Abstract:
A method of tuning the frequency response of filters embedded in or formed on a ceramic substrate, such as but not limited to a low temperature co-fired ceramic substrate (LTCC), by re-firing a previously fired LTCC substrate to a temperature which is greater by a predetermined, relatively small, amount than that of the temperature produced during the original firing profile of the substrate so as to change the dielectric constant of the substrate, and thus cause a desired shift in the filter&#39;s frequency response.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to microwave filters embedded in a co-fired ceramic substrate, and more particularly to a method of tuning the center frequency of filters embedded in a low temperature co-fired ceramic (LTCC) substrate. 
     DESCRIPTION OF RELATED ART 
     The center frequency of a filter embedded in co-fired LTCC is known to vary depending upon the dielectric constant of the fired substrate.  FIG. 1  depicts three sets of simulated characteristic curves which correspond to the expected response of a filter embedded in an LTCC substrate and whose dielectric constant ∈ r  is varied over the range 5.85 to 6.15. As is well known, the center frequency F 0  is inversely proportional to the square root of the dielectric constant ∈ r , i.e., F 0 =1/√{square root over (∈ r )}. In  FIG. 1  two sets of characteristic curves S 21  and S 11  are shown for three values of dielectric constant, i.e., where ∈ r1 =5.85, ∈ r2 =6.0 and ∈ r3 =6.15. The three curves labeled S 21  (input at port  1 , output at port  2 ) depict a measure of filter insertion loss vs. frequency while the second set of curves S 11  (input at port  1 , reflected signal at port  1 ) depicts the amount of signal reflected back to the input port for each frequency and thus is a measure of return loss. 
     In each instance it can be seen that the center frequency F o  is higher for a dielectric constant of ∈ r1 =5.85, relatively lower at ∈ r2 =6.0, and lowest at ∈ r3 =6.15. Thus it can be seen in  FIG. 1  that with all other parameters unchanged, for a filter designed with a center frequency F o =10 GHz, varying the dielectric over the range ∈ r =6.0±0.15 produces a variation in center frequency of ±1.3%. For a center frequency F o  of 10 GHz as shown in  FIG. 1 , a shift of ±130 MHz would be expected in all filters embedded in an LTCC substrate. 
     It should be noted that 1.3% variation in center frequency as shown in  FIG. 1  is greater than that seen in typical combline tuned filter(s) obtained, for example, from a filter manufacturer. Accordingly, the cost of carefully tuned filters is higher due to the labor required to perform the tuning. Furthermore, the relatively large variation in center frequency in LTCC embedded filters currently limits their usage to applications which do not require precise control of the center frequency. 
     The dielectric constant of LTCC green tape cannot be accurately controlled by the manufacturers of the tape, since the fired dielectric constant depends on many variables encountered during the subsequent processing of an LTCC substrate. Currently, LTCC filters are typically being utilized in systems for applications that do not require precise filtering characteristics, such as image rejection filters or local oscillator signal filters. Furthermore, the filter bandwidth is normally designed to be much wider than the bandwidth of the signal of interest. Thus, even after the expected variation of the embedded filter, the signal will always fall within the pass band of the filter. 
     For narrow band applications where system requirements call for rejection very close to a specified pass band, a method for tuning the filters after firing is needed. One approach that has been proposed in the past is to place several filter designs that were purposely designed with offset center frequencies side by side on the same LTCC panel. Once the substrates are fired, the filters are tested and those that are closest to the desired center frequency are selected, while the rest are discarded. While this approach may be acceptable for some applications, there is a large yield penalty that increases the final cost of the filters. Another disadvantage is the large number of filter designs that would be required for a filter bank. 
     SUMMARY 
     Accordingly, it is an object of the present invention to provide a method of tuning the frequency response of a microwave filter. 
     It is another object of the invention to provide an improved method of tuning the frequency response of a microwave filter embedded in or formed on a ceramic substrate. 
     It is yet another object of the present invention to provide a method for accurately tuning the center frequency of an embedded LTCC filter. 
     These and other objects are achieved by a method of tuning the frequency response of filters embedded in or formed on a ceramic substrate, such as LTCC and/or HTCC, by re-firing a previously fired substrate to a temperature which is greater by a predetermined, relatively small, amount than that of the temperature used during the original firing profile of the substrate so as to change, for example, decrease the dielectric constant of the substrate, and thus cause a desired shift, for example, upward in the frequency response. In one aspect of the inventive method, a ridge waveguide bandpass filter embedded in a multi-layer LTCC substrate can be tuned to a higher center frequency by re-firing the substrate to a temperature above the initial firing temperature. 
     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific examples while indicating the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present inventive method will become more fully understood from the detailed description provided hereinafter, and the accompanying drawings, which are provided by way of illustration only, and thus are not meant to be limitative of the method, and wherein: 
         FIG. 1  is a set of characteristic curves illustrative of a simulated frequency response obtained from the same filter with varying dielectric constants; 
         FIG. 2  is a top plan view of an embedded ridge waveguide filterbank including five individual ridge waveguide filters implemented in a multi-layer LTCC substrate; 
         FIG. 3  is a perspective view further illustrative of a single embedded ridge waveguide filter of the type included in the filter bank shown in  FIG. 2 ; 
         FIG. 4  is a plot illustrative of the relationship of dielectric constant as a function of re-firing temperature for a plurality of one particular type of LTCC substrates; and 
         FIGS. 5–8  depict the measured S-parameters of an embedded filter of the type shown in  FIG. 3  tuned to a different center frequency by re-firing the LTCC substrate with different peak profile temperatures as shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the remaining  FIGS. 2–8 ,  FIG. 2  is illustrative of a filter bank  10  including five discrete embedded ridge waveguide bandpass filters  12   1 ,  12   2 ,  12   3 ,  12   4 ,  12   5 , for implementing five separate and distinct sub-bands, arranged side by side and embedded within a common LTCC substrate  14 . Reference numeral  16  denotes a metallization pattern formed on the top surface of the substrate  14  for separating the five embedded filters  12   1  . . .  12   5  and a bypass signal path  18 . A stripline track  20  connects an input port p 1  to one terminal  22  of a single pole six terminal (SP6T) switch  24 . One of the other switch terminals connects to a stripline track at the top of the filterbank which serves as the signal by-pass path  18  with no inherent filtering. The remaining terminals of the switch  24  connect to stripline tracks  26   1  . . .  26   5  which connect to respective input ports of the five filters  12   1  . . .  12   5 . The output ports of the filters  12   1  . . .  12   5  connect to respective stripline tracks  28   1  . . .  28   5  which connect to separate terminals of a second single pole six terminal switch  30  which has one terminal  32  connected to an output port p 2  via a stripline track  34 . 
       FIG. 3  is illustrative of a single embedded ridge waveguide bandpass filter of the type included in the substrate  14  shown in  FIG. 2 . An embedded ridge waveguide structure is well known in the art and typically includes a plurality of ridged waveguide sections, for example, sections  36   1  . . .  36   n  whose side walls are implemented with vias in a substrate formed of multiple layers of LTCC, and with the top and bottom walls of the waveguide sections being comprised of solid metal ground planes printed on the LTCC substrate. The ridge waveguide sections  36   1  . . .  36   n  are appropriately spaced so as to provide an evanescent mode bandpass filter having a predetermined frequency response. Stripline circuits  40  and  42  are further provided at either end of the ridge waveguide sections  36   1  . . .  36   n  for implementing an impedance match to circuitry, not shown, to which it is to be connected. 
     This invention is directed to a method of tuning the center frequency of filters embedded in dielectric material, such as LTCC, such as shown in  FIGS. 2 and 3  by re-firing the previously fired (solid) LTCC substrate to a temperature which is above the temperature achieved during the original firing profile. 
     Typically, the ceramic in LTCC tape is a calcium boro-silicate crystallizing glass ceramic. The sintering of this material occurs in two stages. Viscous sintering occurs first to form a dense ceramic followed by a crystallization of two main phases CaSiO3 and CaBxOy. The material is reported to remain consistent through refires at or below the original firing temperature. Heretofore, there was no apparent need for re-firing at temperature(s) above the original firing temperature following initial fabrication and therefore the effect of re-firing at elevated temperatures was of no concern. However, it was discovered by the subject inventors that when re-firing was performed at temperatures above the original firing temperatures, such a procedure would result in further crystallization of the glass ceramic, resulting in a change in dielectric constant and/or density, and as such could be utilized to selectively tune the center frequency of LTCC embedded filters. 
       FIG. 4  is a graphical representation of a change in the value of the dielectric constant Er when at least four LTCC samples of Ferro A6 were refired from an original firing temperature of 840° C. It can be seen that re-firing at temperatures of 850, 870, 890 and 910° C. resulted in the lowering of the respective dielectric constant. It should be noted that these temperatures are specific to one type of LTCC material, i.e. Ferro A 6  and are different for other types of LTCC and for HTCC. Furthermore, it was found that the change in dielectric constant is not dependent upon hold time, but only on the peak temperature achieved during the re-firing process. 
       FIGS. 5–8  are characteristic curves  44  and  46  illustrating measured S-parameters of an LTCC ridge waveguide bandpass filter before and after re-firing of an x-band filter embedded in an LTCC substrate. S-parameters are well known parameters used by microwave designers to quantify network responses and in this case, where there is a two port device, there are four S-parameters which are defined as follows: S 12  is a measure of the response with voltage incident at port  2 , while measuring the output voltage at port  1 ; S 21  is a measure of the voltage incident into port  1  while measuring the output at port  2 ; S 11  is the measure of the response with voltage incident at port  1  and the reflected voltage is measured at port  1 ; and, S 22  is a response of the response with voltage incident at port  2  and the reflected voltage is also measured at port  2 . 
       FIG. 5  is illustrative of the S 12  frequency response,  FIG. 6  depicts the S 21  frequency response,  FIG. 7  depicts the S 11  frequency response and  FIG. 8  depicts the S 22  frequency response of the same bandpass filter. 
     The measured center frequency F o1  following a first firing is shown to be about 8566 MHz. This is shown clearly in the S 21  frequency response of  FIG. 6 . When the substrate containing the filter was refired at 900° C., the center of frequency F o2  was measured again after cooling and found to be about 8703 MHz, indicating that the re-firing process caused a shift of the center of frequency of the filter up by approximately 136 MHz. 
     In addition to the filter whose characteristics are shown in  FIGS. 5–8 , four other filters embedded in the same LTCC substrate as shown, for example, in  FIG. 2  were subjected to re-firing. Table 1 below summarizes the data measured from the same five filter configurations. In Table 1, the column “Filename” merely indicates an assigned name for each of the filters for the initial firing and the re-firing, along with the measured change (deltas). The adjacent column “IL,min” is the minimum S 21  insertion loss in the pass band of the filter. The column “Fo, MHz” is the center frequency. 
     The next column “BW, 3 dB” is the bandwidth measured at 3 dB down from IL,min and the column “fl, 3 dB”, is the frequency on the low side of the pass band, where S 21  is 3 dB down from IL, min. The column “fh, 3 dB”, is the frequency on the high side of the pass band where S 21  is 3 dB down from IL, min. 
     Next, the column “BW, 20 dB” is the bandwidth of the filter measured from the 20 dB points. The column “fl, 20 dB” corresponds to the frequency on the low side of the pass band where S 21  is 20 dB down from IL, min, and, “fh, 20 dB” is the frequency on the high side of the pass band where S 21  is 20 dB down from IL, min. 
     It should be noted that the measured S 21  data of the third filter design identified by the file name P5tkch3.s2p corresponds to the characteristic curves shown in  FIGS. 5–8 . In all instances, the center frequency F o  shifted upward upon re-firing as shown by the positive deltas in the “F o , MHz” column of Table 1. In all instances, the insertion loss also improved upon re-firing as shown by the positive deltas in the “IL, min” column in Table 1. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Filename 
                 IL, min 
                 Fo, MHz 
                 BW, 3 dB 
                 fl, 3 dB 
                 fh, 3 dB 
                 BW, 20 dB 
                 fl, 20 dB 
                 fh, 20 dB 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 P5tkch1.s2p 
                 −1.3 
                 7327.1 
                 881.6 
                 6886.3 
                 7767.8 
                 1286.9 
                 6670.3 
                 7957.2 
               
               
                 refire2p5_ch1.s2p 
                 −1.2 
                 7435.3 
                 901.4 
                 6984.6 
                 7886.1 
                 1302.4 
                 6770.4 
                 8072.8 
               
               
                 Deltas 
                 0.1 
                 108.2 
                 19.8 
                 98.3 
                 118.3 
                 15.5 
                 100.1 
                 115.6 
               
               
                 P5tkch2.s2p 
                 −1.4 
                 7941.6 
                 887.6 
                 7497.8 
                 8385.5 
                 1274.5 
                 7305.6 
                 8580.1 
               
               
                 refire2p5_ch2.s2p 
                 −1.3 
                 8067.9 
                 907.9 
                 7613.9 
                 8521.8 
                 1289 
                 7423.3 
                 8712.2 
               
               
                 Deltas 
                 0.1 
                 126.3 
                 20.3 
                 116.1 
                 136.3 
                 14.5 
                 117.7 
                 132.1 
               
               
                 P5tkch3.s2p 
                 −1.6 
                 8566.3 
                 893.7 
                 8119.5 
                 9013.2 
                 1306.2 
                 7899.5 
                 9205.7 
               
               
                 refire2p5_ch3.s2p 
                 −1.4 
                 8702.6 
                 917.1 
                 8244.1 
                 9161.2 
                 1318.9 
                 8027.9 
                 9346.8 
               
               
                 Deltas 
                 0.2 
                 136.3 
                 23.4 
                 124.6 
                 148 
                 12.7 
                 128.4 
                 141.1 
               
               
                 P5tkch4.s2p 
                 −2.2 
                 8998.6 
                 697.8 
                 8649.7 
                 9347.5 
                 1111.9 
                 8484.8 
                 9596.7 
               
               
                 refire2p5_ch4.s2p 
                 −2.1 
                 9135.9 
                 701.1 
                 8785.4 
                 9486.5 
                 1138.2 
                 8620.7 
                 9759 
               
               
                 Deltas 
                 0.1 
                 137.3 
                 3.3 
                 135.7 
                 139 
                 26.3 
                 135.9 
                 162.3 
               
               
                 P5tkch5.s2p 
                 −3.6 
                 9689 
                 186.1 
                 9596 
                 9782 
                 1076.2 
                 9242.2 
                 10318.4 
               
               
                 refire2p5_ch5.s2p 
                 −3.2 
                 9837.4 
                 179 
                 9747.9 
                 9926.9 
                 1077.3 
                 9397 
                 10474.4 
               
               
                 Deltas 
                 0.4 
                 148.4 
                 −7.1 
                 151.9 
                 144.9 
                 1.1 
                 154.8 
                 156 
               
               
                   
               
             
          
         
       
     
     Thus what has been shown is a method of shifting the center frequency of a microwave filter embedded in a multi-layer ceramic substrate, such as LTCC, by re-firing the substrate containing the filter to a higher temperature following initial fabrication. It should be noted that this method is not limited to filters embedded in LTCC, but also applicable to HTCC filters. It is also applicable to any stripline filters embedded in a multilayer ceramic substrate as well as microstrip filter structures printed on a single layer of ceramic substrate. This method is further applicable to high pass or low pass filters wherein tuning comprises tuning the cutoff frequency of the filter. 
     The inventive method being thus described, it will be obvious that it may be varied in a variety of ways. Such variations, however, are not to be regarded as a departure from the spirit and scope of the invention. Accordingly, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.