are all temperature sensitive and each requires a specific temperature for optimal operation. The optimal temperature for the three devices is not the same so the temperature used is a compromise of the desired temperatures, making the devices more sensitive to external temperature change. The frequency of the rubidium transition is sensitive to the pressure in the gas cell, which changes with temperature. Rubidium&#39;s melting point is 34° C. so for rubidium to be in the gaseous state, the temperature of the gas cell must be elevated with an oven. Since there are two different isotopes of rubidium there are two different resonances. A filter cell is used to filter out one resonance. A second cell is used to absorb the light of a rubidium lamp or laser diode. This abortion point is very fine providing a high Q frequency reference. For proper operation the filter cell and the absorption cell must be at a specific temperature and the lamp has its specific temperature to provide a zero light shift/zero temperature coefficient (TC) condition (ZLS/ZTC). Deviation from these ideal temperatures means the rubidium frequency standard becomes more dependent on temperature changes. Different types of light sources have been tried in an effort to minimize the effect of the light shifting. 
     The TC of the rubidium frequency standard is closely related to how well the ZLS/ZTC temperature can be maintained. As the frequency standard is miniaturized, the cells and the lamp are brought closer meaning their optimal temperatures must be compromised. Also the room for thermal insulation is minimized. In addition the electronics components are brought closer together, concentrating their heat, causing thermal gradients. All this makes it much more difficult to maintain the optimal temperature conditions required for frequency stability. 
     Previous attempts to stabilize rubidium frequency standards have included ovenization of the components and for even higher stability units double ovenization or cooling by thermal electric cooling. An OCXO is typically used to provide the output frequency. All these attempts occupy large volumes and consume high power in the rubidium frequency standard. 
     For many applications in GPS and radio communications systems miniaturization and low power consumption are required. The high stability of a rubidium frequency standard is also required. What is needed is a miniaturized rubidium frequency standard with temperature compensation to offer high stability without consuming large amounts of power. 
     SUMMARY OF THE INVENTION 
     A rubidium frequency standard with a temperature compensated output signal is disclosed. The rubidium frequency standard has a voltage controlled crystal oscillator (VCXO) that provides the temperature compensated output signal. A frequency synthesizer receives the output signal from the voltage controlled crystal oscillator as a reference and provides a RF signal. A physics package receives the RF signal and provides a light output signal with a null indicating when the RF signal is such that a transition frequency of rubidium is obtained within the physics package. A temperature sensor senses temperature and provides a temperature signal. A microcontroller receives the light output signal, generates a control word for the frequency synthesizer, receives the temperature signal, and provides an error signal to the VCXO to lock the VCXO to the physics package. The microcontroller uses the temperature signal to look up a frequency error in a memory to offset the synthesizer and adjust the VCXO to compensate for temperature. 
     The physics package further comprises a multiplier to multiplying the RF signal to a microwave frequency, a light source for providing light, a gas cell excited by the microwave frequency that passes the light from the light source and reduces the light from the light source when the gas cell is excited by the microwave frequency at the transition frequency. A photo detector detects the light passed through the gas cell and provides a light output signal that is at the null when the gas cell is excited at the transition frequency. 
     The microcontroller calculates a offset control word for the frequency synthesizer by use of a temperature versus control word lookup table, sweeps the synthesizer around the offset control word while sampling the photo detector output, detects the null in the photo detector output, and adjusts the VCXO frequency so that the null frequency matches the synthesizer setting that is recorded in the memory. A compensation table is built and stored in the memory that contains the offset control word for the temperature. 
     During temperature compensation of the rubidium frequency standard a high stability reference oscillator is substituted for the VCXO, the rubidium frequency standard is run over a desired temperature range, and the microcontroller adjusts the synthesizer with a control word to find a null point in the photo detector output and then records the offset control word along with the temperature at which the offset control word was found in the compensation table. 
     It is an object of the present invention to compensate for frequency variations over temperature in a rubidium frequency standard. 
     It is an object of the present invention to correct for frequency variations over temperature in a rubidium frequency standard by allowing the rubidium frequency to varying while maintaining a constant output frequency. 
     It is an advantage of the present invention to allow size reduction of a rubidium frequency standard while maintaining frequency accuracy. 
     It is an advantage of the present invention to eliminate an oven controlled crystal oscillator. 
     It is an advantage of the present invention to reduce power consumption in a rubidium frequency standard, 
     It is a feature of the present invention to allow removal of insulation in ovenized components. 
     It is a feature of the present invention to provide improved frequency accuracy in a compact package with reduced size for radio communications and GPS applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention in conjunction with the appended drawings wherein: 
     FIG. 1 is a block diagram of a rubidium frequency standard known in the art; 
     FIG. 2 is a block diagram of a time compensated clock oscillator (TCCO) technique that the basis for a temperature stabilization scheme of a rubidium standard in the present invention; and 
     FIG. 3 is a block diagram of a temperature compensated rubidium frequency standard of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of a rubidium frequency standard  100  known in the art. Rubidium frequency standards lock a crystal oscillator to the hyperfine transition frequency of 6.8346826128 GHz in rubidium. 
     The output from the rubidium frequency standard  100  is taken from an oven stabilized crystal oscillator (OCXO)  105 . The OCXO  105  is tuned by a digital-to-analog converter  110 . The output from the OCXO  105  is also a reference source to a frequency synthesizer  115  that provides an RF signal to a physics package  120 . The RF signal is multiplied in the physics package  120  by a multiplier such as a step recovery diode  124  to provide the microwave frequency (6.834 GHz) to a rubidium gas cell  122  in the physics package  120 . 
     The physics package  120  also contains a light source  121  that provides light that is passed through the rubidium gas cell  122  and sensed by a photo detector  123 . The light source  121  operates at about 150 MHz and is driven by an oscillator (not shown). The resonance gas cell  122  is inside a magnetic shield to reduce frequency pulling effects of external magnetic fields. The apparent hyperfine transition frequency may be tuned by a magnetic field coil (not shown) in the physics package  120 . 
     The frequency synthesizer  115  has a very fine step size and a low phase noise output. The frequency synthesizer  115  utilizes a dual loop design. An inner loop consists of a VCO  116  that provides the RF signal to the physics package  120  and that is phase locked to a crystal oscillator (not shown) in an RF synthesizer  117 . An outer loop compares the RF signal frequency to the OCXO  105  output in RF synthesizer  117 . The outer loop provides high resolution by dividing the RF signal and the OCXO  105  output by large numbers. The outer loop keeps the inner loop crystal oscillator locked to the OCXO  105  output reference. The frequency synthesizer  115  is set to a frequency above the apparent hyperfine transition for the physics package  120 . The magnetic field coil is used to tune the physics package  120  apparent hyperfine transition frequency to the synthesizer frequency. A magnetic field control signal comes from a control function  130  connected to a microcontroller  140 . 
     A low frequency sine wave modulating signal (typically 70 Hz) is used to phase modulate the inner loop by varying a control word  131  to the RF synthesizer  117  from the microcontroller  140 . This generates an RF output from the VCO  116 , which when multiplied to 6.834 GHz in the gas cell  122 , sweeps with an approximate 300-Hz deviation around the hyperfine transition frequency. By sweeping through the transition at 70 Hz, the output from the photo detector  123  has an ac component at 140 Hz, when centered on the transition frequency. The phase of the 70-Hz component is used to determine if the RF output is above or below the transition frequency. 
     The microcontroller  140  generates the 70-Hz phase modulation of the RF signal that excites the physics package and detects the amplitude and phase of the 70-Hz and 140-Hz signals from the photo detector  123 . The microcontroller  140  also provides an error signal to lock the OCXO  105  to the rubidium hyperfine transition. The 70-Hz digitally synthesized phase modulation waveform is generated by varying a control word input to frequency synthesizer  117  as discussed above. The photo detector  123  signal is amplified and bandpass filtered in photo amp  135  before being converted by an analog to digital converter (A/D)  137 . 
     A time compensated clock oscillator (TCCO) technique forms the basis for temperature stabilization of a rubidium standard in the present invention. The TCCO patented by the assignee of the present invention as U.S. Pat. No. 4,305,041, is incorporated by reference in its entirety. FIG. 2 is a block diagram of a TCCO  200 . In this type of compensation a crystal oscillator with low temperature hysteresis is used as a reference oscillator  210 . The reference oscillator  210  is not compensated for temperature, but its frequency is characterized over temperature and stored in a nonvolatile memory in a microcontroller  240 . In the reference oscillator  210  a crystal is a third overtone SC cut crystal with a Q well over two million. Since this crystal can not be pulled to frequency a second oscillator, a voltage controlled crystal oscillator (VCXO)  220  is used to provide a frequency output. The microcontroller  240  measures the VCXO  220  frequency using the SC-cut crystal as the reference and also at the same time the temperature of the SC-cut crystal by using a temperature sensing oscillator  230 . The microcontroller  240  then calculates the reference oscillator  210  frequency using a look up table in with the reference crystal&#39;s temperature-frequency characteristics in a nonvolatile memory EEPROM in the microcontroller  240 . With this information the microcontroller  240  then calculates the VCXO  220  frequency error and corrects an output word to a digital to analog converter  250  that corrects the VCXO  220  frequency. 
     The digital compensation scheme used in the TCCO with a quartz crystal reference discussed above is adapted for use in a rubidium frequency standard  300  shown in FIG. 3 in the present invention. The function of the temperature compensation in the rubidium frequency standard  300  is similar to that in the TCCO  200 . The rubidium frequency standard  100  in the block diagram of FIG. 1 is modified to include the temperature compensation scheme as discussed below. 
     A temperature sensor  310  is included in the rubidium frequency standard  300  that allows the microcontroller  140  to measure the temperature and find a frequency error setting in a temperature versus frequency error look-up table. The temperature sensor  310  may be a diode or some other method of measuring temperature known in the art including the temperature sensing oscillator  230  of the TCCO  200 . The microcontroller  140  then offsets a modulated fractional divider (MFD) frequency synthesizer  317  by the recorded frequency error setting allowing a VCXO  305  to be set to a frequency reducing the frequency error due to the temperature. The rubidium frequency in the physics package  120  is allowed to shift but the VCXO  305  frequency remains on the correct frequency. The OCXO  105  of FIG. 1 is replaced with the VCXO  305  in the present invention. 
     The RF synthesizer  117  of FIG. 1 may be a modulated fractional divider (MFD) synthesizer  317  due to the fine frequency resolution that can be obtained with this type of synthesizer. The MFD, known in the art, lends itself to integration quite well, which is desirable for the miniaturization of the rubidium frequency standard  300 . Other types of frequency synthesizers that have fine frequency resolution may also be used in the rubidium frequency standard  300 . 
     Initially during temperature compensation a high stability reference oscillator such as a crystal oscillator is substituted for the VCXO  305 . The rubidium frequency standard  300  is then run over a desired temperature range. The microcontroller  140  adjusts the MFD synthesizer  317  with the control word  131  to find a null point in the photo detector  123  output and then records the synthesizer  317  offset control word  131  along with the temperature at which the setting was found. With the conclusion of the temperature compensation run a compensation table is built and stored in a memory (not shown) that may be in the microcontroller  140  that contains the correct synthesizer  317  offset control word for a given temperature. 
     When the rubidium frequency standard  300  of the present invention is in a normal run mode the microcontroller  140  measures the temperature with the temperature sensor  310 . The microcontroller  140  calculates the correct offset control word  131  for the MFD synthesizer  317  using the temperature versus control word look up table for the measured temperature and offsets the synthesizer  317 . The microcontroller  140  sweeps the MFD synthesizer  317  around the correct offset control word while sampling the photo detector  123  output through the analog to digital converter  137 . When a null is detected in the photo detector  123  output, the microcontroller  140  adjusts the VCXO  305  frequency so that the VCXO  305  frequency at the photo detector  123  output null matches the MFD synthesizer  317  correct offset control word  131  that is recorded in the compensation table. 
     The digital compensation scheme of the present invention allows replacement of the ovenized crystal oscillator (OCXO)  105  with a non-ovenized VCXO  305 . This reduces the size required of the rubidium frequency standard  300  and also reduces power by the elimination of the oven used to stabilize the crystal oscillator temperature. The oven associated with the OCXO  105  can use several watts of power. 
     As the light source  121  ages it loses intensity. The intensity of the light source  121  affects the gas cell  122  resonance null point. The null point is found by the microcontroller  140  sweeping the frequency synthesizer  317  though the null point. With the microcontroller  140  constantly monitoring the output of the light source  121 , variations of the light source  121  can also be compensated. The microcontroller  140  can also have a table of the light source  121  intensity versus temperature, which is generated at the same time as the other temperature versus frequency data. In addition a table of the light intensity versus frequency shift must be created. By finding the deviation of the light intensity from the original intensity the aging of the light source can be compensated in the same manner as the temperature effects. 
     The magnetic field sensitivity of a rubidium frequency standard is a result of the hyperfine magnetic resonance on which it depends. The physics package  120  uses an internal longitudinal dc magnetic bias field to orient the Rb atoms and separate the Zeeman sublevels. The frequency varies linearly with the strength of the magnetic field. As the size of the rubidium frequency standard is reduced, the amount of magnetic shielding is reduced, and the proximity of components that may induce magnetic fields is increased. The effect of magnetic fields can be measured by three sensors, one for each plane, and then compensated in much the same way. With inputs from the temperature sensor  310  and the magnetic field sensors, the offset frequency experienced by the physics package  120  to these effects can be minimized. Dynamic temperature effects can also be measured and corrected in much the same manner as the TCCO corrects for temperature effects. 
     With continued miniaturization, the size of the physics package  120  and its associated oven is reduced, as is the insulation surrounding this oven. As the size of the physics package  120  is reduced, so is the magnetic shielding protecting the gas cell. With these changes the frequency accuracy of the rubidium frequency standard  300  is reduced. The rubidium frequency standard digital compensation scheme of the present invention can then recover the accuracy lost though miniaturization. 
     It is believed that the temperature compensated rubidium frequency standard of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages, the form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.