Frequency filter apparatus and method
&#39; United States Patent Graham Apr. 8, 1975 1 1 FREQUENCY FILTER APPARATUS AND OT E P L METHOD H M H RE UB lCATIONS 3 I I I arveyicrowave ngineering&#34; Academic Press, [76] lnventor. Gerald Graham, 105 Caml aren London and New York 973&#39; page 889.  
  Crescent, Kemburg, Ontano, Canada Primary Examiner-James W. Lawrence 1 1 Flledi D c. 3, 1973 Assistant Examiner-Marvin Nussbaum [2|] APPL NO; 421,249 Attorney, Agent, or FirmRid.out &amp; Maybee 1521 11.5. c1. 333/73 R; 333/73 c; 333/73 w; 1571 ABSTRACT 333/82 Bl 333/83 R An electrical filter is provided by a transmission line 1 1 &#39;f 7/10; H011) 7/061 H011J 7/04 from which a coupling probe extends into a quarter [58] Field of Search 333/73 R, 73 C, 76, 1, wave (or Odd multiple f quarter wave) Cavity tuned 333/83 R 82 B 82 R1 325/21-25; to resonate at the frequency that it is desired to pass 343/180? 179/15 AN along the line, the probe being tuned to produce frequency rejection notches in the line. A diplexer may 1 1 References Clled consist of two such filters, each separated by an odd UNITED STATES PATENTS number of quarter wave lengths of transmission line 2.421.033 5/1947 Mason 333/6 from Common terminal. each filter Causing its associ- 2,661,424 12/1953 Goldstcin 1 333/6 ated line to p the fr qu n y t which ts cavity is 2.984.798 5/1961 Bryan 333/9 tuned and to reject the frequency passed by the other 3,733,608 5/1973 McGhuy et a1. 343/180 line.  
  FOREIGN PATENTS OR APPLICATIONS 6 Claims, 7 Drawing Figures 696,394 8/1953 United Kingdom 333/73 C PATENTED 8 5 13. 876 963 sum 1 m? 2 II .hUminwwx KO ZO 40% 1 FREQUENCY FILTER APPARATUS AND METHOD BACKGROUND OF THE INVENTION and reject a second frequency. It&#39;is desirable that the filter be easily tunable to vary either the pass or reject frequencies. Such filters are useful in the diplex operation of two radios on a common aerial.  
 SUMMARY OF THE INVENTION The filters of the present invention provide what is believed to be a novel method and arrangement of components for obtaining the foregoing type of operation. According to the invention a coupling transmission line probe extends from a transmission line into a quarter wave (or odd multiple of a quarter wave) cavity resonant at substantially the desired pass frequency, and the probe is tuned to produce frequency rejection notches in the line at frequencies below and above the resonant frequencies of the cavity resonator and the probe, the result being a band-pass, band-reject frequency response, with the band-pass and band-reject frequencies tunable by adjusting the effective electrical length of the probe or of a central conductor in the cavity.  
 BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are illustrated by way of example in the accompanying drawings in which:  
 FIG. 1 illustrates diagrammatically a tunable filter;  
  FIG. 2 shows graphically the frequency response of the filter of FIG. 1 with the cavity and the probe resonant at the same frequency;  
  FIG. 3 shows the frequency response of the filter of FIG. 1 with its probe detuned from the condition of FIG. 2;  
  FIG. 4 illustrates a diplexer incorporating two filters of the type shown in FIG. 1;  
  FIG. 5 shows graphically the frequency response of the device of FIG. 4;  
 FIG. 6 illustrates another diplexer; and  
  FIG. 7 illustrates diagrammatically another form of tunable filter.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS:  
  The filter illustrated in FIG. 1 includes a through transmission line 10, for example a coaxial line, and a coupling probe 11 connected at one end to the line so as to branch from the line and extend into the chamber ofa coaxial cavity resonator l2, e.g., a resonator of cylindrical or square cross-section. Extending along the axis of the cavity is a conductor 13 made up of a tubular element 13a within which is an adjustable element 13b. The length of element 13b within the cavity is adjustable, as by an external nut 14 rotatably fixed to the cavity, whereby the effective electrical length of the conductor 13, formed by the elements 13a, 1312, can be varied to tune the cavity resonator. The electrical length of the probe 11 can also be varied by means of a tuning capacitor 15 adjustable from outside the cavity, or by loading the probe, or varying its physical length, or by other conventional means, the probe constituting a transmission line having self-inductance and mutual capacitance with the wall of the cavity. Tuning of the cavity may also be achieved by capacitive loading or other known means.  
  In the condition illustrated in FIG. 1 the electrical lengths of the central conductor 13 and of the probe 11 are shown to be All/4 where M is the wave length of the cavity resonant frequency fa. In this condition the frequency response indicated in FIG. 2 can be achieved, pole X indicating that frequency fa is at the center of a narrow band of frequencies that can pass along the line 10, and notches Y and Z representing band-reject notches at lower and higher frequencies f and f If the resonant frequency of the probe is increased slightly, for example by adjusting the capacitor 15, the frequency response indicated in FIG. 3 can be achieved, the reject notch Y moving closer to fa and the reject notch Z moving farther away. Conversely, if the resonant frequency of the probe were decreased slightly, reject notch Z would move closer to fa and reject notch Y would move farther away. Thus. the cavity resonator may be tuned to the desired pass frequency fa and the probe may be tuned to reject, for example, f1. f f  
  If, starting with a curve such as that shown in FIG. 2, one does not alter the resonant frequency of the probe, but instead alters the resonant frequency of the cavity, for example, by adjusting the electrical length of the conductor 13, a curve similar to that shown in FIG. 3 may be obtained but the location of the pole X will be shifted. For example, in one experiment the probe and cavity were both resonant at 180 megahertz, and a response as in FIG. 2 was recorded, with pole X at 180 megahertz and reject notches Y and Z at approximately and 195 megahertz respectively. The cavity was then tuned with the object of bringing the pole X and notch Y closer together, leaving the probe as it was, i.e. resonant at megahertz. Adjustment of the cavity resonant frequency to 145 megahertz resulted in pole X being shifted to 145 megahertz, reject notch Y occurring at approximately 140 megahertz, and reject notch Z at approximately megahertz. The reject notches are thus just below arid just above&#39; the lower and the higher respectively of the resonant frequencies of the probe and the cavity. As the cavity resonant frequency was further reduced, notch Y rose slightly (losing isolation) and notch Z deepened, approaching the probe resonant frequency of I80 megahertz. In a typical application the resonant frequency of the probe may differ from the resonant frequency of the cavity by up to about 20% without serious loss of isolation.  
  FIG. 4 shows how filters of the type shown in FIG. 1 may be applied to a diplexer. In such a device, a transmission line 100 connects a terminal T to a terminal A where a transmitter or receiver (not shown) operates at frequency fa, and a transmission line 200 connects the terminal T to a terminal B where a transmitter or receiver (not shown) operates at frequency fl). An antenna may be connected to the common terminal T. One end of a coupling probe Ill is connected to the line 100 and the probe extends into a coaxial cavity A having its central conductor 113 turned for resonance of the cavity at fa. The electrical length of the probe 111 may be adjusted to provide a reject notch atjb, as indicated graphically in FIG. 5 where the broken line labelled A to T (a replica of the curve shown in FIG. 3) indicates the frequency response between terminals A and T. Similarly, from line 200 a coupling probe 211 extends into a coaxial cavity B whose central conductor 213 is tuned for resonance of the cavity atfb, and probe 211 is tuned to provide a pass response at fb and a reject notch atfa, as indicated by the broken line labelled B to T in FIG. 5. The solid line labelled A to B in FIG. 5 indicates that substantially no signal passes between terminals A and B. In FIG. 4 the probes are shown branching from the lines 100, 200 at a distance \/4 from the common terminal T, where A may be a wave length corresponding to an average offu and fl), i.e. is approximately the wave length of a frequency at the middle of the band of frequencies passed by the lines.  
  Although the drawings indicate components and distances of quarter wave lengths, odd multiples of quarter wave lengths may of course be used.  
 For greater isolation multiple sections may be used on either or both sides of the common terminal, as indicated by way of example in FIG. 6 which shows the addition of cavities A and B resonant at frequencies fa and fb respectively and with their probes 111&#39;, 211 tuned to the same frequencies as probes 111, 211 respectively, to enhance the rejection offb and fa respectively. In practice it will usually be desirable to add, in a similar manner, at least one more cavity to each side.  
  Use of coaxial cavities is preferred, but other forms of cavities may be used, for example, helical resonators as illustrated in FIG. 7. The resonator of FIG. 7 consists of a cylindrical chamber 12&#39; having a central helical conductor 13&#39;, and the resonator is connected to a line by a helical coupling probe 11&#39; tunable by means of a variable capacitor 15, to probe constituting a transmission line having self-inductance and mutual capacitance with the wall of the cavity.  
  Other modifications will be obvious to those skilled in the art and are intended to be covered by the following claims.  
 What I claim is:  
  l. A method of filtering signals in a through transmission line, comprising coupling a cav ty resonator to the line by connecting to the line a probe that extends into the cavity resonator and constitutes a transmission line interacting with the cavity resonator to produce a response in the through transmission line having a frequency that is passed thereby with frequency rejection notches below and above the passed frequency, tuning the cavity resonator frequency to determine the frequency that is passed, and tuning the probe to determine said frequency rejection notches below and above said frequency that is passed.  
  2. A filter comprising a through transmission line, a cavity resonator, a probe coupling the resonator to the line, the resonator being tuned to resonance at a first frequency to be passed by the line, the probe constituting a transmission line that extends from the through transmission line into the resonator and interacts therewith to have an electrical length from the through transmission line that is substantially an odd multiple of one quarter the wave length of its resonant frequency, the probe being tuned to produce frequency rejection notches, below and above said first frequency, at frequencies that are not to be passed by the line.  
  3. A filter as claimed in claim 2, wherein the cavity resonator has a central conductor that is of variable length for varying said first, passed frequency.  
  4. A filter as claimed in claim 2, including means for varying the electrical length of the probe transmission line for varying said second and third, rejected frequencies.  
  5. A filter as claimed in claim 2, wherein the through transmission line and a second through transmission line are connected to a common terminal, a second probe couples a second cavity resonator to the second line, the second resonator being tuned to resonance at a second frequency, corresponding to one of said rejection notches produced byithe first resonator, for pas-- sage of the second frequency by the second through line, the second probe constituting a transmission line that extends from the second through line into the second resonator and interacts therewith to have an electrical length from the second through line that is substantially an odd multiple of the wave length of its resonant frequency, the second probe being tuned to produce frequency rejection notches one of which is at said first frequency, the probes being connected to the through transmission lines at distances from the terminal that are approximately equal to an odd number of quarter wave lengths of a frequency at the middle of the band of frequencies passed by the lines.  
  6. A filter as claimed in claim 5, wherein said cavity resonators coupled to the through transmission lines are each but one of a plurality of similar resonators coupled by probes extending from points on the first and second lines, said points being at intervals that are approximately an odd multiple of one quarter of said middle frequency wave length, the cavities coupled to the first through line all being tuned to resonance at said first frequency and their probes being tuned to produce the frequency rejection notch at said second frequency, and the cavities coupled to the second through line all being tuned to resonance at said second frequency and their probes being tuned to produce the frequency rejection notch at said first frequency.