Abstract:
A rubidium frequency standard control circuit which utilizes an initial sweep hand-off frequency locking process followed by a negative feedback loop for optimization of the stability of the voltage control oscillator employing the rubidium frequency standard.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to electronics, and more particularly relates to voltage-controlled crystal oscillator (VCXO) circuits using a rubidium frequency standard, and even more particularly relates to methods and systems for controlling such circuits. 
   BACKGROUND OF THE INVENTION 
   In the past, it has been well known to use a rubidium frequency standard to enhance the frequency accuracy of certain voltage-controlled oscillators. 
   While these rubidium frequency standards have enjoyed much use and success in many applications in the past, they do have several shortcomings. 
   First of all, it is often necessary to use a dithering process with such standards. Such dithering often results in unwanted noise in the oscillator output. Also, prior art rubidium standards have exhibited difficulty in correcting for changes in ambient temperature and other environmental events. 
   Consequently, there exists a need for improved methods and systems for operating and controlling a VCXO having a rubidium frequency standard. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an oscillator with a rubidium standard with enhanced control characteristics. 
   It is a feature of the present invention to utilize an initial sweep scheme. 
   It is another feature to provide a method to use an initial sawtooth sweep. 
   It is another feature to provide a mechanism for starting a sawtooth sweep initially and terminating the sweep upon acquisition of a predetermined frequency. 
   It is another feature to automatically provide a feedback loop which regulates a voltage-controlled oscillator when the sweep is not being utilized. 
   It is an advantage of the present invention to provide for a low dithering noise, low power, low component part count, control for a VCXO with a rubidium frequency standard. 
   The present invention is a system and method for controlling a VCXO with a rubidium frequency standard which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the already articulated advantages. The present invention is carried out in a dither-noiseless manner in the sense that noise resulting from intentional continuous dithering has been eliminated. 
   Accordingly, the present invention is a rubidium frequency standard control circuit which utilizes a sweep hand-off locking arrangement upon initialization and during recapture events. 

   
     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 VCXO with a rubidium frequency standard control circuit of the present invention. 
   

   DETAILED DESCRIPTION 
   Now referring to the drawings wherein like numerals refer to like matter throughout, there is shown an oscillator system generally designated  100  with a first oscillator system  110  (rubidium loop) outlined by dotted and dashed lines comprising a voltage-controlled crystal oscillator (VCXO)  102 , which is well known in the art, to provide an output frequency which is proportional to the input voltage. Coupled to voltage-controlled crystal oscillator (VCXO)  102  is a multiplier  104  which could be a step-recovery diode such as a Schottkey diode for generating various odd harmonics of the output of voltage-controlled crystal oscillator (VCXO)  102 . Coupled to multiplier  104  is a frequency modulated laser  106  which is coupled to a rubidium cell  108  and a detector  109 . The rubidium cell  108  may be an entrapment of the gas comprised of the isotope rubidium  87  and buffer gases. The detector  109  may be a device that is photo-sensitive to radiation near the visible light region which detector has an output voltage proportional to the intensity of the radiation. The output of detector  109  is applied to feedback inverting amplifier  112  and then back to voltage-controlled crystal oscillator (VCXO)  102  via switch  114  and capacitor  116 . In order for the oscillator system  110  (rubidium loop) to be stable (when using an inverting amplifier), the detector  109  output versus frequency slope must remain positive. As the nominal frequency is swept low to high, the detector  109  output changes from zero to maximum, and then back to zero. It is preferred, to maintain loop stability, that the operating point is on the front edge of the detector  109  versus frequency curve. 
   The oscillator system  110  is initially locked up by sweep generator and hand-off locking system  120  which is outlined with dotted lines. To initially lock up the rubidium loop, switch  114  is open, and switch  129  is closed. The charge circuit  127 , in conjunction with the flip-flop  124  and two comparators, upper sawtooth limit comparator  125  and lower sawtooth limit comparator  126 , provide a sawtooth output. Flip-flop  124  may be of the S-R type where a high input on S sets the output Q to a high state, and a high input on R resets the output Q to a low state. This output slews the voltage-controlled crystal oscillator (VCXO)  102  from low frequency to high when switch  114  is open and switch  129  is closed. Logic input to the AND gate  128  prevents switch  114  from closing and switch  129  from opening (preventing the loop from closing) when the sawtooth is varied from high to low. When detector  109  output starts to rise (as frequency is increased), comparator  122  goes high at its output, which can function to turn off the sawtooth generator by closing switch  114  and opening switch  129 . It is at this point of the sweep that the open loop circuit hands off and locks the rubidium loop. If the rubidium loop is ever opened, by a loss of detector  109  output, comparator  122  will go low and the sawtooth automatically starts again to lock the loop by sweeping the voltage-controlled crystal oscillator (VCXO)  102  output frequency until one of the multiplier  104  harmonic outputs exactly coincides with the 3.47341307 GHz needed by the rubidium cell  108 . At this frequency, the detector  109  output is again maximized and is then again used to open the switch  129  from the sawtooth generator and close the switch  114  that connects the feedback amplifier to the voltage-controlled crystal oscillator (VCXO)  102  input. This automatic initiating process can be referred to as sweep hand-off locking. 
   In an exemplary embodiment, the sweep hand-off locking is allowed to take place only when the sawtooth generator signal is rising. The rubidium loop is then always locked, for maximum sensitivity on the maximum slope of the rising edge of the detector  109  output. The sweep generator and hand-off locking system  120  immediately disable driving the voltage-controlled crystal oscillator (VCXO)  102  when the rubidium loop is locked, thereby decreasing noise and saving power. The sweep generator and hand-off locking system  120  automatically starts driving the voltage-controlled crystal oscillator (VCXO)  102  when the detector  109  output is ever lost or the rubidium loop is no longer locked. There is no dithering of the voltage-controlled crystal oscillator (VCXO)  102  frequency once the rubidium loop is locked. 
   The voltage-controlled crystal oscillator (VCXO)  102  has an output which is a sub-multiple of the rubidium frequency and is often not usable alone. A frequency locked offset VCXO system  130  is locked to the rubidium loop frequency through a series of dividers, a mixer and the microcontroller  134 , thereby allowing standard output frequencies, such as 10 MHz, 50 MHz or any nominal value. Microcontroller  134  can be used to compensate for temperature and other environmental effects by storing a calibrated compensation value in a nonvolatile memory. 
   Several features of the “Rubidium Frequency Standard with Sweep Hand- off Locking” are summarized as follows:
         1. The “Sweep Hand-off Locking” is allowed to take place only when the sawtooth signal is rising. The rubidium loop is then always locked, for maximum sensitivity, on the maximum slope of the rising edge of the detector output.   2. The sawtooth circuitry is immediately disabled when the rubidium loop is locked, saving power and decreasing noise.       

   3. The “Sweep Hand-off Locking” automatically restarts if the detector output is ever momentarily lost or the rubidium loop unlocks. 
   4. There is no dithering of the VCXO frequency once the rubidium loop is locked. This eliminates the dithering noise from the output. 
   5. An integrator-controlled output VCXO which is frequency locked to the rubidium loop VCXO through a series of dividers, a mixer and the Microcontroller counter, allows standard output frequencies (e.g. 10,000,000 Hz, 50,000,000 Hz, or any nominal value required).
         6. The integrator-controlled output VCXO provides a low-noise frequency output.       

   7. The Microcontroller can be used to compensate for temperature and other environmental effects. This compensation can be stored in the Microcontroller nonvolatile memory. 
   The above description is focused around a rubidium standard It should be understood that the present invention could be used with other standards, such as Cesium or any other suitable material. When such changes are made, then the details of voltages, frequencies, etc., would be changed as well. It is believed that the notion of alternating between a feedback control and an acquisition sweep could be readily adapted to various alternate embodiments. 
   In view of the high level of skill in the art known by designers of prior art VCXOs with rubidium frequency standards, it is thought that the method and apparatus of the present invention will be understood from the foregoing description, and that it will be apparent that various changes may be made in the form, construct steps and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.