On-board processor compensated oven controlled crystal oscillator

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a Continuation-In-Part of U.S. Ser. No. 10/037,962, filed Dec. 21, 2001, entitled On-Board Processor compensated Oven Controlled Crystal Oscillator, of David Villella. Field of the Invention

[0002] The present invention relates to crystal oscillators, and more particularly, to controlling an output frequency of an oven controlled crystal oscillator in response to a monitored temperature sensitive parameter and a calculated relationship between the monitored parameter and a frequency compensating signal.

BACKGROUND OF THE INVENTION

[0003] Oscillating circuits play a central and increasingly important role in digital and analog electronic systems. Digital devices require precise system timing, a function provided by oscillators and similar timing sources. Telecommunication and data transmission systems, which have analog and digital components, likewise rely on oscillators for modulation, demodulation, system clocking and other functions.

[0004] A standard choice for a highly stable frequency source in such applications is a crystal-based oscillator or resonator. Although atomic frequency standards are highly accurate, such devices are undesirable in most applications in view of cost and environmental capability.

[0005] While crystal oscillators are stable in comparison with non-crystal-based circuits, the crystal oscillators nevertheless exhibit a degree of variance in frequency and stability owing to the inherent frequency response to temperature changes and other environmental influences of crystals.

[0006] An approach to compensate for the temperature effects on a crystal resonator is to maintain a constant temperature of the temperature sensitive section of the oscillator. Such devices, oven controlled crystal oscillators (OCXO), provide enhanced stability and frequency control. However, OCXOs require increased manufacturing considerations and hence, greater costs so as to match the stabilities associated with their atomic frequency reference counterpart.

[0007] Double oven controlled crystal oscillators (DOCXO) provide the required stability needed for the higher stability applications. However, DOCXOs require increased material costs over the single oven.

[0008] The need exists for further controlling the frequency of an OCXO to provide enhanced stability over greater temperature ranges. In addition, the need exists for an economical method of controlling the frequency of a crystal oscillator without requiring a double oven construction.

SUMMARY OF THE INVENTION

[0009] The present invention provides an apparatus and method for the onboard adjustment of an output frequency of a crystal oscillator in conjunction with a heater and a frequency compensator, by monitoring a parameter corresponding to a power consumption of the heater and providing a frequency compensating signal to the input of the frequency compensator in response to the monitored parameter and a derived relationship between the monitored parameter and a required frequency compensating signal for the given crystal oscillator.

[0010] In a preferred configuration, the derived relationship is set forth in terms of the monitored parameter and a corresponding frequency compensating signal. This relationship can be a linear, non-linear or user defined relationship.

[0011] In one configuration, the monitored parameter corresponding to a power consumption of the heater is determined by the voltage measurement across a resister in series with the heater, where the varying voltage potential signifies a varying current consumption with a constant supply. However, it is understood a secondary relationship, or correspondence can be identified between the monitored parameter and the predetermined relationship.

[0012] Thus, one configuration an oven controlled crystal oscillator (“OCXO”) includes an on-board processor and associated memory to retain the relationship and calculate a frequency compensating signal as a function of the monitored parameter and the retained relationship. The present invention thereby provides for the on-board frequency error correction due to temperature influence on a crystal in an oven controlled crystal oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a block diagram of an oven controlled crystal oscillator having an on-board processor.

[0014]FIG. 2 is a graphical representation of crystal frequency versus temperature (in terms of a temperature sensor voltage).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015] Referring to FIG. 1, an oven controlled crystal oscillator (“OCXO”) <b>10</b> is shown.

[0016] The OCXO <b>10</b> includes a crystal oscillator <b>20</b>; a frequency compensator <b>30</b>; an oven control circuit <b>40</b>; a current sensor <b>50</b>; a heater <b>60</b>; a (temperature) sensor <b>70</b>; a processor (microprocessor) <b>80</b>; a reference clock <b>90</b> and a memory <b>100</b>. In addition, the OCXO <b>10</b> includes a power input <b>110</b> for receiving power from an external or outside source.

[0017] All of the components of the OCXO <b>10</b> are enclosed within a substantially sealed environment by a package. The sealed package advantageously serves to isolate the OCXO <b>10</b> from adverse external temperature fluctuations. Preferably, the sealed package includes an input/output (I/O) interface <b>120</b> for allowing mode selection of the processor <b>80</b>.

[0018] The crystal oscillator <b>20</b> may be any of a variety of commercially available oscillators including SC cut quartz crystals. However, it is understood that other cut crystals can be employed, without departing from the invention.

[0019] The frequency compensator <b>30</b> is operably connected to the crystal oscillator to adjust the frequency of the crystal oscillator <b>20</b> and includes a frequency compensating input <b>32</b>.

[0020] The frequency compensator <b>30</b> can employ any of a variety of known systems including changing the load capacitance of the crystal oscillator <b>20</b> to change the frequency. Alternative frequency compensator schemes, include the frequency compensator <b>30</b>, but is not limited to a varactor diode and a biasing scheme, which are used to vary the load capacitance of the crystal. Varying the load capacitance effectively changes the output frequency of the device. Other methods or biasing could be employed such as utilizing a potentiometer, digital potentiometer, digital-to-analog converter, integrators and adjustable regulators. Along with other means to adjust crystal loading including trim caps and capacitor banks in which capacitors could be switched in and out.

[0021] The thermal control of the crystal oscillator <b>20</b> in the OCXO <b>10</b> is performed by the oven control circuit <b>40</b>, the heater <b>60</b> and the temperature sensor <b>70</b>, as well as a thermal mass. The thermal mass can be an oven block made of a conductive material or simply be the can or package of the crystal.

[0022] The oven portion of the OCXO <b>10</b> is used to regulate the temperature of the temperature sensor <b>70</b>, and hence crystal oscillator <b>20</b> to a set value. That is, as the ambient temperature changes around the OCXO <b>10</b>, a heat flow occurs between the ambient environment and the OCXO <b>10</b>. If the ambient temperature (surrounding OCXO <b>10</b>), the temperature sensor <b>70</b> signals a change in temperature and the oven control circuit <b>40</b> increases or decreases the flow of a heating current to the heater <b>60</b> to bring the oven temperature back to the nominal temperature.

[0023] Because of inherent errors in the control of a temperature of crystal oscillator <b>20</b>, a frequency deviates from a set nominal value. It is said then that the more accurately heater <b>60</b> can hold crystal oscillator <b>20</b> the more stable the control of a frequency.

[0024] The temperature sensor <b>70</b> is typically placed in close thermal proximity to the crystal <b>20</b>. The temperature sensor <b>70</b> can be, but is not limited to, any of a variety of elements including thermistors, and digital and analog temperature sensors. The temperature sensor <b>70</b> is connected to the oven control circuit <b>40</b>. The oven control circuit <b>40</b> is operably connected to the heater <b>60</b> to control an amount of current to the heater for generating resistive heating. However, it is understood the heater <b>60</b> can be any of a variety of heating devices, which can be controlled by the oven control circuit <b>40</b>. In one configuration, the heater <b>60</b> can be a power transistor.

[0025] In the present invention, the current sensor <b>50</b> is operably connected to the heater <b>60</b> in the oven control circuit <b>40</b> for generating a signal corresponding to the amount of energy, power, or current flowing to the heater <b>60</b>, when the supply voltage is held constant. However, alternative methods of obtaining a power signal maybe used for determining the amount of energy the heater <b>60</b> is consuming without departing from the scope of the invention.

[0026] In one configuration, the current sensor <b>50</b> is a high power thick film resister in series with the heater <b>60</b> to measure a voltage drop. The voltage drop is a measured voltage delta across the resistor to which a current can be determined by the equation I=deltaV/R and a power signal can then be determined with the equation P=V*I. However, it is understood different configurations of the current sensor <b>50</b> can be employed such as integrated current monitoring solutions and comparitors.

[0027] Typically, the monitored voltage from the current sensor <b>50</b> is amplified to broaden the voltage change over the available temperature range to achieve a sufficient resolution and thereby increase accuracy.

[0028] The processor <b>80</b> is operably connected to the current sensor <b>50</b>. In one configuration, the current sensor <b>50</b> provides a signal, which is analog and is passed through an analog-to-digital converter (ADC) <b>82</b> for input to the processor <b>80</b>. The output of the processor <b>80</b>, such as the frequency compensating signal, passes through a digital-to-analog converter (DAC) <b>84</b> for communication to a frequency compensating input <b>32</b> of the frequency compensator <b>30</b>. Also, available integrators include multipliers in conjunction with summers, which provide an analog processor solution.

[0029] The processor <b>80</b> is an on-board processor that is integrally retained and dedicated to the specific OCXO <b>10</b>, and hence crystal oscillator <b>20</b>. The memory <b>100</b> is connected to the processor <b>80</b>. However, it is understood the processor <b>80</b> can include integrated memory. The memory <b>100</b> can be a non-volatile EEPROM for data storage.

[0030] The processor <b>80</b> includes, or is operably connected to the memory <b>100</b> for storing and retrieving data. Also, as well known in the art, a reference clock <b>90</b> is operably connected to the processor <b>80</b>.

[0031] The processor <b>80</b> includes an interface <b>85</b> operably connected to the I/O interface <b>120</b> for toggling the processor between a read mode during the calibration process and an operating mode.

[0032] In one configuration the OCXO <b>10</b> employs a calibration run for each unit, or crystal oscillator <b>20</b>. In the calibration run, the OCXO <b>10</b> is cycled through an entire specified temperature range. During the calibration process, the OCXO <b>10</b> is located in a temperature chamber and allowed to achieve thermal equilibrium. At this temperature, the oven control circuit <b>40</b> causes a power flow to the heater <b>60</b> to maintain the crystal oscillator at the desired temperature. At this temperature, the output frequency deviates from nominal. A required compensating signal is determined to accommodate for the deviation from nominal and the required amount of correction. Thus, data associating a given power consumption of the heater to a required compensating signal is obtained.

[0033] During the temperature cycle, current consumption measurements from the current sensor <b>50</b> are taken along with a corresponding voltage level (frequency compensating signal) required by the frequency compensator <b>30</b> to bring the crystal oscillator <b>20</b> frequency back to a nominal value. It is understood that any of a variety of parameters corresponding to the current consumption of the heater <b>60</b> can be measured. Further, the variable relating to the frequency of the crystal oscillator <b>20</b> can be any of a variety of parameters, including, but not limited to the crystal oscillator frequency, or the required frequency compensating signal, which in turn can be a required voltage for the frequency compensator <b>30</b>.

[0034] Thus, during the calibration run, once a thermal equilibrium has been attained in the temperature chamber, the voltage corresponding to the current consumption of the heater <b>60</b> is taken. The frequency of the crystal oscillator <b>20</b> is measured. The frequency compensator <b>30</b> is adjusted to identify the required frequency compensating signal to bring the output frequency of the crystal oscillator <b>20</b> to the nominal value.

[0035] It is understood that during the calibration run, these data points, of current consumption versus required frequency compensating signal, can be taken periodically throughout the temperature slew. Further, the data points during the calibration run may be acquired during either or both the temperature up cycle and the temperature down cycle of the temperature slew. For an OCXO <b>10</b> having a calibration run that includes both the up cycle and the down cycle, the effect of thermal hysteresis can be addressed by determining a correspondence that splits the difference between the up cycle and the down cycle values. Alternatively, the processor <b>80</b> can be configured to employ a separate relationship for the up cycle and the down cycle, wherein during operation of the OCXO <b>10</b>, the processor identifies the existing temperature slew as either an up cycle or down cycle and applies the appropriate relationship to calculate the required frequency compensating signal. In other words, the method contemplates utilizing the defining of two independent relationships corresponding to positive and negatively changing temperature slews.

[0036] During the read mode in the calibration process, the processor <b>80</b> and memory <b>100</b> acquire the corresponding current consumption measurements and required frequency compensating signals (voltages).

[0037] From the data acquired during the calibration run, the relationship between the measured current consumption of the heater <b>60</b> and the required frequency compensating signal (voltage) is calculated by any of a variety of mathematical algorithms such as the best fit method or least squares, and the relationship is determined. The coefficients generated by the curve-fitting algorithm are then stored in the memory <b>100</b> for subsequent use by the processor <b>80</b> during operation of the OCXO <b>10</b>. However, predetermined or determined lookup tables can also be stored into memory to be use for correction.

[0038] That is, the calibration run provides a relationship between a parameter corresponding to the current consumption of the heater <b>60</b>, and the necessary frequency compensating signal (corresponding to frequency deviation from nominal crystal oscillator frequency, and required voltage of the frequency compensator <b>30</b>) over the intended operating temperature range.

[0039] In an alternative configuration, crystal oscillator frequency v. current sensor voltage measurements for a given crystal oscillator <b>20</b> can be accumulated to then calculate a crystal oscillator frequency v. current sensor voltage relationship, wherein the relationship can be calculated by fitting to a function, such as by using the least squares method. Thereby, the frequency deviation of the crystal oscillator <b>20</b> can be predicted using a given current sensor voltage and the derived frequency v. current sensor voltage algorithm. The processor <b>80</b> then calculates a frequency compensating signal corresponding to the calculated frequency deviation. That is, upon monitoring a given current consumption, the processor <b>80</b> performs the mathematical algorithm to determine a frequency deviation and then calculate the necessary frequency compensating signal.

[0040] During operation, the processor <b>80</b> monitors the current consumption of the heater <b>60</b>, via the sensor <b>50</b> and performs the embedded calculation to generate a frequency-compensating signal. It is understood that the processor <b>80</b> can continually or periodically monitor the signal of the current sensor <b>50</b> and generate a corresponding “continuous” or periodic frequency compensating signal.

[0041] Thus, the present OCXO <b>10</b> provides on board monitoring and compensation of crystal oscillator frequency, without requiring external input during operation. That is, the OCXO <b>10</b> self compensates for frequency deviations caused by ambient temperature variations independent of external data input or reliance. As the processor <b>80</b> and determined relationship are carried with the OCXO <b>10</b>, the present correction of frequency error due to temperature influences on the crystal is provided for “on board.”

[0042] It has been found that the present processor compensated OCXO <b>10</b> can achieve double oven crystal oscillator temperature stability without double oven labor costs. Thus, the present OCXO <b>10</b> employs less components and less human intervention and fabrication as well as a decreased set up time.

[0043] While the invention has been described in connection with a presently preferred embodiment thereof, those skilled in the art will recognize that many modifications and changes may be made therein without departing from the true spirit and scope of the invention, which accordingly, is intended to be defined solely by the appended claims.