CROSS-REFERENCE TO RELATED APPLICATION 
   A related application is application Ser. No. 11/107,556, filed concurrently herewith for “Wideband Temperature Variable Attenuator,” the disclosure of which are incorporated herein by reference. 
   FIELD OF INVENTION 
   The present invention relates to a voltage controlled attenuator (VCA) for RF (radio frequency) and microwave applications that is free of intermodulation distortion. More particularly, the present invention relates to an attenuator that is controlled based upon temperature and does not include active devices. 
   BACKGROUND OF THE INVENTION 
   VCAs are a fairly common element of almost any RF or microwave circuit. Their function is to change the amplitude of a signal based on some external signal, usually a voltage or current. A common use is the leveling of a signal so that both strong and weak signals can be adjusted in amplitude to provide a constant level signal to the next stage of the circuit. Another use is the balancing of multiple signal paths so they all have the same gain. A third use would be to use a VCA to control the gain of an amplifier over temperature by varying the control voltage based on a measurement of the ambient temperature. This last use is to counter undesired changes to the gain of the amplifier when the ambient temperature changes. 
   The vast majority of presently available VCAs include either diodes, transistors, or FETs (field effect transistors). These active devices have non-linear transfer characteristics which result in distortion to RF and microwave input signals. This causes additional and unwanted signals to be generated which are not present in the original signal. For example, suppose two people are transmitting a signal (from a cell phone, for instance) on two different frequencies at the same time. If the two signals were applied to a non-linear device, several additional signals would be generated that would be on frequencies that are different from the original two frequencies. This is known as intermodulation distortion. These additional signals have the potential of causing interference to other services, like police or fire departments that use the same frequencies as the additional signals. 
   VCAs are designed to reduce intermodulation distortion to the smallest possible value, but due to the non-linear characteristics of the control devices used, there is no way to eliminate intermodulation distortion entirely. Therefore, there exists a real and present need for a VCA that can control the amplitude of an RF or microwave signal without generating any distortion products which result in intermodulation distortion. 
   U.S. Pat. No. 5,332,981, issued to Joseph B. Mazzochette, et al., issued Jul. 26, 1994, entitled “Temperature Variable Attenuator,” which is incorporated herein by reference, describes an attenuator that includes temperature variable resistors (thermistors) in the attenuating path. As shown in  FIGS. 1A and 1B  which are reproduced from  FIGS. 1 and 3  of the &#39;981 patent. conventional attenuators include a Tee attenuator  10  comprising a pair of identical series resistors R 1  and a shunt resistor R 2  and a Pi attenuator  12  comprising a series resistor R 2  and two shunt resistors R 1  and R 3 .  FIG. 1  C is a plot reproduced from  FIG. 2  of the &#39;981 patent, showing a family of constant attenuation curves from 1 to 10 dB and a constant 50 ohm impedance curve descending from the upper left of the plot to the lower right. The vertical axis on this plot represents the value of shunt resistor R 2  in the T attenuator  10  and the horizontal axis represents the values of series resistors R 1 . The point of intersection between the 50 ohm impedance curve and an attenuation curve gives the value of R 1  and R 2  that produce the attenuation represented by the attenuation curve and a 50 ohm impedance match. 
   In the temperature variable attenuator of the &#39;981 patent, the temperature coefficient of resistance (TCR) of at least one resistor is different such that the attenuation of the attenuator changes at a controlled rate with changes in temperature while the impedance of the attenuator remains substantially constant. Thus, this device changes its attenuation based on the ambient temperature, but because it is constructed entirely of passive components it does not generate any intermodulation distortion. However, the attenuation of this device cannot be set to a predetermined value based upon a constant external voltage or current. 
   U.S. Pat. No. 5,999,064, issued to Robert Blacka, et al., issued Dec. 7, 1999, entitled “Heated Temperature Variable Attenuator,” which is also incorporated by reference, provides a heater in a temperature variable attenuator. The heater allows an external voltage or current to heat the thermistors that are part of the attenuating circuit to affect their resistance, and thus, the attenuation of the device. However, there are a number of limitations with this device which reduces its usefulness as a VCA. 
   SUMMARY OF THE INVENTION 
   The present invention is a VCA for RF and microwave applications that is free of intermodulation distortion. In a preferred embodiment, the present invention has at least first and second thermistors, arranged into a classical Tee, Pi, or Bridged Tee attenuator design, a heating element, a temperature sensor, and a control circuit. The thermistors have different temperature coefficients of resistance and are in close proximity to the heating element and the temperature sensor. The control circuit receives a voltage signal from the temperature sensor, compares that signal with a voltage signal specifying a desired temperature, and applies electrical energy to the heating element until receiving a signal from the temperature sensor that the temperature of the thermistors matches the desired temperature. As a result, the attenuation of the attenuator can be changed at a controlled rate by varying the temperature of the thermistors, while the impedance of the attenuator remains within acceptable levels. 
   In one embodiment, the temperature coefficient of resistance of one thermistor is zero. In another embodiment, the temperature sensor is also a thermistor. In yet another embodiment, the temperature sensor is a resistance temperature detector. 
   In a particular embodiment, the attenuator is constructed using thick-film or thin-film resistors that vary their resistance over temperature. In yet another embodiment, the thick-film or thin-film resistors are deposited onto a substrate of aluminum oxide, aluminum nitride, beryllium oxide, CVD diamond, or epoxy-glass laminate. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features, and advantages of the invention will be more readily apparent from the following detailed description in which: 
       FIGS. 1A–1C  depict aspects of prior art temperature variable attenuators; 
       FIG. 2  is a schematic diagram showing the basic structure of an attenuator in accordance with the present invention 
       FIG. 3  is the top view of an embodiment of the present invention; 
       FIG. 4  is the top view of a heating structure in the embodiment of the present invention shown in  FIG. 3 ; 
       FIG. 5  is the side view of the embodiment of the present invention shown in  FIG. 3 ; 
       FIG. 6  is a graph depicting the attenuation produced at given temperature in an illustrative embodiment of the invention; 
       FIG. 7  is a circuit diagram showing the basic structure of a control circuit of an embodiment of the present invention; and 
       FIGS. 8A–8N  are top views illustrating the sequence of steps in the formation of the attenuator of  FIGS. 3–5 . 
   


   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a schematic diagram of an illustrative attenuator  200  of the present invention. Attenuator  200  includes a pair of identical series thermistors  204  and shunt thermistor  206 . These thermistors are arranged in a classical Tee attenuator design. The attenuator also includes temperature sensor  202  and heating element  208 . Thermistors  204  and  206  are arranged relative to heating element  208  and temperature sensor  202  such that they are simultaneously heated by heating element  208 , and their temperature is detected by temperature sensor  202 . 
   A physical embodiment of the attenuator of  FIG. 2  is shown in  FIGS. 3–5 .  FIG. 3  is a top view of attenuator  300 ,  FIG. 4  is a top view of a heating structure  400  of the attenuator and  FIG. 5  is a side view. As shown in  FIG. 5 , attenuator  300  is formed on substrate  500 . Substrate  500  is an insulating material such as aluminum oxide (alumina), aluminum nitride (ALN), beryllium oxide (BeO), CVD diamond, or epoxy-glass laminate. A ground plane  501  of platinum, silver or a platinum silver alloy is formed on one side of substrate  500 . Optionally, a dielectric layer  502  is formed on the opposite side of substrate  500 . Heating structure  400  is formed on dielectric layer  502 , if present, or on substrate  500 . As shown in the top view of  FIG. 4 , heating structure  400  comprises a dielectric layer  502  in which are formed heater contact areas  404 ,  406  and a U-shaped heating element  408 . Heating element  408  is positioned such that it electrically extends between and contacts first and second heater contact areas  404  and  406 . As best shown in the side view of  FIG. 5 , a layer of insulating material  320  covers most of heating element  408  but not contact areas  404 ,  406 . 
   Temperature sensor  202  and thermistors  204  and  206  are realized in the implementation of  FIGS. 3–5  as sensor  316  and thermistors  310  and  312  which are formed on insulating material  320 . Thermistors  310  and  312  are each positioned to extend at least across a portion of heating element  408 . The thermistors are electrically connected to each other at node  311 , thermistors  310  are electrically connected to contact areas  314  and thermistor  312  is electrically connected to contact area  322 . Contact area  322  is connected to ground plane  501  on the underside of substrate  500  by a ground wrap connector on the outside of the substrate or by a via through the substrate. Temperature sensor  316  is positioned so that it is in close enough proximity to thermistors  310  and  312  to detect their temperature and is an electrical contact with first and second sensor contact areas  318  and  319 . 
   The attenuating characteristics of attenuator  300  as a function of temperature can be determined simply by measuring them over the operating range of the attenuator. For example, in an illustrative embodiment of the inventory, the variation of attenuation with temperature might be determined to be that shown in the graph of  FIG. 6 . Once this functional relationship is known, any attenuation over the operating range of attenuator  300  can be selected by accurately controlling the temperature of thermistors  310  and  312  so as to achieve the attenuation known to correspond to that temperature. This temperature control is accomplished with external circuit  700  of  FIG. 7  which constantly monitors the device temperature with temperature sensor  202 / 316  and controls the heat output from heating element  208 / 408 . 
   Circuit  700  comprises an operational amplifier  710  having an inverting input connected to the node between an input resister R 1  and a feedback resistor R 2  and a noninverting input connected to the node between resistors R 3  and R 4  in a voltage divider network  720 . The resistances of R 1  and R 3  are equal and the resistances of R 2  and R 4  are equal. Input resistor R 1  is connected to a node in a temperature sensing circuit  730  comprising temperature sensor  202 / 316  and resistor R 5 . The voltage at this node is V 1 . The voltage applied to voltage divider  720  is V 2 . As a result, operational amplifier  710  functions as a differential amplifier that receives at its inverting and non-inverting terminals, respectively, signals proportional to V 1  and V 2  and produces an output signal 
             V   ⁢           ⁢   out     =       R1   R2     ⁢       (     V2   -   V1     )     .             
The output of operational amplifier is applied to a transistor  740  in a heating circuit  750  comprising transistor  740  and heating element  208 / 408 .
 
   For the circuit shown in  FIG. 7 , temperature sensor  202 / 316  has a negative temperature coefficient of resistance (TCR). As a result, as the temperature rises, voltage V 1  increases monotonically. Voltage V 2  specifies the desired operating temperature of the attenuator. Thus, the output of the operational amplifier is a signal proportional to the difference between the desired operating temperature and the actual operating temperature; and this signal is used to control the current flow in heating circuit  750  such that the amount of current flow is a function of the difference between the desired temperature and the actual temperature. Since the current flow through the heating circuit increases the temperature sensed by temperature sensing circuit  730 , this increases V 1  and thereby decreases the difference (V 2 −V 1 ) until the temperature sensed by the temperature sensing circuit reaches the temperature specified by voltage V 2 . 
   Alternatively, circuit  700  would function in the same way if the positions of sensor  202 / 316  and resistor R 5  in the temperature sensing circuit were interchanged and if sensor  202 / 316  had a positive TCR. 
     FIGS. 8A–8N  are top views illustrating the sequence of steps in the formation of the attenuator of  FIGS. 3–5 . The starting material is a bare ceramic substrate typically measuring about 3 inches by 3 inches although other sizes of ceramic substrate may also be used in the practice of the invention. As mentioned above, suitable ceramic materials include aluminum oxide (Alumina), aluminum nitride (ALN), beryllium oxide (BeO), CVD diamond, or epoxy-glass laminates such as FR-4 or G-10. Low temperature co-fired ceramic may also be used as substrates in the practice of the invention. Individual devices that measure approximately 0.125 inches by 0.060 inches each are formed simultaneously on the ceramic substrate using screen printing technology in which layers of material are first printed on the substrate and then fired at an appropriate temperature in the range of 600 deg, C. to 900 deg. C. To maximize the number of devices formed on a substrate, the devices are aligned in a rectangular array. For convenience of illustration,  FIGS. 8A–8N  depict the steps performed in making one such device but it will be understood that the same steps are being performed simultaneously on all the devices being made on the ceramic substrate. At the end of the formation process, the ceramic substrate is scribed and the individual devices are separated using well-known techniques. 
   The underside of the ceramic substrate is first metallized as shown in  FIG. 8B  to provide ground plane  501  and first and second dielectric layers optionally are then deposited on the top-side of the substrate as shown in  FIGS. 8C and 8D . Next, individual heater structures  400  are formed in  FIGS. 8E and 8F  by first printing gold contact layers  404 ,  406  and then printing heating elements  408 . Illustratively, the resistance of each heating element  408  is 150 ohms. The heating structures  408  are then covered by one or more dielectric layers in  FIGS. 8G and 8H . 
   Gold contact areas  311 ,  314 ,  318 ,  319  and  322  are then printed in  FIG. 81  and the temperature sensor  316  is printed in  FIG. 8J . Illustratively, the temperature sensor is a thick-film 10K ohm thermistor with a negative temperature coefficient of resistance. Next, the attenuator is formed by screen printing the series thermistors  310  as shown in  FIG. 8K  and then the shunt thermistor  312  as shown in  FIG. 8  L. Illustratively, the thermistors are thick-film thermistors and the series thermistors have a positive TCR and the shunt thermistor has a negative TCR. Alternatively, thin-film thermistors could be used for temperature sensor  316  and the series and shunt resistors. 
   As shown in  FIG. 8M , the thermistors can then be laser-trimmed to adjust their resistance; and in  FIG. 8N  a protective layer is printed on the top surface. Product markings such as the manufacturer&#39;s name and part numbers can then printed on each device and the devices are then ready for testing. Following testing, the ceramic substrate is scribed and the individual devices are separated. Advantageously, the ground plane facilitates the soldering of the attenuator onto a larger substrate and electrical connections to the attenuator are made by wire bonding lead wires to the various contact areas. As will be apparent to those skilled in the art, the order of some of these steps can be varied. In addition, while firing would typically be carried out after each printing step, it may be advantageous to combine some of the firing steps. 
   The attenuators of the present invention are suitable for numerous applications including amplifier gain calibration, the balance of multiple channels and automatic gain control. They can be used to maintain oscillator output constant over frequency or reduce the output of a transmitter if the standing wave ratio is too high. They have an extremely wide frequency operating range being operable from DC to 20 GHz or higher. Since their components are completely passive, they are free of any distortion. 
   Typical specifications for the attenuators of the present invention are: 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               impedance 
               50 
               ohms nominal 
             
             
                 
               frequency range 
               DC to 20 
               GHz or higher 
             
             
                 
               insertion loss 
               1.5 
               dB Max 
             
             
                 
               attenuation range 
               3 
               dB above insertion loss 
             
             
                 
               attenuation flatness 
               +/−0.25 
               to dB to 10 GHz 
             
             
                 
               VSWR 
               1.3 
               Max 
             
             
                 
               response time 
               100 
               mS Max 
             
             
                 
               RF power 
               250 
               mW Max 
             
           
        
         
             
                 
               operating temperature 
               −55° C. to 125° C. 
                 
             
             
                 
                 
             
           
        
       
     
   
   The foregoing description, for purposes of explanation, used specific examples to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention is not limited to these examples. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. Thus, the foregoing disclosure is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. 
   While the invention was described for the example of a Tee attenuator, the invention may also be practiced using other attenuators such as a Pi attenuator or a bridged Tee attenuator in which a thermistor is connected in parallel to the pair of series resistors of the Tee attenuator. Of particular note, it should be observed that a wide range of attenuations can be achieved by appropriate selection of the TCRs of the various thermistors and whether the TCRs are positive or negative. In some cases, it is not necessary for every resistive element on the attenuator to have a resistance that varies with temperature and the invention may be practiced where one of the resistive elements has a zero TCR. As will be appreciated, the impedance that is observed over the operating frequency range and/or operating temperature range of the attenuator will not be precisely constant and the variation in impedance will depend on the amount of attenuation provided by the attenuator. At low attenuation, deviation from the desired impedance may be within +/− a few percent of the desired impedance over the operating range. At higher attenuations, deviation from the desired impedance can be expected to be higher, for example, +/−10%, +/−20%, and even +/−50% or more. In practice, considerable variation in impedance may be tolerated depending on the specific application in which the attenuator is used and the temperature and frequency range of use. As a rule of thumb, the variation in impedance of the attenuator should be such that the Voltage Standing Wave Ratio (VSWR) of the RF power is no more than 2.0:1 over the operating range of the attenuator. 
   It is intended that the scope of the invention be defined by the following claims and their equivalents.
A preferred embodiment of the present invention comprises at least first and second thermistors, arranged into a classical Tee, Pi, or Bridged Tee attenuator design, a heating element, a temperature sensor, and a control circuit. The thermistors have different temperature coefficients of resistance and are in close proximity to the heating element and the temperature sensor. The control circuit receives a voltage signal from the temperature sensor, compares that signal with a voltage signal specifying a desired temperature, and applies electrical energy to the heating element until receiving a signal from the temperature sensor that the temperature of the thermistors matches the desired temperature. As a result, the attenuation of the attenuator can be changed at a controlled rate by varying the temperature of the thermistors, while the impedance of the attenuator remains within acceptable levels.