Patent GB-451633-A (GB)
Title: Improvements in or relating to electrical wave filters
IPC: H03H7/01
Filed: 1935-08-09
Abstract: 451,633. Impedance networks. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A. Aug. 9, 1935, No. 22480. Convention date, Aug. 16, 1934. [Class 40 (iii)] Two or more filters having common input and separate output pairs of terminals are so designed that their combined input impedance is purely resistive and constant at all frequencies. The invention may be applied to a two-way repeater circuit of the kind described in Specification 389,119, in which a high-pass filter and a low-pass filter are interposed in parallel between each line and the amplifier. Thus a simple low-pass filter 10 feeding a resistance Ro, Fig. 1, may have its input terminals in parallel with those of a high-pass filter 11 feeding a similar resistance, and the two filters are so related that they have the same cut-off frequency fo and that the input impedance of the whole system at terminals 1, 2 is equal to Ro at all frequencies. In filter 10 the elements in different sections have different values calculated as described in Specification 353,066 so that the input impedance is Ro {1+ (f/fo)< 2N> } where f is the frequency, and n the number of elements ; each inductance is of the form a2m + 1Lo and each capacity of the form a2mCo, where Lo /Co = Ro<2> and 2#foLo=Ro, while the coefficients a are functions of sin (r# /2n) and cos (r# /2n), r being an integer. Filter 11, on the other hand, has an imput impedance Ro { 1 + (fo /f)<2n> } so that the resultant input impedance of the pair of filters is Ro. To this end each capacity and inductance in one filter is made inversely proportional to a corresponding capacity or inductance in the other. When the filters 10, 11 have their input terminals in parallel, Fig. 1, they are terminated mid-series as shown ; but when they have their input terminals in series they are terminated midshunt, Fig. 2 (not shown). In order to provide a sharper cut-off, the shunt arms of the filters may comprise resonant circuits, Fig. 3 The elements in filter 15 are reciprocally related, as before, to those in filter 14. The resonance frequencies of the arms are proportional and inversely proportional, respectively, to the quantities P1, P2 . . shown in Fig. 3, and these are arbitrarily assigned numerics less than unity. It is shown that any pair of filters will satisfy the requirements of the invention if their input impedances are respectively equal to Ro[1-{ F(z<2>] and Ro[1-{F( Z<-2>] where z=jf/fo. If the function F is so chosen then F(1/z) = 1/F(z), the elements of the two filters are reciprocally related. A method of calculating the factors a when F(z) = z#(P<2>m = z<2>)/#(1 + Pm<2>z<2>) is given in the Specification, where the Ps have arbitrarily assigned values less than unity. It is found that two filters so designed have overlapping characteristics so that at the cut-off frequency, where the pass range of one filter meets the attenuation range of the other, the attenuation may be only 3 decibels. To obviate this disadvantage a band-pass filter 20, Fig. 5, may be added, to short-circuit the component filters 18, 19, at the cross-over frequency. These filters are so designed as to have input impedances equal to those already prescribed for the filters 10, 11, multiplied by a factor (I-N) where N = M<2>/p<2>F(p)F(-p), while p = (z+1/z) and M is arbitrarily chosen. It is then found that the resultant input impedance will be Ro if the band-pass filter 20 is a constant -k filter having an impedance Ro(1-1/N) and designed in a manner indicated in the Specification. Alternatively a sharp cut-off or crossover may be obtained by means of the arrangement shown in Fig. 7. The low-pass channel comprises a filter 24 having a cut-off frequency fo, in tandem with a filter 25 having a cut-off below fo, this latter being shunted by a complementary filter 26 terminating in a resistance Ro which absorbs the power transmitted through 26. The high-pass channel comprises a filter 27 having a cut-off fo, in tandem with one 28 having a cut-off above fo, and provided with a complementary shunt 29. A further alternative for sharpening the cut-off or cross-over comprises the insertion of a band-elimination filter, Fig. 8, (not shown), in tandem with the pair of filters 14, 15 and in shunt with a compensating filter.