Abstract:
An absorptive temperature-variable microwave attenuator is produced using at least two temperature-variable (or fixed) resistors in series with a transmission line. The temperature coefficients of the temperature-variable resistors are selected so that the attenuator changes at a controlled rate with changes in temperature while attenuator remains relatively matched to the transmission line. In one embodiment, the resistors are thick-film resistors and a variety of temperature coefficients can be created for each resistor by properly selecting and mixing different inks when forming the resistors. Furthermore, attenuators can be created having either a negative temperature coefficient of attenuation or a positive temperature coefficient of attenuation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to temperature-variable microwave attenuators. 
   2. Description of the Related Art 
   Attenuators are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the attenuator itself. The latter is often preferred because the mismatch which results from using a reflective attenuator can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive attenuators are more popular, particularly in microwave applications. The important parameters of an absorptive attenuator are: attenuation as a function of frequency; return loss; and stability over time and temperature. 
   It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. In many cases, it is impossible or impractical to remove the temperature variations in some Radio Frequency (RF) components. For example, the gain of many RF amplifiers is temperature dependent. In order to build a system with constant gain, a temperature-dependent attenuator is placed in series with the amplifier. The attenuator is designed such that a temperature change that causes the gain of the amplifier to increase will simultaneously cause the attenuation of the attenuator to increase such that the overall gain of the amplifier-attenuator system remains relatively constant. 
   Prior art temperature-dependent attenuators employ connections between an unbalanced transmission line and ground or between two lines of a balanced line. Such constructions is not always optimal, especially at frequencies above 18 GHz. The reason for this is that parasitic capacitances and inductances can taint (or alter) the response of the device so that its attenuation over frequency and VSWR is no longer useful or desirable. It is typically desirable that the attenuation at any particular temperature is constant over the frequency of interest and the VSWR is as low as possible, usually less than 2.0 to 1. For frequencies exceeding 18 GHz, the prior art is unable to achieve this with any degree of accuracy. Therefore there is a real need for a temperature dependent attenuator that exhibits flat (or nearly flat) attenuation characteristics and low VSWR over selected portions of the frequency range of 18 GHz to 300 GHz. 
   The present invention solves these and other problems by providing a temperature-dependent attenuator that uses two or more temperature-dependent resistors in series with a transmission line. The attenuator can be used at radio frequencies, microwave frequencies, etc. In one embodiment, the temperature-dependent radio-frequency attenuator includes a plurality of temperature-dependent resistors electrically in series with a transmission line. The temperature-dependent resistors are in series with the transmission line approximately one-quarter wavelength apart at a desired frequency. The temperature coefficients of the resistors are configured such that the attenuator changes attenuation at a desired rate with changes in temperature. 
   In one embodiment, the resistors have different temperature coefficients. In one embodiment, the resistors have the same, or similar, temperature coefficients. In one embodiment, temperature coefficient of one or more of the resistors is a negative temperature coefficient of resistance. In one embodiment, temperature coefficient of one or more of the resistors is a positive temperature coefficient of resistance. In one embodiment, one or more of the resistors are film resistors. In one embodiment, one or more of the resistors are thick-film resistors. In one embodiment, one or more of the resistors are thin-film resistors. In one embodiment, one or more of the resistors are printed ink resistors. 
   In one embodiment, the attenuator has a negative temperature coefficient of attenuation. In one embodiment, the attenuator has a positive temperature coefficient of attenuation. In one embodiment, the transmission line includes a microstrip transmission line. In one embodiment, the transmission line includes a stripline transmission line. 
   In one embodiment, the attenuator has three resistors in series with the transmission line and approximately one-quarter wavelength apart. In one embodiment, the middle resistor in the series has a resistance that is approximately twice the resistance of the outer. In one embodiment, the two outer resistors have approximately the same resistance. 
   In one embodiment, an attenuator includes a series combination of temperature-dependent resistors separated by transmission line sections. In one embodiment, the transmission line sections have different lengths and/or characteristic impedances. In one embodiment, the lengths of the transmission line sections are symmetric about an electrical center of the attenuator. 
   In one embodiment, the attenuator uses a microstrip transmission line. In one embodiment, the attenuator uses a stripline transmission line. In one embodiment, the attenuator uses a co-planer waveguide transmission line. In one embodiment, the attenuator uses a grounded co-planar waveguide transmission line. In one embodiment, the attenuator uses a coaxial transmission line. In one embodiment, the VSWR of the attenuator remains below 3 to 1 over a desired operating band. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a series attenuator with three resistors. 
       FIG. 2  is a perspective drawing of a series attenuator represented by the schematic of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic representation of a series absorptive attenuator  100  that includes three resistors  101 – 103  separated by transmission line sections  112  and  123 . The resistor  101  is provided in series between a transmission line  104  and the transmission line section  112 . The resistor  102  is provided in series between the transmission line section  112  and the transmission line section  123 . The resistor  103  is provided in series between the transmission line section  123  and a transmission line  106 . In one embodiment, the resistors  101 – 103  are thick-film resistors. The transmission line section  112  is one-quarter wavelength long at a first desired center frequency. The transmission line section  123  is one-quarter wavelength long at a second desired center frequency. In one embodiment, the first desired center frequency will be the same as the second desired center frequency. In one embodiment, the resistors  101 – 103  are temperature-dependent resistors (thermistors), where the resistance of each thermistor varies with temperature according to a temperature coefficient. In one embodiment, the resistors  101  and  103  have the same resistance and temperature coefficient. In one embodiment, the resistance of the resistor  102  is twice the resistance of the resistors  101  and  103 . In one embodiment, the temperature coefficient of the resistor  102  is twice the temperature coefficient of the resistors  101  and  103 . In one embodiment, the transmission lines  104 ,  112 ,  123 , and  106  have the same characteristic impedance.  FIG. 1  shows three resistors for purposes of illustration. One of ordinary skill in the art will recognize that two, three, four or more resistors separated by transmission line sections can be used. 
   The attenuator  100  behaves as a lossy transmission line, as the resistors  101 – 103  absorb a portion of the energy propagating between the transmission line  104  and the transmission line  106 . If the resistance of the resistors  101 – 103  is different from the characteristic impedance of the transmission lines  104  and  106 , then the resistors  101 – 103  will produce undesired reflections on the transmission lines  104  or  106 . 
   By making the transmission line sections  112  and  123  one quarter wavelength long at a desired frequency, the reflections from the resistors will cancel at the desired frequency, and thus the reflections on the transmission lines  104  and  106  will be reduced or eliminated at the center frequency and in a band about the desired center frequency. 
   One of ordinary skill in the art will recognize that two, three, four or more resistors separated by transmission line sections can be used. The transmission line sections can be of different length and/or different characteristic impedance (e.g., different width). In one embodiment, standard microwave filter design techniques are used to design the attenuator by selecting the parameters that do not vary with frequency (e.g., the number of resistors, the lengths and impedances of the transmission lines, etc.), and then determining the resistor values at a number of temperatures to match the desired attenuation-temperature profile over the desired bandwidth. Once the resistances at a number of temperatures are known, the temperature coefficients of each resistor are selected to produce the desired temperature profile in each resistor. 
   In one embodiment, the resistors  101 – 103  are thick film resistors are produced by inks combining a metal powder, such as, for example, bismuth ruthenate, with glass frit and a solvent vehicle. This solution is deposited and then fired onto a ceramic substrate which is typically alumina but could also be beryllia ceramic, aluminum nitride, diamond, etc. When the resistor is fired, the glass frit melts and the metal particles in the powder adhere to the substrate, and to each other. This type of a resistor system can provide various ranges of material resistivities and temperature characteristics can be blended together to produce many different combinations. 
   The resistive characteristics of a thick film ink is specified in ohms-per-square (Ω/□). A particular resistor value can be achieved by either changing the geometry of the resistor or by blending inks with different resistivity. The resistance can be fine-tuned by varying the fired thickness of the resistor. This can be accomplished by changing the deposition thickness and/or the firing profile. Similar techniques can be used to change the temperature characteristics of the ink. 
   The temperature coefficient of a resistive ink defines how the resistive properties of the ink change with temperature. A convenient definition for the temperature coefficient of the resistive ink is the Temperature Coefficient of Resistance (TCR) often expressed in parts per million per degree Centigrade (PPM/C). The TCR can be used to calculate directly the amount of shift that can be expected from a resistor over a given temperature range. Once the desired TCR for a particular application is determined, it can be achieved by blending appropriate amounts of different inks. As with blending for sheet resistance, a TCR can be formed by blending two inks with TCR&#39;s above and below the desired TCR. One additional feature of TCR blending is that positive and negative TCR inks can be combined to produce large changes in the resulting material. 
   Some thermistors exhibit a resistance hysteresis as a function of temperature. If the temperature of the resistor is taken beyond the crossover point at either end of the hysteresis loop, the resistor will retain a memory of this condition. As the temperature is reversed, the resistance will not change in the same manner observed prior to reaching the crossover point. In one embodiment, to avoid this problem, the inks used in producing a temperature variable attenuator are selected with crossover points that are beyond the −55 deg. C. to 125 deg. C. operating range. 
     FIG. 2  shows one embodiment of attenuator construction wherein a substrate  214  is provided as a base. The substrate can be an insulating material such as, for example, aluminum oxide, Teflon, reinforced Teflon, fiberglass board, aluminum nitride, diamond, or beryllia ceramic, etc. The resistors  101 – 103  are provided as thick-film resistors  201 – 203 . A transmission line section  212  is provided between the resistors  101  and  102 . A transmission line section  223  is provided between the resistors  102  and  103 . The transmission line sections are one quarter wavelength long at a desired center frequency. 
   The length of the resistors is determined by the sheet resistance of the thick-film material and the width of the resistors. In one embodiment, the width of the resistors is similar to the width of the transmission line sections to reduce inductive effects. 
   In one embodiment, the transmission line sections are made from thick film conductor which is deposited on the substrate  214 . Thick film resistors  201 – 203  having the specifications described above and of the desired width and length are then formed. In one embodiment, the resistors  201 – 203  are then protected by a protective coating  222 . 
   The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and accordingly reference should be made to the appended claims rather than to the foregoing specification as indicating the scope of the invention.