Patent GB-910793-A (GB)
Title: Improvements in or relating to apparatus for determining the transfer function of anelectrical system
IPC: G01R27/32
Filed: 1960-01-11
Abstract: 910,793. Electric analogue computers; determining impedance/frequency characteristics. WAYNE KERR LABORATORIES Ltd., GONRIET, G. G., and LEVINGE, R. W. Jan. 6, 1961 [Jan. 11, 1960], No. 903/60. Class 37. Apparatus for the determination of the transfer function comprises means for producing a test signal consisting of an electric signal of varying amplitude and at least its first derivative which is fed both to the system under test and then to one input of a phase and amplitude comparator and directly to the other input of the comparator, it being shown that by adjusting the test signal so that the inputs to the comparator are equal, the coefficients of the transfer function can be determined from said adjustments. In the embodiment of the Fig. 1, an oscillator of the type described in Specification 910,794 provides a sinusoidal electrical signal and outputs corresponding to the first, second, third and fourth derivatives thereof to provide a test signal which is fed, via potentiometers 40, 42, 44, 46, 48 and an adder 50 both to the system 85 under test, and directing via an adder 60 to one input of a C.R.O. 70, the other input of which is connected to the output of system 85. Fifth and sixth derivative terms to form part of the test signal fed to system 85 are derived from the output of 50 by differentiating circuits 71, 72 and through potentiometers 76, 77 are applied to the input of adder 78. The potentiometers are then adjusted so that the inputs to the C.R.O. are of equal phase and amplitude, when the settings of potentiometers 41, 43, &c. and of potentiometers 40, 42, &c. and 84, 76, 77 will determine the # and α coefficients respectively. Theory.-The signal fed directly to the comparator can be adjusted to have the sinusoidal signal and the derivatives in the proportion # 0 , # 1 , # 2 , &c. and that to the system under test in the proportions α 0 , α 1 , α 2 , &c. If the system has a transfer function = F(p) then the output of the system = F(p) Î (α 0 +α 1 p+αnpn) i.e. F(p)xtest signal equal to # 0 +# 1 p+ . . . # m p<SP>m</SP> i.e. equal in phase and amplitude to the test signal is provided directly to the comparator. Therefore, if the comparator inputs are equalized, values for α, # can be derived from the potentiometer settings. In a further embodiment (Fig. 5, not shown), the output from an oscillator is differentiated to provide outputs fed to an adder to provide one input to the comparator and, via an adder and two further differentiating circuits to the system under test and to the other comparator input. Alternatively, the sinusoidal signal only could be fed to the system under test and the output then differentiated. In the embodiment of Fig. 6, the output from the system and the reference signal are subtracted in a potentiometer 233, 232 and applied to the Y-plates of a C.R.O., the reference signal being also fed to the X-plates, so that when the inputs are equal, a spot only is shown on the C.R.O. A further embodiment (Fig. 7, not shown) includes attenuators so that a non-linear system, or one test will saturate can be tested and filters may be included in the apparatus to reduce the effects of noise (Figs. 8-10, not shown).