Patent Publication Number: US-2007096839-A1

Title: Temperature compensation circuit for a surface acoustic wave oscillator

Description:
FIELD OF THE INVENTION  
      This invention relates to surface acoustic wave oscillators and, in particular, to temperature compensated high-frequency surface acoustic wave frequency oscillators.  
     DESCRIPTION OF THE RELATED ART  
      High capacity data networks rely on signal repeaters and sensitive receivers for low-error data transmission. To decode and/or cleanly re-transmit a serial data signal, such network components include components for creating a data timing signal having the same phase and frequency as the data signal. This step of creating a timing signal has been labeled “clock recovery.” 
      Data clock recovery requires a relatively high purity reference signal to serve as a starting point for matching the serial data signal clock rate and also requires circuitry for frequency adjustment. The type, cost and quality of the technology employed to generate the high purity reference signal vary according to the class of data network application. For fixed large-scale installations, an “atomic” clock may serve as the ultimate source of the reference signal. For remote or movable systems, components including specially configured quartz resonators have been used. As communication network technology progresses towards providing higher bandwidth interconnections to local area networks and computer workstations, the need has grown for smaller, higher frequency, and less-expensive clock recovery technology solutions.  
      For higher frequency applications now in demand, e.g., above 500 MHz, more conventional resonator technologies such as standard AT-cut quartz crystals have not been fully successful. The recognized upper limit for fundamental-mode, straight blank AT-cut crystals is about 70 MHz and the upper limit for mesa crystal technologies is about 200 MHz. In order to utilize crystals in these higher frequency applications, PLL circuits or analog multipliers are used to increase the base crystal frequency by factors of 2×, 4×, 8×, etc. While crystal-based oscillators provide good frequency versus temperature characteristics, the multiplication of the crystal frequency creates adverse sub-harmonics and degrades phase noise performance.  
      Another solution, in recent years, for frequencies above 500 MHz has been the implementation of surface acoustic wave (SAW) oscillators. While this technology requires no multiplication and hence does not result in any sub-harmonics or degradation in phase noise, the frequency versus temperature performance thereof is on the order of 2× to 10× worse than its uncompensated crystal counterpart.  
      Therefore, a method to temperature compensate a SAW-based oscillator would provide the benefits of the absence of sub-harmonics and phase noise degradation with the added benefit of good frequency versus temperature response.  
      A generalized topology to temperature compensate a SAW oscillator is shown in U.S. Pat. No. 4,011,526 to Kinsman which discloses the use of a temperature sensitive voltage source Vs with a SAW oscillator. Unfortunately, the device of Kinsman is not adapted to be efficiently manufactured in significant quantities because the mass production of temperature compensated SAW oscillators requires a circuit topology and method of temperature compensation which accounts for component tolerance variances between individual SAW resonators as well as other circuit components. There thus remains a need for a SAW oscillator which addresses these shortcomings.  
     SUMMARY OF THE INVENTION  
      It is thus a feature of the invention to provide a temperature compensation circuit for a surface acoustic wave oscillator that includes a temperature sensor that is structured to generate a temperature sensor signal and is coupled to a temperature signal conditioner. The temperature signal conditioner receives the temperature sensor signal and generates a conditioned temperature sensor signal. A reactance generator is coupled to the temperature signal conditioner. The reactance generator receives the conditioned temperature sensor signal and generates a compensation signal. A resonator device such as, for example, a surface acoustic wave device is coupled to the reactance generator. An oscillator circuit is coupled to the surface acoustic wave device. The oscillator produces a stable output frequency over a temperature range.  
      There are other advantages and features that will be more readily apparent from the following description of the invention, the drawings, and the appended exemplary claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other features of the invention can best be understood by the following description of the accompanying drawings as follows:  
       FIG. 1  is a schematic diagram of a surface acoustic wave oscillator with a temperature compensation circuit in accordance with the present invention;  
       FIG. 2  is a graph of frequency change versus temperature of an uncompensated SAW oscillator;  
       FIG. 3  is a graph of capacitance versus temperature for the varactor configuration of the oscillator shown in  FIG. 1 ;  
       FIG. 4  is a graph of the voltage versus temperature response of a transistor configured as a forward biased diode;  
       FIG. 5  is a schematic diagram of an alternate embodiment of a reactance generator for the oscillator of the present invention;  
       FIG. 6  is a graph of the resultant varactor capacitance versus applied voltage at different Voffset voltages;  
       FIG. 7   a  is a graph of the voltage versus temperature response of the temperature sensor of the present invention;  
       FIG. 7   b  is a graph of the voltage versus temperature response of the temperature sensor signal conditioning circuit of the oscillator of the present invention;  
       FIG. 7   c  is a graph of the capacitance versus temperature response of the reactance generator of the oscillator of the present invention; and  
       FIG. 7   d  illustrates the resulting frequency versus temperature response of the oscillator of the present invention. 
    
    
      It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. The invention will be described with additional specificity and detail through the accompanying drawings.  
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Oscillator Circuit  
       FIG. 1  is a schematic diagram of a surface acoustic wave oscillator  10  with a temperature compensation circuit in accordance with the present invention. Oscillator  10  includes a temperature sensor  20 , a temperature signal conditioner  30 , a reactance generator  40 , an oscillator circuit  50 , and a surface acoustic wave resonator (SAW)  60 .  
      Temperature sensor  20  can comprise a transistor Q 1  having a base Q 1 B, an emitter Q 1 E and a collector Q 1 C. The base Q 1 B is connected to collector Q 1 C. Collector Q 1 C is connected to a DC power source Vcc through a resistor R 1 . Power source Vcc is preferably set above Voffset and can be approximately 5 volts. Emitter Q 1 E is connected to node N 1 . Node N 1  is connected to resistor R 2 . The other end of resistor R 2  is connected to ground G.  
      Transistor Q 1  is adapted to change output voltage in response to a change in temperature. The voltage developed at node N 1  therefore is proportional to the temperature that transistor Q 1  is subjected to. The voltage at node N 1  can be called a temperature sensor signal. Preferably, transistor Q 1  is mounted in an electronic package close to SAW resonator  60 . In this manner, the temperature of transistor Q 1  closely tracks the temperature of SAW resonator  60 .  
      A temperature signal conditioner  30  comprises a differential amplifier U 1  having a pair of input terminals A and B, an output terminal C, a power supply terminal D and a ground terminal E. Node N 1  is connected to Node N 2  through a resistor R 3 . Input terminal A is connected to node N 2 . Input terminal B is connected to node N 4 . Node N 4  is connected to resistor R 6   a  and variable resistor R 6 . Resistor R 6  is further connected to ground. Node N 4   a  is connected to a variable resistor R 5  and resistor R 6   a . Resistor R 5  is further connected to Vcc. Node N 4   a  is further connected to Voffset.  
      Power supply terminal D is connected to power supply Vcc and ground terminal E is connected to ground G. Variable resistor R 4  has one end connected to node N 2  and the other end connected to node N 5 . Output terminal C is connected to node N 5 . Resistor R 7  is connected between node N 5  and ground. Capacitor C 7  is connected between node N 3  and ground G. Resistor R 8  is connected to node N 3 . Node N 3  is connected to node N 5 . Temperature signal conditioner  30  receives the temperature sensor signal from temperature sensor  20  at node N 1  and generates a conditioned temperature sensor signal at node N 6 . The conditioned temperature sensor signal provides the correct gain and offset voltages to be supplied to reactance generator  40 .  
      Reactance generator  40  comprises a pair of varactors V 1  and V 2 . Varactor V 1  has an anode V 1 A and a cathode V 1 C. Varactor V 2  has an anode V 2 A and a cathode V 2 C. Cathode V 2 C and anode V 1 A are connected to each other at node N 6 . Anode V 2 A is connected to ground G. Cathode V 1 C is connected to node N 7 .  
      Reactance generator  40  receives the conditioned temperature sensor signal at node N 6  and generates an oscillator compensation voltage at node N 7 .  
      Oscillator  50  is arranged in a Colpitts configuration. Oscillator  50  includes a transistor Q 2  having a base Q 2 B, an emitter Q 2 E and a collector Q 2 C. The base Q 2 B is connected to node N 8 . Collector Q 1 C is connected to node N 11 . Inductor L 3  is connected between node N 11  and power source Vcc. Node N 11  is further connected to oscillator output terminal  70 . Emitter Q 2 E is connected to node N 10 . Resistor R 9  is connected between node N 10  and ground G.  
      A capacitor C 3  is connected between node N 8  and node N 9 . Capacitor C 4  is connected between node N 9  and ground G. Node N 9  is connected to node N 10 . Capacitor C 5  has one end connected to node N 8  and the other end connected to SAW resonator  60 . Inductor L 2  has one end connected to node N 7  and the other end connected to SAW resonator  60 .  
      SAW resonator  60  can be a surface wave acoustic resonator that is commercially available from TAI-SAW Corporation of Taiwan. SAW resonator  60  resonates at a nominal frequency of 600 MHz. RF choke inductor L 1  is connected between node N 7  and Voffset. Capacitor C 6  is connected between power supply Vcc and ground G.  
      Oscillator Operation  
      Individual SAW resonators  60  have variances in their resonant frequency, turning points, and frequency versus temperature response due to manufacturing variations between individual resonators. The components of oscillator  50  and temperature sensor  20  also have variances in their circuit values due to manufacturing variations between individual components.  
      Signal conditioning circuit  30  matches the response of an individual temperature sensor  20  to an individual saw resonator  60 . In order to accurately compensate each individual resonator  60 , a compensation technique/circuit must be flexible enough to account for the component tolerances. Signal conditioning circuit  30  is programmable or adjustable in order to match the response of individual SAW resonators  60  (regardless of frequency or manufacturer) to its respective temperature sensor  20 . Therefore, each oscillator  10  can be provided a unique temperature compensation solution that results in lower frequency drift with temperature.  
      Referring to  FIG. 2 , the frequency change versus temperature response of an uncompensated SAW oscillator can be modeled with the mathematical form of a parabola: 
 
 f ( T )= AT   2   +BT+C   (Equation 1) 
 
 where T is temperature, f is frequency, and A, B, and C are constants which adjust the shape of the parabolic response.  FIG. 2  shows the frequency versus temperature response of a SAW-based 622.08 MHz voltage controlled SAW oscillator by CTS Corporation having a part number VCS1001A along with its best fit to a parabola. 
 
      In order to compensate for the inherent frequency versus temperature SAW resonator response, reactance generator  40 , coupled via signal conditioner  30  to temperature sensor  20 , is used to produce a capacitance versus temperature response of the form: 
 
 C ( T ) XT   2   +YT+Z   (Equation 2) 
 
 where C is capacitance, T is temperature, and constants X, Y, and Z adjust the shape of the parabolic response.  FIG. 3  shows this capacitance versus frequency response utilizing SMV1251 varactors. 
 
      The compensation technique of this invention can be described by Equation 3 below where decreasing capacitance (C load ↓) corresponds to an increase in frequency (freq ↑) and increasing capacitance (C load ↑) corresponds to a decrease in frequency (freq ↓):  
             Frequency   =     1     2   ⁢   π   ⁢     LC                 (     Equation   ⁢           ⁢   3     )             
 
      In Equation 3 above, capacitance C is the reactance generator and hence a component of the load that the SAW resonator  60  is subjected to.  
      Referring to  FIG. 2  and utilizing Equation 3, it can be seen that at the respective extreme cold and hot temperatures, the capacitive loading (CL) seen by the SAW resonator would need to be lower than the capacitance seen at the nominal frequency referenced to 32 degrees Centigrade. Therefore, the capacitance versus temperature response of a properly temperature compensated SAW oscillator would require the form shown in Equation 2. The present invention provides a circuit and method to create the reactance response described by Equation 3.  
      The graph of  FIG. 2  was produced from actual circuit measurements using an HP model 53132 frequency counter.  
      Temperature compensation circuit  30  of the present invention can provide frequency versus temperature performance that can be +/−5 ppm or better over a temperature range of −40 to +85 degrees Centigrade.  
      Temperature sensor  20  creates a voltage versus temperature response that is similar to the mathematical form of a line: 
 
 V ( T )=+/− M *( T )+ B   (Equation 4) 
 
 where V is voltage, T is temperature, M is the slope of the line, and B is the voltage offset. Temperature sensor  20  should be located as close as possible to SAW resonator  60  in order to capture the true thermal gradient of SAW resonator  60 . 
 
      A forward biased transistor Q 1  is used in temperature sensor  20 . Other temperature sensors can be used such as thermistors or IC temperature sensors provided the response described in Equation 4 is adequate.  
       FIG. 4  illustrates the voltage versus temperature response of a single forward biased diode. The diode can be a model number MMBT3904 BJT made by ON Semiconductor of Phoenix, Ariz.  
      With reference to  FIG. 1 , signal conditioning circuit  30  receives the temperature sensor signal from temperature sensor  20  at node N 1  and adjusts the slope and/or the offset of the temperature sensor signal before sending or applying a conditioned temperature sensor signal to the reactance generator  40  at node N 6 .  
      Differential amplifier U 1  can be a model number LM324 amplifier made by Texas Instruments of Dallas, Tex. Variable resistors R 4  and R 5  preferably are digitally adjusted potentiometers that are commercially available from Xicor Corporation and adapted to control the gain, slope, and offset of the temperature sensor signal received from temperature sensor  20 . Resistors R 4  and R 5  controlling the gain, slope, and offset are adjusted by another computer (not shown), while monitoring the output frequency. In this manner, the optimum compensated temperature signal is derived. Once the optimum values for resistors R 4  and R 5  are found, the computer either permanently sets resistors R 4  and R 5  to those values or records the optimum discrete resistor value to be placed in component positions R 4  and R 5 .  
      Reactance generator circuit  40  comprises two identical type varactors V 1  and V 2  having V 1 A&#39;s anode connected to V 2 C&#39;s cathode with the conditioned temperature sensor signal being applied to node N 6 .  
       FIG. 6  depicts a sweep of the conditioned temperature sensor signal voltage from 0 to “Voffset” volts across varactors V 1  and V 2  with respective Voffset potentials of 1.5V, 2.0V, and 3.0V. In other words,  FIG. 6  shows the output of reactance generator circuit  40 .  
      It is noted that resistor R 5  of the signal conditioning circuit  30 , in conjunction with the reactance generator  40 , has the primary function of adjusting the Voffset voltage seen by the reactance generator. The Voffset voltage selects capacitance offset seen in the oscillator loop  50 . Resistor R 4  of the signal conditioning circuit  30  has the primary function of selecting the range of the parabolic capacitance change required, over the temperature range of interest.  
      For a more compact package and to reduce cost, the temperature sensor  20 , the signal conditioning circuit  30 , the varactors  40  and oscillator  50  could be integrated into a single integrated circuit (IC).  
       FIGS. 7   a - 7   d  illustrate the temperature compensation of oscillator  10 .  FIG. 7   a  shows a graph of the voltage versus temperature response that is generated by temperature sensor  20 , i.e., the temperature sensor signal which is provided to temperature signal conditioner  30 . The temperature sensor signal is applied to the input of the signal conditioning circuit  30  at node N 2 , where R 4  and R 5  are adjusted to “condition” the input signals. Based on the best solution for R 4  and R 5 , a conditioned temperature sensor signal is generated. This signal is shown in  FIG. 7   b  which shows a graph of voltage versus frequency at node N 6 , i.e., the output of signal conditioning circuit  30 . The conditioned temperature sensor signal is provided or applied to the reactance generator  40  at node N 6 .  
      Reactance generator  40  generates a compensation signal which is shown in  FIG. 7   c .  FIG. 7   c  shows how the varactor capacitance versus temperature response of the present invention as applied at node N 7  with the use of Equation 3, can be used to temperature compensate SAW resonator  60 . The compensation signal is provided to SAW resonator  60  at node N 7 .  
      Osillator circuit  50  generates an output frequency that is stabilized around a nominal frequency at which SAW resonator  60  resonates.  FIG. 7   d  illustrates the resulting oscillator output frequency versus temperature response of oscillator  10  showing no frequency drift with temperature.  
       FIG. 5  depicts an alternative reactance generator configuration. Reactance generator  240  is similar to reactance generator  40  except that a series combination of an inductor L 244  and capacitor C 242  have been connected in parallel across the series connected combination of varactors V 1  and V 2 . Capacitor C 242  can have a value of 0.01 microfarads. Capacitor C 242  is connected to cathode V 1 C at node N 7 . Inductor L 244  is connected to anode V 2 A at node N 146 .  
      The addition of inductor L 224  in parallel with the varactors provides for the adjustment of the apparent overall inductance of inductor L 224  rather than simply the varactor capacitance of reactance generator  240 .  
      The present invention provides an improvement over previous temperature compensated ocillators. The present invention allows the frequency versus temperature sensitivity of a SAW oscillator to be reduced significantly and provides a method to set the oscillator onto its desired frequency.  
      The use of signal conditioning circuit  30  and reactance generator  40  provides the ability to temperature compensate any SAW oscillator. The ability to select the voltage versus temperature function applied to the varactors provides the ability to compensate each individual SAW to its respective tolerance and oscillator tolerance.  
      The oscillator shown in the present specification is of a Colpitts configuration. However, this is not a requirement of the invention, and the oscillator may be of other oscillator configurations including, but not limited to Clapp, Driscoll, Butler, Pierce, and Hartley oscillator configurations.  
      The oscillator shown in the present specification utilizes a SAW resonator. However, it is contemplated that any other type of resonator could be used including, but not limited to, quartz crystals, FBAR, lithium niobate, lead zirconium titanates and other piezoelectric materials.  
      Oscillator assembly  10  would be packaged and assembled using conventional electronic manufacturing techniques.  
      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.