Abstract:
A FET oscillator with increased frequency stability. This is accomplished by using a controlled voltage supply with error correction to power the amplifier stage of the oscillator. This voltage changes as the oscillator temperature increases in order to reduce the variation in frequency, caused by the amplifier and other frequency determining components changes. By using this compensated amplifier as the active section of an oscillator, the oscillator frequency stability is increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The benefits of filing this invention as Provisional application for patent “SUPPLY VOLTAGE CONTROLLED VOLTAGE AND TEMPERATURE COMPENSATED OSCILLATOR”, U.S. PTO 60/631353 filed Nov. 29, 2004 by Fred Mirow are claimed. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to FET oscillators in which the oscillation frequency is relatively independent of supply voltage and ambient temperature. The term FET is used to refer to CMOS, MOSFET, JFET and other variation of the Field Effect Transistor.  
         [0003]     One of the problems associated with FET oscillators at high frequency is that the oscillation frequency is very sensitive to changes in ambient temperature and power supply voltage. To reduce this instability some form of compensation is necessary. One of the methods used is to use a FET as a resistor to control the charging time of a capacitor. The FET resistance value is controlled by a temperature dependent voltage which varies to maintain a constant capacitor charging time. This is described in U.S. Pat. No. 4,547,749 issued to Clinton Kuo. Another method is to use a constant current source circuit, which is designed to be temperature independent, to charge and discharge a timing capacitor. This is described in U.S. Pat No. 4,714,901 issued to Jain et al.  
         [0004]     In these methods the variation in oscillator frequency has been reduced by controlling the charging time of capacitors, but nothing has been done to correct an other large error source, the high sensitivity of the FET amplifier to temperature and supply voltage change.  
         [0005]     A solution to this was taught in U.S. Pat. No. 5,241,286 issued to Mirow on Aug. 31, 1993. Mirow taught a oscillator in which the frequency stability is increased by reducing the change in the amplifier circuit gain and phase shift due to variations in ambient temperature and power supply voltage. This reduction is accomplished by powering the amplifier from a power supply in which the output voltage level varies with temperature. However Mirow did not take into account non linear frequency shift or show how to reduce errors in the supply voltage verses temperature curve.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of this invention is a FET oscillator in which the frequency stability is increased by reducing the change in the circuit gain and phase shift due to variations in ambient temperature and power supply voltage. In addition, a controlled change in the amplifier circuit gain and phase shift can also be used to further increase frequency stability by canceling the effects due to variations in the feedback network with ambient temperature and power supply voltage. This increased frequency stability is accomplished by powering the amplifier from a voltage supply in which the output voltage level varies with temperature.  
         [0007]     In one embodiment, the voltage supply includes a temperature responsive voltage regulator to maintain the oscillator frequency constant as operating temperature and power supply voltage changes. In another embodiment, the voltage supply includes a look-up table embodying a desired supply voltage versus temperature relationship. In another embodiment, the voltage supply includes a temperature sensitive voltage supply and a additional signal from the error corrector. The error corrector may be a look-up table embodying a desired correction to the supply voltage versus temperature relationship or temperature responsive comparators for supply voltage adjustments at predetermined temperatures.  
         [0008]     In multivibrators type oscillators further improvements in oscillator frequency stability are obtained by limiting the oscillator feedback signal voltage level in response to temperature.  
         [0009]     In addition to using the compensated oscillator to provide a constant frequency the oscillator can be modulated to provide a frequency signal that varies about a stable center frequency creating a spread spectrum oscillator. The oscillator is modulated by using an additional oscillator that causes the voltage supply output voltage level to vary in response to this oscillators frequency. The voltage supply&#39;s output voltage level now changes in response to temperature and also the additional oscillator.  
     
    
     BRIEF DESCRIPTION OF THE  
       [0010]     The invention will be described in detail hereinafter with reference to the accompanying drawings; in which  
         [0011]      FIG. 1  is a schematic representation of the circuit of the present invention;  
         [0012]      FIG. 2  is a schematic representation of Error Corrector  7 A;  
         [0013]      FIG. 3  is a schematic representation of Error Corrector  7 B;  
         [0014]      FIG. 4  is a schematic representation of an alternate Voltage Supply  80 A;  
         [0015]      FIG. 5  is a schematic representation of Astable Multivibrator  81 A; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Shown in  FIG. 1  is compensated oscillator  100  comprising oscillator  81 , buffer amplifier  12 , and voltage supply  80 . The oscillator  81  consist of feedback network  10  and amplifier  11 . Feedback network  10  is connected to the input and output of amplifier  11  by lines  14  and  15 . Amplifier  11  has a phase shift of about 180 degrees and feedback network  10  supplies the remaining phase shift necessary to make the total phase shift at the frequency of oscillation 360 degrees. If amplifier  11  is not an inverter, than the feedback network  10  will provide the required phase shift amount to have 360 degrees total. The phase shift provided by feedback network  10  varies with frequency. There are many well known phase shift networks that can be used such as the twin T and the Wien bridge. One practical means well known by those skilled in the art of implementing amplifier  11  is to use a CMOS inverter. Buffer Amplifier  12  provides isolation between the oscillator  81  output on line  14  and any load connected to oscillator output  13 . It may be overdriven to provide a square wave output signal. The voltage to power amplifier  12  may come from terminal  4  instead of line  5 . Amplifier  11  phase shift and gain are effected by the voltage on line  5  and ambient temperature. Amplifier  11  and feedback network  10  are thermally coupled and essential at the same temperature. Line  5  supplies the voltage to power the amplifier from voltage supply  80 . The effect of temperature is that as the temperature increases the phase shift of amplifier  11  and feedback network  10  changes causing the frequency of oscillation to change. The effect of temperature on amplifier  11  and feedback network  10  is substantially canceled by changing the voltage on line  5 . Thus the voltage on line  5  serves as a frequency control signal to adjust the output frequency of oscillator  81  to it&#39;s original frequency and remain constant over temperature and input voltage.  
         [0017]     Voltage supply  80   FIG. 1  receives a DC input voltage at terminal  4  and provides a predetermined voltage on line  5  in response to Amplifier  11  temperature and oscillator  6 . Voltage supply  80  consist of voltage regulator  1 , voltage reference  3 , temperature sensor  2 , error corrector  7  and oscillator  6 . Voltage regulator  1  may use an operational amplifier or an other well known voltage regulator circuit along with voltage reference  3  and temperature sensor  2 . Temperature sensor  2  can be a semiconductor such as a diode or a temperature sensitive resistor. Voltage regulator  1  may instead use other well known voltage reference circuits into which temperature sensor  2  and voltage reference  3  are integrated. Voltage regulator  1  output on line  5  is a DC voltage that is nominally set by voltage reference  3  and varies in a controlled manner with the temperature change of temperature sensor  2  which is thermally coupled to amplifier  11  and also feedback network  10 . Temperature sensor  2  can be formed on the same substrate as Amplifier  11  transistors to provide good thermal coupling. Error corrector  7  causes an additional predetermined change to Voltage regulator  1  output in response to temperature sensor  2  at temperatures where additional frequency correction is needed due to the variation of line  5  voltage with temperature not exactly following the correct variation required to maintain constant frequency. Error corrector  7  output correction signal on line  8  is applied to the input of voltage regulator  1 . Oscillator  6  is used only when it is desired to have oscillator  81  provide a frequency signal that varies about a stable center frequency. The Oscillator  6  AC output voltage is connected to the input of voltage regulator  1  causing the voltage on line  5  to have an AC voltage added to the predetermined DC voltage.  
         [0018]     Referring now to  FIG. 2 a  practical means of implementing Error corrector  7  is shown. Error corrector  7 A consist of temperature sensor  2 , temperature references  62  and  64 , and comparators  61  and  63 . Temperature sensor  2  applies a signal level corresponding to the measured temperature to the inputs of comparators  61  and  63 . Comparator  61  turns on when the temperature of temperature sensor  2  produces a signal level exceeding the temperature reference  62  signal level connected to the other input of comparator  61 . When Comparator  61  is on a correction signal of predetermined value is applied to line  8 . The signal level of temperature reference  62  is set to a value that corresponds to the minimum temperature that comparator  61  is desired to be turned on at. The correction signal level was predetermined to be the value needed to maintain maximum frequency stability verses temperature. Conversely, comparator  61  may also be connected so as to turn off when the temperature increases and the signal level at its input exceeds the signal level of temperature reference  62 . The operation of comparator  63  is identical to that of comparator  61  except that it&#39;s other input is connected to temperature reference  64 . Temperature reference  64  signal level may be set to a different level than temperature reference  61 . Additional temperature references, and comparators may be added with all the output correction signals added together on line  8  to provide additional frequency correction at different temperature levels to maintain the least variation of frequency with variation of temperature.  
         [0019]     An other approach to implementing error corrector  7  is error corrector  7 B  FIG. 3 . The output of temperature sensor  2  which provides an signal corresponding to the measured temperature to the input of address generator  72 . Address generator  72  provides a digital output corresponding to the input signal level. Address generator  72  digital output is connected to the input of memory  73 . Memory  73  output is a stored digitally coded signal level corresponding to each input signal value. Memory  73  digital output is connected D/A  74  which converts the digitally coded signal level at it&#39;s input to an output analog correction signal applied to line  8 . The signal applied to line  8  provides additional frequency correction at different temperature levels to maintain the least variation of frequency with variation of temperature.  
         [0020]     Also, as shown in  FIG. 4  corrector  7 B may be used to directly supply the voltage on line  5  without using regulator  1  when Memory  73  has a large enough memory to store enough correction signal values at corresponding temperatures to maintain the desired oscillator  81  frequency accuracy. The result is that for each temperature level measured by temperature sensor  61  a predetermined error correction signal is applied to line  5 . The correction signal level was predetermined to be the value needed to maintain maximum frequency stability at different temperature levels.  
         [0021]     Referring now to  FIG. 5 , oscillator  81 A is shown. Oscillator  81 A is one possible alternate embodiment of oscillator  81 . Oscillator  81 A is configured as a astable multivibrator. The astable multivibrator is well known by those skilled in the art. Three CMOS inverting amplifiers are used to provide the necessary gain. The first amplifier input is connected to line  15  and consist of PMOS  87  and NMOS  86 . The output of which is connected the input of the second amplifier consisting of PMOS  89  and NMOS  88 . The output of which is connected to capacitor  92  and also to the input of the third amplifier consisting of PMOS  91  and NMOS  90 . The output of which is connected to resistor  93  and line  14 . The oscillator frequency is primarily determined by the time constant of the feedback network consisting of resistor  93  and capacitor  92 . This oscillator has been modified to obtain additional improvements in oscillator frequency stability by adding one or more diodes that limit the oscillator feedback signal voltage level. The diodes are thermally coupled to the Amplifier  11  and the feedback network so as to be at essentially the same temperature.  
         [0022]     The oscillator frequency is additionally effected by diodes  84  or  83  conducting and limiting the peak voltage level on line  15 . Diode  83  is connected between a voltage supply  82  and line  15  such that the diode only conducts when the voltage on line  15  is greater than voltage supply  82  level plus the diode  83  forward voltage drop. Diode  84  is connected between voltage supply  85  and line  15  such that the diode  84  only conducts when the voltage on line  15  is less than the sum of voltage supply  85  and the diode  84  forward voltage drop. Voltage supply  82  is a voltage greater than  12  the level on line  5  and voltage supply  85  is a voltage less than ½ the level on line  5 . As an example when line  5  equals 4 volts diode  83  conducts when line  15  exceeds 4 volts and when line  15  is less than 0 volts diode  84  conducts. As the resistance levels of resistor  93  and the PMOS  9  and NMOS transistors increase with temperature the oscillator frequency is reduced. However diodes  83  and  84  forward voltage drop is also temperature sensitive. The voltage drop across diodes  83  and  84  decreases with temperature . The reduced feedback signal amplitude on line  15  causes the oscillator frequency to increase. By setting the voltage levels of voltage supply  82  and  85  to predetermined value the temperature effects on oscillator frequency is reduced and a more constant frequency is maintained as temperature varies. The circuit can also maintain a constant frequency verses temperature by using only one of the diodes  84  or  83 . The use diodes  84  or  83  is also applicable to other well known multivibrator circuits.