Abstract:
An oscillator having improved frequency stability which includes an oscillator circuit and an SC-cut resonator connected with the oscillator circuit. The SC-cut resonator has a first turning point. A temperature compensation circuit is connected with the oscillator circuit. The temperature compensation circuit is adapted to adjust a reference frequency generated by the oscillator circuit according to a frequency response associated with a second turning point of an AT-cut resonator.

Description:
TECHNICAL FIELD 
   This invention relates to oscillators that provide a stable reference frequency signal in electronic equipment and, more specifically, to a temperature compensated crystal oscillator that is contained within an ovenized enclosure and that compensates only a portion of a Bechmann curve of frequency change with temperature. 
   BACKGROUND OF THE INVENTION 
   Various devices are well known for providing a reference frequency or source. Such devices are called oscillators and typically incorporate a quartz crystal or other type of resonator and electronic compensation circuitry to stabilize the output frequency. 
   Various methods are known to stabilize the output frequency as the temperature of the oscillator changes. Temperature compensated crystal oscillators (TCXOs) typically employ a thermistor network to generate a correction voltage which reduces the frequency variation over temperature. The correction voltage is usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a small amount. TCXO stability can approach 0.1 PPM but several problems must be addressed. 
   A TCXO that resides at one temperature extreme for an extended period of time may exhibit a frequency shift when returned to normal room temperature. Usually this hysteresis or “retrace” error is temporary but a seemingly permanent offset is common. Retrace errors are usually less than about 0.1 PPM but can be much higher, especially if the mechanical tuning device (trimmer capacitor or potentiometer) is shifting. This hysteresis makes the manufacture of TCXOs with specifications approaching 0.1 PPM quite difficult. The high precision crystals found in oven oscillators exhibit less retrace but they are unsuitable for use in TCXOs because they often exhibit activity dips at temperatures below the designed oven temperature. 
   Further SC-cut and high overtone type crystals cannot be tuned by a sufficient amount to compensate for the frequency excursion with temperature. In addition, SC-cut crystals are very expensive. 
   TCXOs are preferred to oven oscillators in low power applications and where a warm-up period is not acceptable. The only warm-up time is the time required for the components to reach thermal equilibrium and the total current consumption can be very low—often determined by the output signal power requirements. Older TCXO designs employ from one to three thermistors to flatten the crystal temperature frequency curve. Newer designs employ digital logic or a microprocessor to derive a correaction voltage from values or coefficients stored in memory. 
   Ovenized oscillators heat the temperature-sensitive portions of the oscillator which are isolated from the ambient to a uniform temperature to obtain a more stable output. Ovenized oscillators contain a heater, a temperature sensor and circuitry to control the heater. The temperature control circuitry holds the crystal and critical circuitry at a precise, constant temperature. The best controllers are proportional, i.e., providing a steady heating current which changes with the ambient temperature to hold the oven at a precise set-point, usually about 10 degrees above the highest expected ambient temperature. 
   Temperature-induced frequency variations can be greatly reduced by an amount approaching the thermal gain of the oven. The crystal for the oven is selected to have a “turning-point” at or near the oven temperature, further reducing the sensitivity to temperature. The combination of the high oven gain with operation near the turning point yields temperature stabilities approaching 0.0001 PPM over a temperature range that would cause the crystal to change by 10 PPM. 
   Highly polished, high-Q crystals which often have significant activity dips may be designed with no activity dips near the operating temperature, providing better stability and phase noise than crystals designed for wide temperature ranges. Ovenized oscillators allow the use of SC-cut crystals which offer superior characteristics but which are impractical for ordinary TCXOs because of their steep frequency drop at cooler temperatures. Unfortunately, SC-cut crystals are much more expensive to produce than AT-cut crystals typically used in TCXOs. 
   Oven oscillators have a higher power consumption than temperature compensated oscillators. Oven oscillators require a few minutes to warm up, and the power consumption is typically one or two watts at room temperature. SC-cut crystals stabilize as soon as they reach the operating temperature, but AT-cut crystals exhibit a significant thermal transient effect, which can take many minutes to settle. A typical AT-cut crystal will drop in frequency well below any point on the static crystal curve due to the sudden application of oven heat. In most oscillators, the frequency will exponentially drift back up to the nominal frequency. 
   In some designs, the oven controller overshoots significantly during initial warm-up and then cools back down to the final oven temperature. This cooling transient can kick the AT-cut crystal in the other direction and may actually result in a shorter warm-up time even though the controller takes longer to settle. Hand-tweaked designs can achieve fairly acceptable warm-up times with carefully selected overshoot, but the advent of quick-settling SC-cut crystals has made this approach obsolete. 
   Despite the advantages of prior art oscillators, an unmet need exists for an oscillator that has improved frequency stability over temperature and time and that can be manufactured at a reasonable cost. 
   SUMMARY OF THE INVENTION 
   It is a feature of the invention to provide an oscillator that includes an oscillator circuit and a resonator connected with the oscillator circuit. The resonator has a first turning point. A temperature compensation circuit is connected with the oscillator circuit. The temperature compensation circuit is adapted to adjust a reference frequency generated by the oscillator circuit according to a frequency response associated with a second turning point. 
   The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Further, the abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     These and other features of the invention can best be understood by the description of the accompanying drawings as follows: 
       FIG. 1  is a diagrammatic view of a temperature compensated crystal oscillator (TCXO) located inside a temperature controlled oven in accordance with the present invention. 
       FIG. 2  is a schematic view of one embodiment of a TCXO in accordance with the present invention; 
       FIG. 3  is a schematic view of another embodiment of a TCXO in accordance with the present invention; 
       FIG. 4  is a schematic view of an additional embodiment of a TCXO in accordance with the present invention; 
       FIG. 5  is a schematic view of the integrated circuit of  FIGS. 2 and 3 ; 
       FIG. 6  is a schematic view of a Colpitts oscillator circuit; 
       FIG. 7  is a graph of frequency change versus temperature for several SC-cut crystals at different cut angles; 
       FIG. 8  is a graph of frequency change versus temperature for several AT-cut crystals at different cut angles; and 
       FIG. 9  is a flowchart of a method of operating an oscillator in accordance with the present invention. 
   

   It is noted that the drawings of the invention are not to scale. The invention will be described with additional specificity and detail through the accompanying drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a diagrammatic view of a temperature compensated crystal oscillator (TCXO) located inside a temperature controlled oven is shown. Oscillator assembly  10  includes an oven  12  which contains the oscillator components. Oven  12  can include a metal housing  14  with a cavity  15  that contains foam insulation  24 . A temperature compensated crystal oscillator (TCXO)  50  is located inside oven  12 . TCXO  50  can be any type of oscillator that uses any type of resonator. For example, TCXO  50  can be a Colpitts oscillator using an AT-cut quartz crystal resonator. TCXO  50  generates and provides a stable reference frequency at output terminal  16 . 
   A heater  18  is located in oven  12 . Heater  18  is typically a transistor in which the dissipated power is proportionally controlled to heat and maintain a constant temperature inside oven  12 . A temperature sensor  22  is located inside housing  14 . Sensor  22  is a negative-coefficient conventional thermistor. The temperature sensor monitors the temperature of TCXO  50 . 
   Connected to sensor  22  and heater  18  is a control circuit  20  which controls heater  18 . Control circuit  20  receives a temperature signal as an input from sensor  22  and provides a heater control signal as an output to heater  18 . When the temperature falls below the selected setpoint for the oven, control circuit  20  increases power to heater  18  to increase the temperature in oven  12 . When the temperature is above the setpoint for the oven, control circuit  20  reduces power to heater  18  to allow a decrease in the temperature in oven  12 . Power is applied to the oscillator assembly through terminal  26 . Terminal  26  is connected with TCXO  50  and the heater control circuit  20 . 
   Oven  12  can be a single oven or a double oven. Oven  12  may include an evacuated region in order to improve the temperature performance of the oscillator. 
   Oscillator assembly  10  can be operated where the operation of TCXO  50  is optimized for temperatures around the setpoint of the oven and has a frequency stability of about 20 PPB. Oven  12  will consume approximately 1 watt of power during operation. 
   Oscillator Circuit 
   Referring to  FIG. 2 , a schematic diagram of an embodiment of a temperature compensated crystal oscillator (TCXO)  100  in accordance with the present invention is shown. TCXO  100  can replace TCXO  50  of  FIG. 1 . TCXO  100  is adapted to be mounted in oven  12 . TCXO  100  includes a temperature compensation integrated circuit IC 1 . Integrated circuit IC 1  can be designed for use with oscillators including, for example, part number MAS9279 integrated circuit that is commercially available from Micro Analog Systems Oy of Espoo, Finland. Integrated circuit IC 1  is optimized for use with an AT-cut quartz crystal. Integrated circuit IC 1  is produced in large quantities and therefore can be purchased at a reasonable cost. Integrated circuit IC 1  can contain an oscillator circuit  410  and a temperature compensation circuit  420 . Integrated circuit IC 1  can operate as a TCXO with only the addition of a resonator or crystal. 
   Integrated circuit IC 1  includes terminals Vdd, Out, X 1 , X 2 , Vss, and TE 1 . Terminal Vdd is connected with a 3.3 volt power source and terminal Vss is connected to ground. Crystal terminals X 1  and X 2  are connected across the resonator  102 , which typically is a quartz crystal. Resonator  102  preferably is an SC-cut quartz crystal. Resonator  102  could also be a mesa-type crystal or other bulk resonator such as lithium niobate. Terminal TE 1  is the temperature input/output terminal. Terminal Out is the reference frequency output terminal. 
   An external temperature sensor  104  is connected with terminal TE 1 . A temperature sensor (not shown) is also located within integrated circuit IC 1 , but is not used in this embodiment. Temperature sensor  104  is mounted in close proximity to resonator  102 . Temperature sensor  104  includes a pair of serial connected resistors R 1  and R 2  that are connected at node N 1 . Resistor R 2  is a thermistor that changes resistance with temperature. Resistor R 1  is a fixed resistor. One end or resistor R 2  is connected to node N 1  and the other end is connected to ground. One end of resistor R 1  is connected to power source Vdd and the other end is connected to node N 1 . Node N 1  is connected to terminal TE 1 . 
   During the operation of the oscillator with TCXO  100 , oven  12  is maintained at a substantially constant temperature by sensor  22 , heater  18  and control circuit  20 . Integrated circuit IC 1  contains oscillator circuit  410  that produces the reference frequency that is stabilized by resonator  102 . Temperature sensor  104  generates a temperature signal that is proportional to the temperature to which resonator  102  is exposed. The output voltage of temperature sensor  104  can be adjusted to a desired voltage range by the selection of appropriate values for resistors R 1  and R 2  and voltage Vdd. 
   Integrated circuit IC 1  contains a temperature compensation circuit  420  that uses the temperature signal to adjust the reference frequency that is produced at terminal Out. The temperature compensation circuit maintains the reference frequency within a determined tolerance. 
   Turning now to  FIGS. 7 and 8 , a graph of frequency change versus temperature, Bechmann curve for several SC-cut crystals at different cut angles is shown in  FIG. 7  and a Bechmann curve for several AT-cut crystals at different cut angles is shown in  FIG. 8 .  FIG. 7  shows that the rate of change of frequency with temperature for an SC-cut crystal is minimized by operating around the maximum  702  or minimum  704  values of the sinusoidal curve. This is called the turning point of the crystal.  FIG. 7  also has an inflection point  706 . In other words, the frequency response line has a small slope. For an SC-cut crystal, the preferred operating or oven set-point temperature value typically is around 85 degrees Centigrade. 
   Integrated circuit IC 1  is designed to compensate the frequency change versus temperature curve for an AT-cut crystal shown in  FIG. 8 .  FIG. 8  has turning points  802  and  804  and an inflection point  806 . It is noted that if an SC-cut crystal is operated at the SC-cut turning point of 85 degrees Centigrade in  FIG. 8 , there is a change in frequency with a change in temperature. In other words, the frequency response line has a large or steep slope. 
   In order to use the integrated circuit IC 1  that was designed for use with both an AT-cut crystal and an SC-cut crystal and obtain good frequency response over temperature, the integrated circuit must be compensated, tricked or provided with the illusion that it is operating at another temperature, when in reality it is not. 
   Integrated circuit IC 1  is adjusted to operate around the 60 degree point in  FIG. 8  by the selection of resistors R 1  and R 2  in  FIG. 2  such that the voltage generated at node N 1  or terminal TE 1  is approximately 1.15 volts as shown in  FIG. 8 . This voltage would typically be about 1.0 volts for use with an AT-cut crystal as shown by the value of 1.0 volts located at the tuning point in  FIG. 8 . 
   The present invention provides the illusion to integrated circuit IC 1  that the resonator  102  is operating at the turning point  804  of an AT cut quartz crystal when in reality the resonator  102  is operating at the turning point  702  of an SC cut quartz crystal. 
   This allows the use of an integrated circuit designed for an AT-cut crystal to be used with an SC-cut crystal. There are many advantages to this design. It avoids the need to design a new integrated circuit specifically for use with an SC-cut crystal and is lower in cost, since AT-cut crystals are used in higher volume applications than SC-cut crystals. 
   First Alternative Oscillator Circuit 
   Referring to  FIG. 3 , a schematic diagram of another embodiment of a temperature compensated crystal oscillator (TCXO)  200  is shown. TCXO  200  can replace or be used for TCXO  50  of  FIG. 1 . 
   TCXO  200  is similar to TCXO  100  except that a separate external oscillator circuit  210 , low pass filter  220  and varactor diode  230  have been added. In  FIG. 4 , oscillator circuit  410 , internal to integrated circuit IC 1 , is not used. 
   Oscillator circuit  210  is connected with crystal terminals X 1  and X 2 . In  FIG. 3 , the internal oscillator circuit of integrated circuit IC 1  is not used and is bypassed by the use of oscillator circuit  210 . Oscillator circuit  210  can be a conventional oscillator circuit such as a Pierce or Colpitts oscillator circuit as will be discussed later in  FIG. 6 . Oscillator circuit  210  has terminals  211 ,  212 ,  213 ,  214  and Fo. Terminal  212  is connected to crystal terminal X 1 . Terminals  213  and  214  are connected across resonator  102 . Terminal Fo is the output frequency terminal. 
   A varactor diode  230  is connected between ground and node N 2 . Varactor diode  230  has an adjustable capacitance that can be used to tune the operating frequency of the oscillator. Node N 2  is connected with terminal  211 . A low pass filter  220  is connected between node N 2  and terminal X 2 . Low pass filter  220  delays the correaction voltage supplied to varactor diode  230  to better match the thermal profile of crystal resonator  102 . 
   During the operation of the oscillator with TCXO  200 , oven  12  is maintained at a substantially constant temperature by sensor  22 , heater  18  and control circuit  20 . Oscillator circuit  210  produces the reference frequency that is stabilized by resonator  102 . Oscillator circuit  410  is not used. 
   Temperature sensor  104  generates a temperature signal that is proportional to the temperature to which resonator  102  is exposed. Integrated circuit IC 1  contains the temperature compensation circuit  420  that uses the temperature signal to adjust the reference frequency that is produced at terminal Fo. The temperature compensation circuit  420  maintains the reference frequency within a tight tolerance. Integrated circuit IC 1  operates the same as previously discussed for TCXO  100 . 
   Referring to  FIG. 6 , a Colpitts oscillator circuit  210  is shown. Oscillator circuit  210  includes a transistor Q 1  that has a base Q 1 B, a collector Q 1 C and an emitter Q 1 E. Base Q 1 B is connected to node N 8 . Collector Q 1 C is connected to node N 9  and resistor R 3 , which is connected to power supply Vdd. Node N 9  is further connected to output terminal Fo. Emitter Q 1 E is connected to Node N 11 , which is connected to resistor R 4 . Resistor R 4  is further connected to ground G. Capacitor C 1  is connected between node N 8  and node N 10 . Capacitor C 2  is connected between node N 10  and ground. 
   Resistor R 1  is connected between node N 7  and power supply Vdd. Resistor R 2  is connected between node N 7  and ground. Node N 7  is connected to terminals  212  and  214 . 
   Second Alternative Oscillator Circuit 
   Referring to  FIG. 4 , a schematic diagram of another embodiment of a temperature compensated crystal oscillator (TCXO)  300  is shown. TCXO  300  can replace or be used for TCXO  50  of  FIG. 1 . 
   TCXO  300  is similar to TCXO  200  except that the connection between terminal X 1  and terminal  212  has been eliminated. 
   Integrated Circuit 
   Referring to  FIG. 5 , a block diagram of integrated circuit IC 1  is shown. Integrated circuit IC 1  includes a power supply terminal Vdd that provides power to the chip. Programming input terminal PV is used for programming the temperature compensation registers after they have been calibrated. Clock input terminal CLK is used only during calibration. Data input terminal DA provides digital serial data to the internal registers. 
   Temperature input/output terminal TE 1  can provide an output voltage from the internal temperature sensor T or can accept an externally generated temperature sensitive voltage. In the present invention, the voltage at terminal TE 1  is set such that integrated circuit IC 1  can be used with an SC cut quartz crystal. Test multiplexer output terminal TE 2  is used for testing IC 1 . Voltage control input terminal VC is used to tune the varactor voltage to the respective frequency within the application. Crystal terminals X 1 , and crystal/varactor terminal X 2  can be connected with a resonator. Ground terminal Vss is connected to ground. Buffer output terminal out provides an output frequency. 
   Several internal circuits and registers are contained within integrated circuit IC 1 . Cubic register CUB sets the scaling of the cubic control voltage part of the varactor control voltage. The inflection point register INF sets the inflection point of the cubic control voltage of the varactor control voltage. The sensitivity register SENS sets the scaling of the overall varactor control voltage. The linear compensation register LIN sets the slope of the varactor control voltage. The CDAC 1  fine offset compensation register compensates the crystal offset by changing the load capacitance through variable capacitor CV 1 . The CDAC 2  offset compensation register is used for coarse tuning of the output frequency by changing the load capacitance through variable capacitor CV 2 . 
   The values of the cubic, inflection point, sensitivity and linear registers are summed in the summing register Σ. The output of the summing register is provided to node N 5  as a compensation voltage. Node N 5  is connected to terminal X 2 , the input of buffer B 1  and varactor diode V 1 . Node N 6  is connected to the output of buffer B 1 , variable capacitors CV 1  and CV 2  and the input of buffer B 2 . Terminals X 1  and X 2  can be connected to resonator  102 . 
   Method of Operation 
   Turning now to  FIGS. 1 and 9 , a method of operating an oscillator in accordance with the present invention is shown. Method  500  includes decision step  502 . At decision step  502 , the control circuit  20  checks to see if the TCXO  50  is at the proper temperature. If the oven  12  is not at the correct temperature, method  500  proceeds to step  504  where the heater  18  is turned on or off depending upon the temperature. After the oven reaches the proper temperature, step  504  returns to decision  502  to confirm the proper temperature of the oven. 
   If the oven is at the correct temperature, method  500  proceeds to step  506  where the reference frequency is generated by the TCXO  50 . Next, the generated frequency is compared to the target frequency at decision step  508 . If the generated frequency is equal to the target frequency, the reference frequency is outputted or provided at step  512 . If the generated frequency is not equal to the target frequency, method  500  proceeds to step  510 . At step  510 , the generated frequency is adjusted by TCXO  50 . The resulting reference frequency is outputted or provided at step  512 . 
   While the invention has been taught with specific reference to these embodiments, someone skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.