Patent Publication Number: US-2005122190-A1

Title: VHF band pass filter built with ceramic coaxial resonator

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a transmission line filter and more particularly to a tunable transmission line band pass filter, suitable for use in the VHF range. The transmission line can be coaxial cable, microstrip line or strip line, which provides relatively high-Q. The transmission line along with a capacitor (shunt or in series) forms a resonator.  
      2. Description of the Prior Art  
      Microwave filters are generally known in the art. Examples of such microwave filters are disclosed in U.S. Pat. Nos. 4,180,787; 4,352,076; 4,578,656; 4,641,116; 4,839,617; 5,066,933; 5,021,757; 5,241,291; 5,392,011; 5,485,131 and 5,138,288. Some known microwave filter circuits, for example, band pass filter circuits, are known to include a pair of resonators and a tuning element disposed between the resonators to define the pass band. Examples of such microwave filters are disclosed in U.S. Pat. Nos. 4,352,076; 4,578,656; 4,641,116 and 4,839,617.  
      Various devices are used to provide tuning of the filter circuit. For example, U.S. Pat. Nos. 5,021,757, 5,138,288 and 5,241,291 disclose the use of varactors for tuning the resonators. The performance of such varactor-tuned resonators (VTR) is also discussed in an article entitled, “Modeling Varactor Tunable Transmission Line Resonators for Wireless Applications” by Boris Kapilevich and Roman Lukyanets,  Applied Microwave and Wireless , pages 32-34, September 1988.  
      In known band pass filters; two resonators are known to be coupled by way of a varactor. In such filters, the zero transmission frequency (i.e. frequency at which the RF energy transmission is a minimum) is tuned to exclude frequencies outside the pass band. In particular, the varactors are used to control the capacitance coupling between the resonators. By controlling the voltage applied to the varactors, the center frequency of the filter can be controlled thus providing the ability to tune the filter.  
      In the VHF range, relatively high-Q components are required. As such, resonators, which use discrete inductors, for example, as disclosed in U.S. Pat. Nos. 4,180,787, 4,839,617 and 5,392,011, are unsuitable, since such resonators include discrete inductors, which require a sufficiently high-Q. As such, for VHF frequencies, filters are known to use transmission line (or strip line) components formed on relatively expensive PC boards, such as Duroid. Examples of such filters are disclosed in U.S. Pat. Nos. 4,352,076; 4,578,656; 5,021,757; 5,066,933; 5,138,288 and 5,241,291. Duroid PCB (printed circuit board) significantly increases the cost of the device. Quarter wavelength Hi-Q ceramic coaxial resonators are also known to be used in filter circuits. Unfortunately, the ceramic λ/4 and λ/2 resonators in the VHF frequency range are not available for frequencies below 400 MHz because of physical size.  
      U.S. Pat. No. 5,484,131 discloses a transmission line, which includes a pair of generally parallel transmission lines capacitively coupled together. One end of each of the transmission lines is directly coupled to ground. An opposing end of each of the transmission lines is coupled to ground by way of a capacitive coupling. Unfortunately, for VHF frequencies, the physical size of the transmission lines required would be too large for the device to be practical. As such, there is a need for a relatively low cost microwave filter for use at VHF frequencies, which eliminates the need for expensive ceramic substrates.  
     SUMMARY OF THE INVENTION  
      Briefly, the present invention relates to a microwave filter for use at VHF frequencies which utilizes a coaxial resonator. In accordance with another aspect of the invention switched capacitor arrays are used in place of the conventional varactors diode to achieve a high IP3 and digital control without a D to A converter. As such, a microwave filter, suitable for VHF frequencies, is provided using relatively low cost off-the-shelf ceramic coaxial resonators which can be used to build a relatively low insertion loss high rejection band pass filter in the frequency range of 100 MHz to 200 MHz. 
    
    
     DESCRIPTION OF THE DRAWINGS  
      These and other advantages of the invention will be readily understood with reference to the following specification and attached drawings wherein:  
       FIG. 1  is a block diagram of the filter in accordance with the present invention.  
       FIG. 2  is a schematic diagram of the capacitor array, grounding switch array and digital interface illustrated in  FIG. 1 , which forms a part of the present invention.  
       FIG. 3  is a schematic diagram of an alternate embodiment of the invention.  
       FIG. 4  is a graphical illustration of the measured S 21  parameter as a function of frequency for various combination of digital input, which changes the capacitance values.  
       FIG. 5  is a plot of test result of a passband filter built as illustrated in  FIGS. 1 and 2 . 
    
    
     DETAILED DESCRIPTION  
      The present invention relates to a band pass microwave filter formed from off-the-shelf low cost components, which provides a relatively high-Q and low insertion loss in the VHF frequency range of 100 MHz to 200 MHz. An important aspect of the invention is that a ceramic coaxial resonator (as opposed to an λ/4 or λ/2 resonator) is used in place of known microstrip and strip line resonators in known VHF band pass filters. As is known in the art, such coaxial resonators  38  normally have high-Q. An alternate embodiment of the invention is shown in  FIG. 3 , in which a single large value fixed capacitor  72  replaces the resonators. In accordance with another important aspect of the invention, switched capacitors, as shown in  FIG. 2  may be utilized in place of the varactors for tuning the resonator.  
      Turning to  FIG. 1 , a tunable VHF band pass filter, generally identified with the reference numeral  20 , is illustrated which provides low insertion loss in the pass band and high rejection ratio outside of the pass band. The band pass filter  20  is formed from a resonator, shown within the dashed box  22 , which includes a pair of transmission lines  40  and  42 , a coaxial resonator  38 , one or more capacitor arrays  44 ,  46  and  48  as well as a grounding switches array and digital interface  50 . The filter  20  also includes a pair of bidirectional input/output ports, represented by the arrows identified with the reference numerals  26  and  28 . The bidirectional input/output ports  26  and  28  are capacitively coupled to the filter  20  by way of a pair of capacitors  30  and  32  respectively. A pair of transmission lines  34  and  36 , for example, 9.4 Ω, 30° transmission lines, are provided for synthesizing transmission zeros, such that the S-parameter S 11 =0. In particular, the capacitors  30  and  32  are placed in series with the transmission lines  34  and  36  for impedance matching and for narrowing the bandwidth. The characteristic impedance Z 0  of transmission line  34 ,  36  is quite low as compared to that of the transmission line  40 ,  42 , effectively shortening the length of the transmission lines  34  and  36 . As shown in  FIG. 1 , the two high impedance transmission lines  40  and  42  are coupled together and to the coaxial resonator  38 .  
      The coaxial resonator  38  may be an off-the-shelf ceramic coaxial resonator with a relatively high-Q and an electrical wavelength as small as 20°, for example as manufactured by Trans-Tech, Inc, Adamstown Md., part number SR9000SPH037SBY. The transmission lines  40  and  42  may be a pair of relatively high characteristic impedance Z 0  transmission lines, which can be a 100 Ω or higher Z 0  cable, about 40 mm, for example, part number R-1200-N37-0411-OA, available from SMI Electronic Devices of America, Inc, Hollister, Calif.  
      Rather than using varactor diodes for tuning the filter as is known in the art, a plurality of switched capacitor arrays  44 ,  46  and  48  are provided which, in turn, are coupled to a grounding switch array and a digital interface  50 . This PIN diode-switching scheme will improve the third order intercept (IP 3 ) to +50 dBm making the filter  20  suitable for high power transmitter use.  
      The configuration in accordance with the invention provides a transmission zero at about 10 MHz below the low turning point; see  FIG. 5 , Marker  1 . This is important because it provides more than 41 dB rejection at 108 Mhz, the frequency of VHF TV stations. Also it helps shaping the Q of the filter in the aviation band.  
      The capacitor arrays  44 ,  46  and  48  together with the grounding switch array and digital interface  50  are used to tune the resonator  22 . In particular, the capacitor arrays  44 ,  46  and  48  together with grounding switch array and digital interface  50  form a switched capacitor circuit whose capacitance varies as the control signals applied to the grounding switch array to enable the center frequency of the resonator  22  to be tuned to select a particular pass band for the filter  20 .  
      One capacitor array  44  is connected at a node defined between the transmission lines  34  and  40  and ground. Similarly, the capacitor array  46  is connected between the node between transmission lines  36  and  42  and ground. The capacitor array  48  is serially coupled to the coaxial resonator  38 , which, in turn, is coupled between a node defined between the transmission lines  40  and  42 . The capacitor array  48  is also connected to ground.  
      A schematic diagram of the capacitor arrays  44 ,  46  and  48 , as well as the grounding switch array and digital interface  50 , is illustrated in  FIG. 2 . It is to be understood that the specific values and configuration illustrated in  FIG. 2  are merely exemplary and that other component values and configurations are within the broad scope of the invention. Referring back to  FIG. 1 , the capacitor array  44  includes a fixed capacitor C  188  as well as four switched capacitors C  169 , C  170 , C  171  and C  172 . As shown, exemplary component values for the capacitors C  169 , C  170 , C  171  and C  172  are provided as 2 pF, 4 pF, 8 pF and 16 pF, respectively. The capacitor array  46  is similar.  
      The capacitor array  48  includes a fixed capacitor C  187  and includes the parallel capacitors C  175 ; C  176 ; C  177  and C  178 . Based upon the capacitance values illustrated, the capacitor array  48  includes 180 pF fixed capacitance and 3 pF, 6 pF, 12 pF and 24 pF switched capacitance.  
      The switching of the capacitors in the capacitor arrays  44 ,  46 , and  48  is under the control of the grounding switch array in digital interface  50 , which includes a four bit digital input F 0 , F 1 , F 2 , and F 3 . The four bit digital input is applied to the base terminals of four transistors  52 ,  54 ,  56  and  58 , for example, bipolar junction transistors, by way of current limiting resistors R 97 , R 98 , R 99  and R 100 . As shown, the grounding switch array and digital interface  50  thus provides 4-bit control of the capacitor arrays  44 ,  46  and  48 . Each of the four PNP transistors  52 ,  54 ,  56  and  58  is used to switch any combination of the four-capacitor stages  51 ,  53 ,  55  and  57  for each of the capacitor arrays  44 ,  46  and  48 . For example, the switch  52  controls the switching of stage  51  and thus switches the capacitors C 169 , C 175  and C 181 . Thus, when a logical “1” is applied to the input F 0 , the capacitor array  44  will include the fixed capacitor C 188  plus the switched capacitor  169 . Similarly, the capacitor array  48  will include the fixed capacitor C 189 , as well as the switched capacitor C 181 . The capacitor array  148  will include the fixed capacitor C 187  plus the switched capacitor C 169  and C 175 . The capacitor&#39;s values are in binary order, which means that the 4 control lines, driven by a 4-bit binary number, will generate 3 sets of 16 different values of capacitance.  
      Control of the capacitance is accomplished by way of one or more pairs of reverse connected diodes. For example, in capacitor array  44 , a fixed capacitor  188  is connected on one end to the group capacitors C 169  to C 172 . A pair of diodes, D 38 A and D 38 B is connected to the other end of the capacitor C 169 . In particular, the cathode of the diode D 38 A is connected to the capacitor C 169  while the anode is connected to a current limiting resistor R 118  and to the switching transistor  52 . The cathode of the diode D 38 A is coupled to the anode of the diode D 38 B inside package. The anode of the diode D 38 B is connected to ground. The configuration of the other sets and the capacitor array  46  is similar.  
      The same digital lines control the three sets of capacitor arrays  44 ,  48  and  46 . For example, capacitors C 175  and C 169  in stage  51  and C 181  are turned on or off as a group by the control signal F 0 . Similarly, the capacitors C 170 , C 182  and C 176  in stage  53  are controlled as a group by the control signal F 1 . The switched capacitors C 171 , C 183  and C 177  in stage  55  are controlled as a group by control signal F 2 . Finally, the capacitors C 172 , C 184  and C 178  in stage  57  are controlled as a group by the control signal F 3 . At the highest frequency band, the least capacitance is required. As such, only the fixed capacitors C  187 , C  188  and C  189  are connected. The switched capacitors are switched off by way of a negative voltage, for example—5.0 volts, applied to the anodes of the diodes in each of the four stages  51 ,  53 ,  55 , and  57  by way of current limiting resistors R  90 , R  91 , R  92  and R  93 . The negative voltage applied to the anodes maintains all of the diodes in a reverse biased condition. In order to switch the diodes on, a positive voltage, for example 3.3 volts, is connected to the emitters of the transistors  52 ,  54 ,  56  and  58 . A logical “0”, that is “LOW” or ground, applied to the base of PNP BJTs  52 ,  54 ,  56  and  58  will cause these transistors to turn on and apply the positive voltage (i.e. 3.3 volts) to the anodes of the diodes in order to forward bias the diodes and connect the switched capacitors to the circuit. In particular, assuming a logical “0” is input as the control signal F 0 , the transistor  52  will be turned on and forward bias the diodes D 26 A, D 26 B, D 32 A, D 32 B and D 38 A, D 38 B. Once all the 6 diodes are turned on, the capacitors C 181 , C 175  and C 169  are AC grounded. During such a condition the capacitor array  44  will include the fixed capacitor C 188 , shown as 8 pF, and a parallel capacitor C 169 , 2 pF. The capacitor  46  will operate in a similar manner.  
      A logical “0” is applied as the input F 0  will also cause the capacitor C 175  to be in parallel with the fixed capacitor C 187 . The other stages operate in a similar manner. Table 1 illustrates the fixed and switchable capacitance values for all four stages  51 ,  53 ,  55  and  57  for the capacitance arrays  44 ,  46  and  48 . Note that the capacitor values are binary numbers. Thus, there would be 2 4  total tuning center frequencies.  
                           TABLE 1                          Capacitor   Fixed   Switched Capacitance(pF)   Total Switched                                         Array   Capacitance(pF)   STAGE 51   STAGE 53   STAGE 55   STAGE 57   Capacitance (pF)                                                 44   8   2   4   8   16   30       46   8   2   4   8   16   30       48   180   3   6   12   24   45                  
 
      Referring to  FIG. 2 , the capacitor arrays  44 ,  46  and  48  are connected to the terminals VAR_CAP 1 , VAR_CAP 2  and VAR_CAP 3  on the coaxial resonator  38 . The GND 1 -GND 5  terminals are connected to ground. The transmission lines  34 ,  36 ,  40  and  42  are not shown for clarity.  
      An alternate embodiment of the invention is illustrated in  FIG. 3 . In this embodiment, the ceramic resonator  38  and the capacitor array  48  may be replaced with a single-layer relatively high value capacitor  72 , for example 9,000 pF. In this embodiment, the resonator portion is shown within the dashed box  74  and includes a pair of serially connected transmission lines  76  and  78  which have a different characteristic impedance Zo relative to the corresponding transmission lines  40  and  42 , shown in  FIG. 1 . For example, the transmission lines  76  and  78  may be 37 mm with a characteristic impedance Zo=50 ohms. The capacitor  72  is connected to the node formed between the serially coupled transmission lines  76  and  78 . A pair of switching capacitor arrays  80  and  82 , similar to the arrays  44  and  46 , illustrated in  FIG. 1 , is connected between ground and the transmission lines  76  and  78 , respectively. These capacitor arrays  80  and  82  are for tuning the band-pass center frequency. The resonator  74  is connected on each end to a transmission line  84 ,  86 , for example 20.5 mm, with a characteristic impedance zo=6.2 ohms. The transmission lines  84  and  86  are coupled to the ports  88  and  90 , respectively, by way of a coupling capacitor  92 ,  94  for example 8 pF. Unfortunately, the Q and self-resonate frequency of the capacitor  72  is low thus will not provide high rejection on the adjacent channel (such as an adjacent TV station).  
       FIG. 4  illustrates the frequency response of an exemplary passband filter as illustrated in  FIG. 3  utilizing different capacitor values for the coaxial resonator  38 , for example 0 to 70 pF. As shown, varying the value from the ceramic resonator  38  varies the center frequency as well as the passband of the filter  20 .  
       FIG. 5  illustrates the frequency response for a passband filter  20  using the exemplary values illustrated in  FIG. 2 . As shown, the filter  20  will have a center frequency of about 118 MHz and a bandwidth of about 5 MHz.  
      Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.