Abstract:
The present invention provides a simple and inexpensive method of calibrating radios. The present invention automatically corrects for crystal frequency drift by determining the true crystal frequency. Once the true crystal frequency has been measured and calculated, the proper multiplier required to produce a desired frequency can easily be found and implemented. The present invention allows for a constantly changing desired frequency without recalibration and without readjusting the crystal frequency once the true crystal frequency is known. Once the true crystal frequency is found, a proper multiplier can be calculated to produce the desired frequency. Continuously adjusting the multiplier thus allows for a changing desired frequency, thereby facilitating clear communications between radios, regardless of whether they are in motion or not.

Description:
FIELD OF THE INVENTION 
   This invention relates to radios and crystals used in radio. Specifically, the invention relates to methods and devices used to calibrate radio crystals and to correct for errors in the crystal frequency or errors due to Doppler effects. 
   BACKGROUND TO THE INVENTION 
   Modern equipment which use radio waves for communications, such as cellular telephones, generally operate at very high frequencies. These devices use frequency synthesizers to produce the high frequencies required. However, crystals are still needed for the high stability frequency they produce. Frequency synthesizers, such as phase locked loop (PLL) frequency synthesizers phase lock with the highly stable signals of crystals to produce stable high frequency signals. 
   Essentially, frequency synthesizers, especially PLL frequency synthesizers, phase lock with a crystal signal and multiplies the frequency of that crystal signal to produce a utilizable high frequency stable signal. Thus, the high frequency signal used by the radio equipment is a multiple of the crystal frequency. It should be noted that the multipliers used by frequency synthesizers are not limited to integers. Fractional-N frequency synthesizers can use fractional numbers as multipliers. This feature allows highly accurate frequencies to be produced. 
   During production, radios have to be tuned to perform at expected frequencies. However, tuning radios to the proper frequency can be time consuming and expensive. Frequently, radios have a tunable component which is adjusted to obtain the correct frequency by a technician during production. 
   One problem with crystals is that, under certain conditions, the frequency of their signals tends to drift away from the expected frequency. The phase of the crystal signal is still stable; however, the frequency may now be different. Temperature, age of the crystal, and other known factors can cause this unfortunate phenomenon. When this occurs, given that the high frequency signal generated by the frequency synthesizer is a multiple of the crystal frequency, the high frequency signal drifts as well. This can lead to poor transmission and/or reception between radios as the frequency being used is no longer the desired frequency. To remedy this problem, periodic calibration of radios is performed. This entrails adjusting the crystal frequency to obtain the correct high frequency signal. Clearly, such methods can be expensive, requiring time consuming disassembly of radios and shop time for technicians to perform the calibration. 
   Another measure currently in use to compensate for frequency drift in crystals is the use of Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO). Using a VCTCXO, the new frequency after drift is determined to be higher or lower than the desired frequency. After this determination, a correction voltage is sent to the VCTCXO to compensate for the drift. However, for the most part, radio manufacturers allow their crystals to drift without correction or compensation. To minimize the possibility of such drifts, manufacturers can use more expensive crystals which are either less prone to drift or which do not drift as much. 
   Another problem with radios concerns the well known and well documented Doppler effect. Given two communicating radios, with at least one radio in motion relative to the other, the transmission frequency perceived by either radio from the other is constantly changing. This frequency varies as the velocity between the two radios changes. Clearly, the changing frequency perceived by either of the two radios makes for problematic transmissions. 
   What is therefore needed is a method and an apparatus that allows for fast and inexpensive calibrations of radios. Also needed is a method which compensates for Doppler effects. 
   SUMMARY OF THE INVENTION 
   The present invention provides a simple and inexpensive method of calibrating radios. By implementing the invention in a lab or production facility, quick, automatic calibrations are possible. Furthermore, the invention allows for automatic calibrations and corrections for crystal drift without extensive shop time or technician time. 
   The present invention automatically corrects for crystal frequency drift by determining the true crystal frequency. Once the true crystal frequency has been measured and calculated, the proper multiplier required to produce a desired frequency can easily be found and implemented. The present invention allows for a constantly changing desired frequency without recalibration and without readjusting the crystal frequency once the true crystal frequency is known. Once the true crystal frequency is found, a proper multiplier can be calculated to produce the desired frequency, what ever that may be. Continuously adjusting the multiplier thus allows for a changing desired frequency, thereby facilitating clear communications between radios, regardless of whether they are in motion or not. 
   In one embodiment, the present invention provides a method of calibrating a radio having a frequency source which produces a first signal having a first frequency by multiplying a second signal having an original frequency by a multiplier value, the method comprising determining the original frequency, utilizing the original frequency to determine a corrected multiplier value, and producing an output signal having an output frequency approximately equal to a desired frequency by adjusting the multiplier value to the corrected multiplier value. 
   In another embodiment, the present invention provides a method of adjusting an output frequency of a signal produced by a frequency source, the said frequency source producing the signal by multiplying an input signal having an original frequency by a multiplier value, the method comprising:
         aa) measuring a preliminary frequency of the signal   ab) adjusting the multiplier value based on the original frequency and a measurement of the preliminary frequency to produce a corrected multiplier value   ac) outputting a signal having an intermediate frequency based on the corrected multiplier value   ad) repeating steps aa) to ac) to obtain a final signal with a final frequency such that a difference between the final frequency and the desired frequency is a minimum.       

   In a third embodiment, the invention provides a device for adjusting an output frequency of a signal produced by a frequency source comprising:
         a frequency source which produces the signal by multiplying:
           an input signal having an input frequency and   a multiplier having a value   
           a controller coupled to the frequency source, said controller controlling the value of the multiplier   a frequency measurement device coupled to the frequency source, said frequency measurement device measuring the output frequency of the signal and producing measurement data relating to the output frequency of the signal
 
wherein the controller is coupled to receive the measurement data produced by the frequency measurement device.
       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention will be obtained by considering the detailed description below, with reference to the following drawings, in which: 
       FIG. 1  is a block diagram illustrating the main components of a hardware implementation of the invention; 
       FIG. 2  is a flow chart detailing the different steps of one embodiment of the invention; 
       FIG. 3  is a flow chart detailing the different steps of a second embodiment of the invention; 
       FIG. 4  is a flow chart detailing the different steps of a third embodiment of the invention; and 
       FIG. 5  is a more detailed block diagram of a hardware implementation of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a block diagram of a hardware implementation of the invention is illustrated. A controller  10  is coupled to a frequency source  20  which, in turn, is coupled to a frequency measuring device  30 . The frequency source  20  is controlled by the controller  10 . The controller  10  controls the multiplier by which the frequency source  20  multiples the input frequency  40  to produce the output frequency  50 . 
   To determine the input frequency  40 , one need only know the output frequency  50  and control the multiplier supplied by the controller  10 . The controller  10  sets the multiplier to a known value and uses this known multiplier value so that the frequency source  20  produces a signal with an output frequency  50 . This signal with the output frequency is accurately measured by the frequency measuring device  30 . The measurement data produced by the frequency measuring device  30 , which is a measurement of the output frequency  50 , is sent to the controller  10 . Since the controller  10  knows the multiplier value used by the frequency source  20  to produce the output frequency  50 , the controller  10  can easily calculate the value of the input frequency  40 . This is done simply by dividing the value of the output frequency  50  by the known multiplier value, yielding the true input frequency. 
   Once the true input frequency is known, the controller can thereby calculate the multiplier value required by the frequency source  20  to produce a signal with an output frequency  50  approximately equal to a desired frequency  60 . Once this corrected multiplier value is found by the controller  10  by dividing the desired frequency to the true input frequency, the corrected multiplier is transmitted to the frequency source  20  so that it can be used to produce the desired frequency. 
   At this point, the radio device utilizing the invention is calibrated as the controller  10  and the frequency source  20  will continue to use the corrected multiplier value until the input frequency  40  or the desired frequency  60  changes. If desired, the true input frequency can be stored in suitable storage means, such as a read only memory (ROM), for future use. 
     FIG. 2  illustrates the logic flow diagram followed by the controller  10  described above. 
   If the input frequency  40  changes or drifts, perhaps because of the age of a crystal producing the signal with the input frequency  40  or because of temperature effects, the output frequency  50  also changes. This new output frequency  50  is measured by the frequency measuring device  30  and is communicated to the controller  10 . The controller  10  then compares the measured new output frequency  50  with the desired frequency  60 . If the difference between the desired frequency  60  and the new output frequency  50  is not within preprogrammed tolerances, the controller  10  calculates a new multiplier based on the new output frequency  50 . Once this new multiplier value is used by the frequency source  20 , the radio device is once again properly calibrated as it produces approximately the desired frequency  60 . If the difference between the output frequency  50  and the desired frequency  60  is not within tolerance limits, the calibration starts all over. 
   Once the difference between the output frequency  50  and the desired frequency  60  is within tolerance limits, the input frequency  40  can now be found. The output frequency  50  is divided by the new multiplier to obtain the input frequency. This input frequency can then be stored in suitable storage means, such as a ROM, for future use. 
   A logic flow diagram followed by the controller as described above  10  is illustrated in  FIG. 3 . 
   In another embodiment of the invention, the controller  10  can be used to compensate for a changing desired frequency  60 . If, as mentioned above, the radio equipment utilizing the invention is in motion relative to another piece of radio equipment it is communicating with, the output frequency  50  must continually change as the frequency required to communicate with the other piece of radio equipment changes. This means that the desired frequency  60  is constantly changing. When this occurs, the controller  10  must continuously compare the output frequency  50  measured by the frequency measuring device  30  with the changing desired frequency  60 . Depending on whether the output frequency  50  is higher or lower than the desired frequency  60 , the controller  10  adjusts the multiplier value accordingly. If the output frequency  50  is higher than the desired frequency  60 , the multiplier value is lowered from its previous value. Alternatively, if the output frequency  50  is lower than the desired frequency  60 , the multiplier value is increased from its previous value. 
   The amount by which the multiplier value is incremented or decremented can be preprogrammed into the controller. Thus, by having the controller  10  continuously sample, compare, and either increase or decrease the multiplier value, the output frequency  50  approaches the desired frequency  60 . This process can terminate once a final output frequency is found. Such a final output frequency is a frequency that, if not exactly equal to the desired frequency, is within tolerance limits to the desired frequency. 
   The above embodiment can also be used to determine the original or input frequency  40  that was multiplied by a multiplier to synthesize the output frequency  50 . Once the final output frequency has been obtained by the above method, the multiplier value used to obtain that final output frequency is noted. The final output frequency is then divided by that multiplier value to determine the original or input frequency  40 . This original or input frequency can then be stored in suitable storage means for future use. Once the true original frequency is known, changes in the desired frequency  60  can be accommodated by adjusting the multiplier value. 
   The logical flow diagram followed by the controller  10  in implementing the above is shown in  FIG. 4 . 
   Alteratively, the controller  10  can be programmed to find the correct multiplier in one iteration of the method. Given a desired frequency  60  and a true input frequency, the controller  10  can skip numerous iterations by simply, as above, dividing the desired frequency  60  by the input frequency  40  to obtain the appropriate multiplier value. While this method may seem more efficient, if the desired frequency  60  is continuously increasing or continuously decreasing, depending again on the Doppler effect, incrementing or decrementing the multiplier value by a set value may be more computationally efficient. 
   A more detailed block diagram of a possible embodiment of the invention is pictured in  FIG. 5 . In this embodiment, the frequency source  20  is a high resolution PLL frequency synthesizer which uses a Delta Sigma modulator  70 . The delta sigma modulator  70  allows the use of multiplier values having both an integer part  80  and a fractional part  90 . Such a frequency source provides an ideal source for high performance radio equipment and is ideally suited for this invention. A Delta-Sigma PLL frequency synthesizer would be ideally suited for this invention as the Delta-Sigma PLL frequency synthesizer allows for high resolution multipliers, thereby allowing for a correction of small errors in the original frequency. However, it should be noted that any high resolution frequency synthesizer with a controllable multiplier would be suitable for use in this invention. 
   It should be noted that the controller  10  referred to above can be implemented using well known processors. A dedicated microcontroller or a dedicated general purpose central processing unit can be implemented inside radio equipment to perform the controller functions. In this case, the software required to control the logic of the processor would be resident in a non-volatile memory such as read-only-memory (ROM) with the true input frequency and the multiplier value being stored in either a rewritable non-volatile memory such as an electronically erasable programmable read only memory (E 2 PROM) or, for a less expensive implementation, in random access memory (RAM). However, for mobile applications, where the invention is resident in a radio, the preferred method of storing the true original frequency is a ROM. In such a case, moving from one wireless network that uses one frequency to another that uses another frequency, would only involve changing the true original frequency by either replacing the ROM. 
   In a production facility implementation of the invention, to be used to calibrate mass numbers of radios, the controller  10  could be a general purpose computer such as a personal computer. An appropriate interface between the controller  10 , which in this implementation would be external to the radio equipment, allows the controller  10  to send multiplier values to the frequency source  20  resident in the radio equipment. Once the calibration has been completed, the controller can either send the true original or input frequency  40  to the radio or the radio can determine the true original or input frequency  40  from the multiplier value. In this implementation, the frequency measurement device  30  would also be external to the radio equipment. 
   The frequency measurement device  30 , in either an internal radio implementation or a production facility implementation, is a suitable high resolution device. In a production facility, suitable frequency counters or spectrum analyzers can serve as frequency measurement devices. In mobile applications, numerous methods or devices can be used. If a radio base station transmits a reference signal, suitable hardware can determine whether the output frequency of the radio is higher or lower than the reference signal. Obviously, the higher the resolution and the better the accuracy of the frequency measuring device, the more accurate and hence the more useful the invention. 
   The input frequency can be supplied by a crystal which oscillates at a certain frequency. This option is illustrated in  FIG. 5 . 
   It should also be noted that the invention can be implemented on a single integrated circuit. All the components listed above, including the code which runs the controller  10 , can be stored on a single application specific integrated circuit (ASIC). 
   A person understanding the above-described invention may now conceive of alternative designs, using the principles described herein. All such designs which fall within the scope of the claims appended hereto are considered to be part of the present invention.