Patent GB-795584-A (GB)
Title: Improvements in high frequency wave transmission filters
Inventor: CUSTIN THOMAS GRANT
IPC: H01P1/209
Filed: 1955-04-15
Abstract: 795,584. Filter networks. GENERAL ELECTRIC, CO. April 15, 1955, No. 10969/55. Class 40 (8). [Also in Group XL (c)] A filter for high frequencies, e.g. 470 to 890 Mc/s., comprises a hybrid network 1, Fig. 1, having two pairs 2, 3 and 4, 5 of conjugate terminals, one pair of which are connected to a source 8 and a load 9, the other pair being connected to two circuits 6, 7 tuned to two different but adjacent frequencies. The hybrid network may be constructed from four 3#/4 lengths of coaxial line (where A is the wavelength of the frequency utilized), one pair of opposing branches being arranged to have an admittance equal to the root of twice the admittance of the second pair of branches. The branches of lower admittance connect the conjugately related pairs of terminals. It can be shown that energy can be transferred between the source 8 and load 9 if the conjugate terminals 2, 3 are connected to reflecting impedances and that the voltage attenuation between the source and the load its a function of the reflective coefficients of these impedances. If the difference of the coefficients is zero, as in the case of two circuits tuned to the same frequency, then the attenuation is substantially zero and is independent of frequency. If, however, the impedances 6, 7 comprise two circuits tuned to different frequencies there will be differences of phase and magnitude between the reflection coefficients. The rate of change of the vector difference between the two coefficients (and hence the rate of change of attenuation) can be so arranged that it is greatest just above and below the resonant frequencies of the tuned circuits. In Fig. 2 (b) the curves 12, 13 indicate the attenuations arising individually from each resonant element 6, 7 whilst curve 14, having steeper sides than the curves 12, 13, shows the total attenuation. In a modification (Fig. 3, not shown), coupling circuits are inserted between the resonant circuits 6, 7 and the terminals of the hybrid circuit. If a steep attenuation slope is desired on the low-frequency side of the band, the coupling to the circuit tuned to the lower frequency is reduced considerably below critical coupling whilst that to the other circuit is retained at a nearly critical value. The result is shown in Fig. 4 where the same reference numerals as Fig. 2 (b) are used. In a further modification (Fig. 5, not shown), the tuned circuit 6 is replaced by a double-tuned circuit which is coupled to the hybrid terminal by a coupling which has the property of varying the coupling to at least one of the tuned circuits independently of the coupling to the other. The resonant frequency of the tuned circuit 7 is arranged to lie between the resonant frequencies of the double tuned circuits 6. In order to increase the attenuation slope on the low-frequency side of the band, the coupling of the lower frequency portion of the doubletuned circuit may be reduced and the attenuation characteristic 22, Fig. 6, is obtained. The circuit of Fig. 7 is used to obtain the composite attenuation characteristic of Fig. 9 which may be used to reject the lower sideband and part of the upper sideband of the visual signal of a television transmitter. The attenuation band 41 has the form of the curve 22, Fig. 6, and is obtained by means of a double-tuned circuit 30 with variable coupling 28 and a singletuned circuit 34 with variable coupling 32. The rejection band 39 has the form of the curve 14, Fig. 4, and is obtained by single-tuned circuits 31, 35 having variable couplings 29, 33. The pairs of tuned circuits 30, 31 and 34, 35 are connected in parallel to the conjugate terminals 24, 25 of the hybrid 23. In practice the tuned circuits take the form of cylindrical tanks arranged to resonate at one or both ends as quarter-wavelength concentric lines. Thus the double-tuned circuit 30 consists of a tank 68, Fig. 8, having axially movable central conductors 72, 73 forming the inner members of two resonant lines. Coupling of the resonators to transmission line 74 may be varied by means of a laterally movable conductive post 77. The tuned circuits 31, 34, 35 each consist of a tank and an adjustable inner conductor 63. Variable coupling is in each case provided a lateral adjustable member 67. In order to prevent adverse effects due to interaction the pairs of tuned circuits are spaced one halfwavelength from the junctions 76, 80. The hybrid circuit 23 is constructed from concentric lines, the correct impedance relations between the arms being obtained by forming arms 60, 61 of three quarter-wave transformers. In the modified filter of Fig. 11 the hybrid network 83 is similar to that of Fig. 1, the conjugate terminals 84, 85 being connected as before to tuned circuits 86, 87. Terminal 88 is connected to a line 90 forming a source 89 and a load 91 by a line 93 which has a length of one half-wavelength at the transmitted frequency. No external connections are made to the terminal 94.