Abstract:
A bandpass filter makes use of at least one waveguide cavity that is thermally compensated to minimize drift of a resonant frequency of the cavity with thermal expansion of cavity components. The compensation relies on deformation of the shape of at least one cavity surface in response to thermally-induced dimensional changes of the cavity. A control rod is used to limit the movement of a point on the deformed surface, while the rest of the surface moves with the thermal expansion. The control rod is made of a material having a coefficient of thermal expansion that is significantly different than that of other filter components. The rod may also be arranged to span more thermally expandable material than defines the filter such that, as the filter expands, the point of deflection is moved toward the interior of the filter beyond its original position. A similar effect may be accomplished by connecting the control rod to an end deflecting rod that does the actual limiting of the movement of the deflection point. If the end deflecting rod has a coefficient of thermal expansion that is higher than that of the control rod, the end deflecting rod will expand with temperature relative to the end of the control rod, forcing the deflection point inward.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of electromagnetic signal communication and, more particularly, to the filtering of high power signals for broadcast communications. 
     BACKGROUND OF THE INVENTION 
     In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers. 
     For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. In particular, the introduction of dual-mode cavities for microwave filters in 1971 made a significant contribution to the art. Dual-mode filters allowed for a reduction in filter size and mass, and could realize more complex filter functions by their ability to easily couple non-adjacent resonators. Later reductions in size and mass were achieved with the introduction of triple and quadruple mode filters. 
     While dual-mode waveguide cavity filters have been used often for space and satellite communications, they have also been used for terrestrial television broadcast applications. Indeed, for transmitters operating in a common amplification mode (i.e., a mode in which both audio and video signals are being amplified together), dual-mode filters have become predominant because of their low loss and ability to realize complex filter functions. Moreover, dual-mode filters have been favored for the transmission of analog television signals because of their flexibility in realizing wide pass bandwidths to compensate for frequency drift due to RF heating and ambient temperature changes. However, with the advent of digital television, system requirements have changed. The FCC emissions mask for digital television broadcast stations is very restrictive for power radiated into adjacent channels or out-of-band frequencies. These requirements will not be satisfied by filters that have wide pass-bands that are allowed to drift. 
     In the past, waveguide cavities have been developed that are adjustable to compensate for thermal expansion. Paul Goud in Cavity Frequency Stabilization with Compound Tuning Mechanisms, Microwave Journal, March 1971 discloses a waveguide cavity that may be adjusted to compensate for thermal expansion. In  FIG. 2  of the article, Goud shows a compound tuning mechanism that may be used to change the effective length of the filter cavity. However, this tuning mechanism requires a manual adjustment of a screw device to make the necessary changes. Moreover, the movable surface is based on a two-section choke. This choke must be unconnected to the sides of the filter, so that it may be moved relative to them. As such, the cavity is unsealed, and is prone to leakage and poorer performance than a sealed filter. 
     More recently, filter design has addressed the need for narrower bandwidth filters by constructing filters from materials with lower thermal expansion coefficients to minimize the effect of heating on the filter dimensions. In particular, the nickel/steel alloy Invar® (a registered trademark of Imphy, S.A., Paris, France) has been used as a cavity material. Because of its extremely low degree of thermal expansion, the cavities built with Invar® suffer less of a dimensional change with heating, and therefore maintain a narrower, more stable passband. However, Invar is also very expensive, and consequently drives up the overall cost of the filter. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a bandpass filter is provided that uses the deformation of a cavity surface in response to thermal changes to compensate for the resonant frequency shifting effects of thermal expansion. The filter has at least one waveguide cavity in which an input electrical signal resonates at a desired resonant frequency, and a plurality of surfaces, each with a predetermined geometric shape. For example, in a preferred embodiment, the filter has a cylindrical outer surface and a circular end plate. A thermal compensator is provided that responds to thermally induced changes in dimensions of the cavity by distorting the shape of one of the cavity surfaces, thereby minimizing any resulting drift in the resonant frequency. 
     Typically, the thermally induced changes in the cavity are an increase in cavity dimensions, and the thermal compensator deflects one of the cavity surfaces inward, such as in the case of a concave deflection of the cavity end plate. In the preferred embodiment, the thermal compensator includes a control rod that limits the movement of at least a first point on an end plate of the cavity in a first direction. That is, the control rod prevents movement of that point in the direction of thermal expansion. Thus, as the cavity expands, outer portions of the end plate move in the direction of the expansion, but the first point is restricted by the control rod. As a result, the end plate is deformed from its original shape. The control rod has a coefficient of thermal expansion that is significantly different (typically lower) than that of a material from which the cavity is constructed. 
     In one embodiment, the control rod fixes a point on the cavity end plate relative to a different location on the filter. This different location may be such that the control rod spans more thermally expanding material than that which defines the waveguide cavity. In such a case, the thermal expansion causes the point of deflection to be moved relative to its original position. In other words, whereas the deflection point initially resides in a first plane perpendicular to the direction of thermal expansion, the expansion of the thermally expanding material spanned by the control rod forces the deflection point out of its original plane toward an interior of the cavity. In another embodiment, a similar inward movement of the deflection point may be accomplished by using an end deflecting rod that connects the control rod to the deflection point. If the end deflecting rod has a coefficient of thermal expansion that is significantly higher than that of the control rod, its expansion will force the deflection point inward relative to the control rod. Naturally, these two techniques may also be combined. 
     In determining the appropriate amount that a cavity surface point should be deflected, a theoretical model may be used to first establish how far a movable end plate would have to be moved to compensate for an expansion of the waveguide cavity without the end plate being distorted. The resulting deflection distance may then be augmented to compensate for the fact that, in the present invention, the entire surface is not being moved. This additional deflection may be determined empirically, and can provide a more accurate compensation for control of the cavity resonant frequency. 
     In a preferred embodiment, the waveguide cavity is one of two cavities, which are coupled together via an iris plate. Each of the cavities may be thermally compensated in the manner described herein. One particularly preferred embodiment is a six section filter consisting of two thermally compensated waveguide cavities, each with two orthogonal resonant modes, and two coaxial resonators, each coupled to one of the waveguide cavities via an impedance inverter. The signal to be filtered is input through one of the coaxial resonators to one of the waveguide cavities and output through the other coaxial resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a bandpass filter according to the present invention. 
         FIG. 2  is a cross sectional perspective view of the filter of FIG.  1 . 
         FIG. 3  is a schematic model useful in making a determination of deflection distance for thermal compensation of the filter of FIG.  1 . 
         FIG. 4  is a cross-sectional side view of the filter of  FIG. 1  in a high temperature state. 
         FIG. 5  is a perspective view of an alternative embodiment of the invention in which additional deflection of the filter cavity end plates is provided using extension disks. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Shown in  FIG. 1  is a perspective view of a temperature compensated pseudo-elliptical function mixed mode bandpass filter  10 . The filter of  FIG. 2  is particularly suitable for high power broadcast applications, and has an aluminum TE 11  cavity consisting of cavity portion  12  and cavity portion  14 . The filter  10  also has an input stage  16  containing a coaxial resonator, and an output stage  18  containing a coaxial resonator. The filter uses a set of thermal control rods to control the position of the center point of each of two cavity end plates  22  relative to an opposite end of the cylindrical cavity housing. This causes the end plates  22  to deflect when the aluminum cavity housing expands, thereby minimizing thermal drift of the filter pass band due to dimensional changes of the filter cavities. 
     The filter  10  is shown in cross section in  FIG. 2. A  coaxial cable (not shown) is connected to filter input stage  16  to allow signal input to the filter. Likewise, the filter output is directed to a coaxial cable (not shown) via output stage  18 . Each of the input and output stages consists of a respective TEM coaxial resonator  24 ,  26 . The coaxial resonators use inner conductors of a material with a low coefficient of thermal expansion, such as Invar, to provide them with good temperature stability. That is, the use of Invar inner conductors gives the TEM resonators good dimensional stability, and therefore good frequency stability, with changes in temperature. The input coaxial resonator  16  also uses an impedance inverter  28  for coupling into the waveguide cavity. Likewise, the output coaxial resonator uses an impedance inverter  30  for coupling out of the cavity. 
     Impedance inverters are found in most microwave RF filter designs, and not discussed in any great detail herein. In the filter of  FIG. 1 , the impedance inverters have the effect of converting the shunt inductance-capacitance of the each of the coaxial resonators to an inductance-capacitance in series with the waveguide cavity stages. That is, the impedance inverters enable the resonant filter characteristics of the coaxial resonators to be series coupled with the resonant filter characteristics of the waveguide cavity filter stages. Similarly, the iris plate  36  separating the two waveguide cavities functions as an impedance inverter between those two stages. The use of TEM mode coaxial resonators with the dual cavity resonator provides a particular mixed mode that increases the spurious suppression as compared to a filter based on a pure TE 11   n  mode design, since the filter band of each coaxial resonator blocks noise outside of the pass band it defines. 
     As mentioned above, control rods provide thermal stability to the cavity waveguide. In the embodiment of  FIGS. 1 and 2 , the filter includes side bracing control rods  20  and end deflecting rods  23 . Unlike the other components of the waveguide stages, such as the aluminum cavity housing, the side bracing control rods are made of a material having a very low coefficient of thermal expansion, such as Invar. Meanwhile, the end deflecting rods  23  are preferably aluminum, for reasons that are discussed in more detail hereinafter. The control rods  20  and end deflecting rods  23  are arranged in two control assemblies that control the position of the center of each end plate  22  relative to the edge of the cavity housing at the opposite end of the adjacent cavity. 
     As shown, each control assembly has two side bracing rods  20 , each of which is secured at one end by a mounting clip  32  to the edge of the cavity housing. At the opposite end, the bracing rods  20  are fixed to a lateral support  34 . The side bracing control rods  22  each reside within a pair of “pass-through” holes in mounting plates  25 . Mounting plates  25  provide the means by which to fasten the two cavity housings  12 ,  14  together and to secure the iris plate  36  separating the cavities. The center of each of the lateral supports  34  is secured to an end deflecting rod  23  that maintains a fixed distance between its respective support and the center of the adjacent end plate  22 . Thus, a first bracing assembly establishes a bracing frame between the edge of cavity  12  and the center of the end plate of cavity  14 , while the other bracing assembly maintains a bracing frame between the edge of cavity  14  and the center of the end plate of cavity  12 . 
     Because of its relatively small thickness in the axial dimension of the filter (i.e., in a direction parallel to the longitudinal axis of the control rods), the thermal expansion of the lateral supports is negligible for the expected operating temperature range of the filter. Furthermore, the embodiment of  FIGS. 1 and 2  shows only two bracing assemblies of three control rods each. However, those skilled in the art will recognize that additional control rods may be used, if desired. However, the use of only two assemblies helps to minimize the amount of low expansion coefficient material, the cost of which represents a significant manufacturing expense. 
     It is known in the art that the resonant frequency f of a cylindrical TE 11   n  cavity may be expressed as: 
       f   =     c   ⁢             (     x   π     )     2     ⁢     1     D   2         +         (     n   2     )     2     ⁢     1     L   2                   
 
where c is the speed of light, D is the cavity diameter, L is the cavity length, n is the number of half wavelengths that contained in the distance L, and x is a zero of a Bessel function dependent on the mode being considered. For example, if n=1 (i.e., the cavity is a T 111  cavity), x=1.841. It has also been shown that this equation may be differentiated with respect to temperature to give the relationship: 
           1   f     ⁢       Δ   ⁢           ⁢   f       Δ   ⁢           ⁢   T         =       -         (     D   L     )     +       d     Δ   ⁢           ⁢   T       ⁢     (     D   L     )               (     D   L     )     2     +     A   2           -       1   D     ⁢       Δ   ⁢           ⁢   D       Δ   ⁢           ⁢   T               
         where   ⁢     :     ⁢           ⁢   A     =       2   ⁢   x       n   ⁢   π           
 
From this, some of the desired parameters of the waveguide may be determined.
 
     Since the equation above represents the frequency changes in a cylindrical cavity filter with changes in temperature, a stable cavity construction may be determined by setting this equation equal to zero. In other words, when 
             1   f     ⁢       Δ   ⁢           ⁢   f       Δ   ⁢           ⁢   T         =   0     ,       
 
the filter cavity is stable with temperature. By substitution and reduction, the following relationship results: 
           (     1   L     )     ⁢       Δ   ⁢           ⁢   L       Δ   ⁢           ⁢   T         =       -         A   2     ⁡     (     L   D     )       2       ⁢     (     1   D     )     ⁢       Δ   ⁢           ⁢   D       Δ   ⁢           ⁢   T             
 
Notably, the coefficient of thermal expansion for the length of the cavity (CTE L ) is proportional to: 
         (     1   L     )     ⁢       Δ   ⁢           ⁢   L       Δ   ⁢           ⁢   T           
 
and the coefficient of thermal expansion for the cavity diameter (CTE D ) is proportional to: 
         (     1   D     )     ⁢       Δ   ⁢           ⁢   D       Δ   ⁢           ⁢   T           
 
Thus, for a thermally stable cylindrical cavity, the ratio of CTE L  to CTE D  may be expressed as: 
           CTE   L       CTE   D       =     -         A   2     ⁡     (     L   D     )       2           
 
     The relationship above may be used to modify the length of the cavity to compensate for changes in cavity diameter so as to keep the resonant frequency of the cavity stable. A particular cavity design has a predetermined length and diameter, as well as a particular value for each of the mode-specific variables x and n that make up A. Thus, for that cavity, a particular value for the ratio of CTE L  to CTE D  can be found. Given that ratio, one may determine how one of those parameters must be changed relative to the other in order to maintain a stable resonant frequency. This provides the basis for the thermal compensation of the cavity. For example, if a cavity had a diameter D=17″ and a length L=18″, and a value for A of 1.172 (given, e.g., x=1.84 and n=1), then the ratio of CTE L  to CTE D  would be −1.54. Therefore, to maintain the resonant frequency of the cavity, an increase in its diameter must be met with a reduction its length (since the ratio is negative), where the length change has a magnitude of 1.54 times the diameter change. 
     While an adjustment mechanism might be used to physically move one or both of the end plates of the filter cavity in response to changes in its diameter, this would require the use of chokes or “bucket shorts” so that the mechanical changes in the cavity shape could be made. Such movable end plates tend to reduce the performance of the filter, and are therefore undesirable. Therefore, in the present invention, rather than moving the cavity end plates, the cavity shape is deformed to compensate for the frequency shifts. The preferred embodiment accomplishes this by using a combination of materials having different coefficients of thermal expansion in such a way as to force a particular deformation in response to temperature changes. 
     Because of the use of cavity deformation, the mathematical analysis provided above may not apply precisely for temperature compensation. In the preferred embodiment, empirical data is used to augment an initial determination of how the cavity would be modified if a cylindrical shape were maintained. The following example demonstrates such a design, and represents a preferred embodiment of the invention. 
     One prominent area of use for waveguide cavity filters is in broadcast communications. In particular, ultra-high frequency (UHF) channels for digital television (DTV) have frequency allocations in the United States from approximately 473 MHz (channel  14 ) to 749 MHz (channel  60 ). It is known in the art that the optimum Q is achieved in TE 111  mode cavity filters with a D/L ratio of approximately 1 to 3. Given this characteristic, it has been found that reasonable performance may be achieved using a filter cavity having a diameter of 17″ for channels  14  through  40  (frequencies from 473 MHz to 629 MHz). In these filters, the length of the cavity is dependent on the desired center frequency. Similarly, it has been found that a cavity filter having a diameter of 15″ is satisfactory for channels  41 - 60  (frequencies from 635 MHz to 749 MHz). The ranges for desirable filter parameters for UHF communications systems is shown in the following table: 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Channel No. 
                 Frequency (MHz) 
                 Diameter (in.) 
                 D/L 
                 CTE L /CTE D   
               
               
                   
               
             
             
               
                 14 
                 473 
                 17 
                 0.70 
                 −2.80 
               
               
                 40 
                 629 
                 17 
                 1.38 
                 −0.72 
               
               
                 41 
                 635 
                 15 
                 1.11 
                 −1.10 
               
               
                 60 
                 749 
                 15 
                 1.50 
                 −0.62 
               
               
                   
               
             
          
         
       
     
     As shown, the ratios of CTE L  to CTE D  for these filters range from −0.62 to −2.80. Thus, using the formulae above, the change in length to compensate for diametric expansion can be calculated. However, because the preferred embodiment relies on cavity deflection, rather than a movable end plate, an adjustment must be made to the calculated value. 
     The foregoing analysis may be applied to a filter construction as shown in  FIGS. 1 and 2 . In that embodiment, the control rods  20  control the position of the center of one cavity end plate  22  relative to the opposite side of the adjacent cavity  14 . As mentioned previously, the aluminum of the cavity housings and the end deflecting rod  23  has a much higher coefficient of thermal expansion than the Invar, and so each cavity is forced to deform as the temperature increases. The appropriate parameters for constructing a UHF filter according to the embodiment of  FIGS. 1 and 2  may be demonstrated using the model shown in FIG.  3 . 
       FIG. 3  provides a model that corresponds to the design of one of the cavities  12 ,  14  of the filter  10  of  FIGS. 1 and 2 . It will be described in the context of cavity  12  to demonstrate how the different filter components affect the cavity deformation with temperature. As shown in  FIG. 3 , the center point of the model is the iris plate  36 , and it has a fixed position for the purposes of this analysis. The distance l ALUM  corresponds to the length of the aluminum material of the waveguide cavity and the end deflecting rod  23 . The overall length l ALUM  is the sum of l ALUM1 , which is the length of the aluminum housing that affects the end plate, and l ALUM2 , which is the length of the aluminum end deflecting rod  23 . The distance l INVAR  corresponds to the length of the Invar rods  20 . 
     As can be seen from  FIG. 3 , an increase in temperature will cause a thermal expansion in both the aluminum material and the Invar material. However, this expansion will be greater for the aluminum material, since the coefficient of thermal expansion of aluminum is much higher than that of Invar. Indeed, the net change per degree Celsius in the distance between iris plate  36  and the center point of end plate  22  of cavity  12  is may be written as:
 
CTE CP =CTE ALUM −CTE INVAR  
 
To determine an optimum length for the two materials given a filter having a particular center frequency, an approximation is first made using the filter adjustment relationships described above for a cavity in which end plate position may be adjusted without cavity deformation. Known filter parameters are also used, such as those shown above in Table 1, to optimize for the desired frequency. This is demonstrated by the following example.
 
     If a filter having a center frequency of 749 Mhz is desired, a 15″ cavity may be used. From Table 1, the ratio of CTE L  to CTE D  for this frequency is −0.62. Substituting this into the equation above gives the following relationship:
 
−0.62(CTE D )(D)=(CTE ALUM )(l ALUM )−(CTE INVAR )(l INVAR ) 
 
The thermal expansion coefficient for aluminum is CTE ALUM =24.7×10 −6 , while the thermal expansion coefficient for Invar is CTE INVAR =1.6×10 −6 . Since the cavity is aluminum, CTE D =CTE ALUM . The foregoing equation may therefore be written as:
 
−0.62(24.7×10 −6 )(15)=(24.7×10 −6 )(l ALUM )−(1.6×10 −6 )(l INVAR ) 
 
or, if (l alum +L) is substituted for l INVAR ,
 
−0.62(24.7×10 −6 )(D)=(24.7×10 31 6 )(l ALUM )−(1.6×10 −6 )(l ALUM +L)
 
Given the D/L ratio from table 1, L=10 may be used, and the equation solved to give a value of l ALUM =9.25. For an initial cavity length L=10, this corresponds to an Invar rod length of l INVAR =19.25.
 
     These values could be used in the filter of  FIG. 1  to provide an approximate solution for thermal compensation. However, as discussed above, the filter of  FIG. 1  does not use an end plate that moves in its entirety, and does not maintain the cylindrical shape of the cavity. Instead, to make the filter simpler and less costly to manufacture and to prevent degradation of the filter Q, the end plate  22  of cavity  12  is allowed to deform in a concave manner. Experimentation has shown that, for the filters having center frequencies in the UHF range, an additional 15% deflection of the end plate  22  of cavity  12  increases the accuracy of the compensation, and provides the resonance frequency with better stability. 
     As mentioned above, the present invention currently makes use of some empirical steps in determining an appropriate degree of deformation to be applied to the cavity end plate. The formulaic method above may be used to determine what an appropriate adjustment to the position of the end plate would be if no deformation of the surface was taking place. This provides a cavity parameter, in this case length, that serves as a starting point for determining the appropriate degree of cavity deformation. Thereafter, heating of the cavity and minor adjustment in the deformation, combined with measurement of the filter characteristics, allow fine-tuning of the degree of deformation. Given the description herein, such modifications are well within the ability of one skilled in the art. An example of this process is described below. 
     After determining an initial deflection amount from the formulae, a low power signal from a network analyzer is input to one port of the filter, and received at the other port. The scattering parameters (“S-parameters”) and temperature of the filter are then measured and recorded. From the S-parameters, the center frequency is found and recorded. The filter unit is then heated in a chamber in order to obtain a change in temperature. Once the frequency response and temperature of the filter have stabilized, the S-parameters and filter temperature are again recorded. At this point, the resonant frequency of the filter will have drifted down a small amount. To compensate, the value of l ALUM  is increased relative to l INVAR . To increase l ALUM , the length of the end deflecting rod  23  may be increased. Alternatively, the length of the invar rods  20  may be increased. This has the same effect, since the larger the distance between the end plate being deflected and the opposite connection point of the rods  20  on the housing, the more length of the aluminum housing there is to move the outer portions of the end plate as it expands. 
     By readjusting the length of l ALUM  relative to l INVAR  according to the measured resonant frequencies at different temperatures, the optimum length may be determined. As mentioned, for the embodiment above, this required an additional 15% deflection of the end plate. However, those skilled in the art will recognize that for other filter dimension, resonant frequencies, or even types and locations of cavity deformation, different degrees of variation may apply. Nevertheless, by applying empirical modifications, as described above, to a theoretically ideal surface movement model, the appropriate filter characteristics may be achieved. 
     In one variation of the preferred embodiment, the effective length of l ALUM  is increased by attaching an extension, such as a disk, to the outside of the end plate being deflected. For example, as shown in  FIG. 5 , disk  38  may be used to increase the degree of deflection provided to the end plate  22 . The magnitude of this increase may be controlled through selection of the material used for disk  38 . For example, in the embodiment of  FIG. 5 , the disk  38  may be made of aluminum. In such a case, the thermal expansion of the disk would result in a much higher deflection of the end plate  22  for a given temperature than it would if it was made of a material having a lower coefficient of thermal expansion. Naturally, selection of the disk material, given the foregoing description, is well within the ability of those having ordinary skill in the art. 
     While the invention has been shown and described with regard to a preferred embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.