Patent GB-786189-A (GB)
Title: Improvements in or relating to attenuators
Inventor: WIGAN EDMUND RAMSEY
IPC: G01R1/20
Filed: 1953-08-13
Abstract: 786,189. Resistance networks. MUIRHEAD & CO., Ltd. Aug. 12, 1954 [Aug. 13, 1953], No. 22293/53. Class 37. An adjustable 4-terminal constant impedance bridged T attenuator comprises first interconnected input and output terminals, second such terminals interconnected by two equal fixed series impedances shunted by plural series connected switchably variable impedances, and plural shunt connected switchably variable impedances connecting the first input and output terminals to the junction of the fixed series impedances; the corresponding series and shunt impedance switches being ganged in pairs to vary the network attenuation against numerically indexed scales and the series and shunt impedances being so proportioned that the sum of the scale indices equals the numerical attenuation ratio, and the product of the variable series and shunt impedance is always constant and equal to the square of the required constant input impedance of the network. Fig. 3 shows a bridged T network having input terminals 21, 22 and output terminals 23, 24 connectible to the instrument. Terminals 22, 24 are directly interconnected over line 25 and terminals 21, 23 are interconnected over series resistances 26, 27, each of 1000 ohms, shunted by a resistance 28 variable in nine 1000 ohm steps by a tap 32 adjustable by a control knob (not shown) graduated with indices 1-10; in series with a resistance 29 variable in ten 100 ohm steps by a tap 33 adjustable by a control knob (not shown) graduated with indices 0-10 also in series with a resistance 30 variable in ten 10 ohm steps by a tap 34 adjustable by a control knob (not shown) graduated with indices 0-10; the circuit being completed by a resistance 31 continuously and linearly variable from 0 to 10 ohms by a tap 35 adjustable by a further graduated control knob. Between the line 25 and the junction of resistances 26, 27 are connected four adjustable shunting resistances 40, 45, 50, 56, in parallel, respectively variable by the control knobs of resistances 28, 29, 30, 31 above. Resistance 40 is adjustable by tap 41 movable through steps 1 to 10 in the first of which the value is infinite while in subsequent positions a resistance value is inserted such that its product with the corresponding value of resistance 28=1000<SP>2</SP>, so that resistance 42 in position 2= 1000 ohms, resistance 43 in position 3=500 ohms, while resistance 44 in position 10 ==111.1 ohms. Resistance 45 is adjustable by tap 46 movable through steps 0 to 10, having infinite value at zero, while resistance 47 in position 1 has value 10,000 ohms, resistance 48 in position 2 has value 5000 ohms, and the resistance in position 10 has value 1000 ohms. Similarly resistance 50 is adjustable by tap 5 movable through steps 0 to 10 to have infinite value at position 1, 100,000 ohms at position 2 and 10,000 ohms at position 10, and linear continuously variable resistance 56 is adjustable by tap 55 ganged for operation with the control knob of resistance 31 to insert an infinite shunt resistance until tap 35 has been varied <SP>1</SP>/ 5  the range of resistance 31 from zero, and then a value of 300,000 ohms linearly decreasing to 100,000 ohms when the tap 35 has traversed its full range. This shunt resistance variation is shown to be a satisfactory approximation to the required law (Fig. 4, not shown). The network is shown to have input and output impedances of 1000 ohms and may be fed from a cathode follower amplifier of low impedance, when the successive control knobs represent attentuation ratios of unity, 0.1, 0.01 and 0.001 so that any desired ratio may be set with an accuracy of 1 in 10,000. For other values of input impedance, if the fixed series resistances have values Ro, the variable series resistance has value Ra, and the shunt resistance has variable value Rb, where RaRb=Ro<SP>2</SP>, the input and output impedances of the network remain constant at Ro while the voltage attentuation ratio L = 1 + N where In modifications, the second fixed series resis. tance may be made infinite, whereby the input impedance =Ro, where RaRb=Ro<SP>2</SP>, although the output impedance is then variable; while both connections between the input and output terminals may each comprise a series pair of fixed resistances shunted by a bridge resistance comprising plural graduated adjustable stepped resistances and having corresponding sets of selectible shunt impedances connected in parallel between the junctions of the respective pairs of series resistance, corresponding controls being ganged together. The variable resistance 28 comprising 9 equal unit resistance elements in series adjustable by a single 10-position tap switch may be replaced (Fig. 5, not shown) by a set of 4 resistances of values 1, 2, 3 and 6 units selectible by a 10-position 2-gang tap switch in series combinations providing total resistance values of from 0 to 9 units. The variable resistances 29, 30 may similarly be replaced (Fig. 6, not shown) by a set of 5 resistances of values 1, 2, 3, 4 and 6 units (selectible by a 11-position 2-gang tap switch) in combination providing total resistance values of from 0 to 10 units. In the same manner, the selectibly switchable shunt resistances 41, 45, 50 may be replaced respectively by sets of resistances of values 1, ¢<SP>1</SP>/ 3 , <SP>1</SP>/ 6  units or 1, ¢<SP>1</SP>/ 3 ,“ and <SP>1</SP>/ 6  units (selectible in parallel by 2-gang 10- or 11-position tap switches) to provide the required total shunt resistance values (Figs. 7, 8, not shown).