Patent GB-520289-A (GB)
Title: Improvements in or relating to electrical impedance networks
IPC: H03H7/24
Filed: 1938-10-18
Abstract: 520,289. Sets of resistances. STANDARD TELEPHONES & CABLES Ltd., and BARNARD, R. M. Oct. 18, 1938, No. 30138. [Class 37] [Also in Group XL] A constant impedance unbalanced attenuator, particularly for use in testing and measuring at high frequencies of one megacycle or more, comprises a number of attenuator pads 1-8, Fig. 2, each housed in a screening casing 16, Fig. 3, which casings are mounted on an earth busbar 9. This is slit so as to be of zigzag formation in order that the pads may be connected thereto in the same order in which they are inserted into the circuit. The earth busbar 9 is mounted on a panel 12 with the interposition of an insulating plate 11 of phenol fibre, the panel having a portion 13 cut away to minimize capacity between the panel and the earth busbar 9. External connections, preferably by coaxial leads, are made to input and output terminals 14, 15, which are of coaxial formation with the outer conductors directly connected to the earth busbar so as to form earth terminals. Owing to the close proximity of the first and last pads 1, 8 to the terminals 14, 15 the resistance and inductance of the earth connection to these pads is kept very low, the resistance being less than 0.0002 ohms and the inductance less than 0.02 microhenries. Either, end of the bus-bar 9 may be connected to the plate 12, forming the rack earth, by a U-link inserted in holes 32, 33 or 32, 34. When the U-link is inserted in the holes 31, 32, the busbar 9 is not earthed. The pads 1-8 are constructed as shown in Fig. 3. The resistors R1, R2, R3, forming the pad are mounted in a metal frame 16.- A key 17 for inserting the pad in circuit is mounted in the frame 16, four contact springs being provided of which one pair 18, 19 is shown, the other pair being behind these. With the key in normal position the two upper contacts are shorted and provide a straight-through connection. When the key is operated the contact springs 18, 19 and also the corresponding rear pair are connected, providing a path through the pad since the resistor R1 is connected to the contact spring 19 and the resistor R2 is connected to the corresponding rear contact spring. The earth lead is taken direct from the resistor R3 to the frame 16, which is mounted on the earth busbar. In a modification, Fig. 4, the earth busbar 9 is arranged at the end of the pads remote from the panel 12. The pad formed of the resistors R1, R2, R3, corresponds to the pad 1, Fig. 2, at the input end of the attenuator and is connected to the input terminal corresponding to the terminal 14, Fig. 1. The terminal comprises a central conductor 20 insulated from an outer coaxial conductor 21 and connected at 22 to the central conductor 23 of a length of coaxial conductor' forming the input lead. The outer conductor 21 is connected to the outer conductor 24 of the coaxial connector by means of a strap 25, the earth lead 28 of the resistor R3 being connected to the outer conductor 24 at a common point 29 with the earth busbar 9. The pads are arranged so that the input terminal is connected to the pad giving the highest attenuation and the load or output to the pad giving the lowest attenuation, the intermediate pads being arranged in descending order of attenuation.