Multi-filter device and method of making same
FIELD OF THE INVENTION 
     This device relates generally to multi-filter devices, and particularly to ceramic block filters. 
     BACKGROUND OF THE INVENTION 
     Multi-filter devices are well understood in the art. For example, many two-way radios, including cellular telephones, utilize ceramic block duplex filters. Such ceramic block filters typically provide both a transmitter and receiver filter function. The passband for the transmit filter can be, for example, 824-849 MHz in a cellular telephone. The receive passband for a corresponding receiver filter can be 869-894 MHz. 
     Typically, during the manufacturing process, such ceramic filters must be milled during a coarse tuning process. Many times, this milling process is accomplished through use of a double-sided lap mechanism. When using this process, material is removed from two opposing sides of the ceramic filter simultaneously. This milling process compensates for shifts in the dielectric material itself, and effectively moves the passbands of the filter. 
     Unfortunately, to date, double-sided lapping techniques will cause both filter passbands in a multi-filter device to be simultaneously changed, but the degree of change in each filter will be different. This can render the coarse tuning process more difficult, and can considerably complicate the fine tuning process that follows. 
     Accordingly, a need exists for a way to better allow double-sided lapping processes to be utilized when coarse tuning a multi-filter ceramic block. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 comprises a side elevational view of a prior art multi-filter ceramic block; 
     FIG. 2 comprises a graph depicting frequency characteristics of a prior art multi-filter ceramic block during a double-sided lap procedure; 
     FIG. 3 comprises a side elevational view of a multi-filter ceramic block as configured in accordance with the invention; 
     FIG. 4 comprises a graph depicting frequency characteristics of a multi-filter ceramic block as configured in accordance with this invention during a double-sided lap procedure; 
     FIG. 5 comprises a prospective view of an alternative embodiment of a multi-filter ceramic block in accordance with the invention; 
     FIG. 6 comprises a block diagram of a radio that makes use of a multi-filter device in accordance with the present invention. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     Prior to describing an embodiment of the present invention, it will be helpful to the reader to first understand in more detail the relevant prior art. Referring to FIG. 1, a prior art multi-filter ceramic block (10) includes a first filter A and a second filter B. The first filter A has a first surface (11) that is offset from an opposing surface (12) by amount X. Similarly, filter B has a first surface (13) that is offset from the same opposing side (12) by amount Y, which is less than amount X. In effect, the first surface (13) of filter B comprises a notch formed in the ceramic block (10). Each filter A and B then includes a plurality of cavities (14) that extend from the opposing surface (12) to the appropriate corresponding filter surface (11 and 13, respectively). 
     The above described ceramic block configuration can be formed through a variety of well known and understood prior art techniques. Hence, the precise method of forming this block will not be described here. 
     The ceramic block, when completed, should preferably have final desired operating characteristics. In this particular example, the desired operating characteristics are the two passbands as described earlier. Depending upon the precise characteristics of the dielectric material that comprises the ceramic block, however, the ceramic block as initially formed will not yield the desired final operating characteristic. Therefore, those skilled in the art will mill material away from the ceramic block. 
     When using a double-sided lapping technique, material will be removed from both the opposing side (12) and the first surface (11). It should be clear from the figure that material will be removed from the bottom of the block in a way that will simultaneously effect the length of the cavities (14) for both filter A and filter B. Hence, the frequency characteristics of both filters will be simultaneously effected. The milling process on the opposing side, however, will not initially impact both filters. Rather, the filter A surface (11) will be subjected to milling prior to the filter B surface (13). As a result, when double-sided lapping, filter A will be modified at a different rate than will filter B. 
     This dichotomy is depicted in FIG. 2, where it can be seen that the original frequency characteristic of filter A moves, as a function of milling, to a new frequency characteristic A&#39; as a relatively linear function. Filter B, however, moves initially at a slower rate of change because material effecting its performance is only being removed from one side. As a result, while the initial difference in frequency between the two filters is originally depicted as Z, the final difference in frequency is depicted as Z&#39;, which is less than the original difference in frequency between the two filters. 
     It can therefore be seen that coarse tuning such a device with double-sided lapping is made more difficult because the characteristics of the two filters change at different rates. 
     Referring now to FIG. 3, a first embodiment of the present invention will be described. A ceramic block (10) is again provided, which block (10) includes a first filter A and a second filter B, wherein both filters have a plurality of resonator cavities (14) that extend between the opposing surface (12) and the filter A and B surfaces (11 and 13). In this embodiment, however, both of the filter A and B surfaces (11 and 13) comprise notches that have been formed in the side (31) that comprises the upper surface in this depiction. So configured, filters A and B are flanked by portions (32) of the ceramic block (10) that extend outwardly further than do the upper surfaces (11 and 13) of either filter A or B. 
     Because of this configuration, when double-sided lapping is utilized to mill the ceramic block (10) both filters A and B change their frequency response at the same rate. This occurs because milling on the common opposing surface (12) affects both filters in a similar manner. Milling on the upper portion (32) affects neither filter A or B because both filters A and B have individual frequency responses that are substantially independent of the upper portion (32). As a result, and referring to FIG. 4, it can be seen that the original difference in frequency between the two filters (depicted here as Z) remains constant throughout the milling process. Of course, if the milling process were to continue such that milling began to occur on the filter A surface (11), then a differing rate of response alteration would of course result. By providing an appropriate amount of flanking material (32), however, this potential situation can be readily and easily avoided and an appropriate multifilter device (30) provided. 
     Referring now to FIG. 5, a second embodiment can be seen to include a first filter A having a plurality of notches (11) and a second filter B having a second plurality of notches (13). In this embodiment, block extension material (32) flanks each of the cavities (14). Depending upon the dimensions involved, such an embodiment may provide a sturdier construction. 
     Those skilled in the art will recognize that various alterations to the embodiments described could be made. For example, the notches that make up a particular filter need not all be of the same depth. Instead, they could be of different depths. 
     Referring now to FIG. 6, a radio that makes use of the multi-filter device (30) described above can be seen as generally depicted by reference numeral 60. This radio (60) includes an antenna (61) that couples to the multi-filter device (30) in accordance with well understood prior art technique. Whichever of the filters in the multi-filter device that comprises a receive filter in this particular application couples to a demodulator (62). (If desired, and depending upon the particular radio configuration used, the filter output may first pass through an intermediate frequency stage (63) as well understood in the art.) The output (64) of the demodulator (62) of course comprises the information output as well understood in the art. The radio (60) also includes a modulator (65) that receives information from an information input (66) (such as voice or data to be transmitted) and modulates this information on to a carrier as obtained from a carrier source (67), all as well understood in the art. The output of the modulator (65) then couples to a transmitter (68), which transmitter (68) couples to whichever of the filters in the multi-filter device (30) is serving as a transmit filter. 
     The above elements of a radio are well understood, and hence no further description need be provided. So configured, it should be appreciated that the multi-filter device (30) as disclosed herein can be provided for use in such a radio, which filter (30) can either be more appropriately tuned than prior art devices, or that can be equally as well tuned with less difficulty during the manufacturing process. So configured, the applicant has provided a multi-filter device that is amenable to existing double-sided lapping processes while simultaneously ensuring a more accurately controlled coarse tuning process.