Patent GB-909546-A (GB)
Abstract: 909,546. Electrical signal filtering systems. GULF RESEARCH & DEVELOPMENT CO. Aug. 3, 1960 [Sept. 29, 1959], No. 26832/60. Class 40 (4). [Also in Group XL (c)] An apparatus for filtering out an unwanted sinusoidal signal from a composite signal, e.g. removing " ringing " frequency from a composite seismic signal, comprises a first signal channel having a broad pass-band and second signal channel having a bandpass filter, the frequency of whose pass-band is controllable and whose amplification at the peak frequency of the pass-band is substantially the same as the amplification of said first channel. Fig. 1 shows a seismogram in which the wanted seismic impulses 3 are shown superimposed on a substantially sinusoidal waveform 2. The reduction in amplitude of the ringing frequency after passing through the apparatus, drawn in block diagram form in Fig. 4, is shown at Fig. 9, the wanted impulses being indicated at 140. The input signal is applied to terminal 12, to which are connected in parallel a wide band amplifier 10, a band-pass filter 11 and a frequencysensitive control device comprising elements 60-66. A suitable wide-band amplifier is shown within the dotted rectangle 10 (Fig. 8), while the band-pass filter is shown in rectangle 11. A saturable core reactor 39, resonated by capacitors 40, 41 to the ringing frequency is placed in the grid circuit of pentode 43, which itself forms part of a Q multiplier circuit 42, the Q value of the circuit being adjustable by the resistors 140, 141 and 142. The output of circuit 42 is taken from the cathode of pentode 43 and is applied to the grid of a triode amplifier 28. Since the range, over which the ringing frequency is liable to vary, is known, the capacitors 40, 41 are chosen so that the resonant frequency of filter 11 which is adjustable by means of the control winding 55, will lie within this range. The amplitude of the wide band signals from amplifier 10 and the narrow band signals from filter 11 are adjusted to be equal so that when the combined signals are passed through differential amplifier 15 (Fig. 4), the output signal on terminal 135 will be substantially free from the unwanted frequency, i.e. ringing, which has been selected by filter 11. Because the ringing frequency is liable to vary over a small range, then the resonant frequency of filter 11 must vary in step with it and a control circuit which is sensitive to such changes in ringing frequency is shown in elements 60-66 of Fig. 4. A portion of the incoming signal on terminal 12 is passed through a clipper unit 60 (Fig. 5). The latter consists of several sets of Zener diodes 72, 82 and 89 connected back-to-back and across the inputs of pentodes 74, 86 and 92 while a further pair 95 are placed across the output circuit of pentode 92. The output on terminal 97 is essentially a signal whose frequency corresponds only to the zero ordinate crossing of an input waveform, i.e. it is independent ofinput signal amplitude, and corresponds to the ringing frequency. The output signal is then applied to two band-pass filters one of which 61 is tuned to a frequency f 1  above the expected ringing frequency while the other 62 is tuned to a lower frequency f 2 . Saturable core reactors tuned by a capacitor and followed by a triode amplifier are used in both filters, the outputs being transformer coupled into fullwave rectifiers and conventional smoothing circuits 63 and 64. The gains of each amplifier are initially adjusted so that the corresponding frequencies passed by each filter have equal amplitudes and consequently after combination in a differential amplifier 65, the output corresponding to a frequency f 0 , lying midway between f 1  and f 2 , will be zero. Now if an incoming signal on terminal 12 has a ringing frequency fr greater than f 0  a positive voltage will appear on terminal 121, while a lower frequency will produce a negative voltage. By mixing the output signal on terminal 121 with another D.C. signal obtained from potentiometer 127, in a differential amplifier 66, a signal on lead 18 is obtained which will control the resonant frequency of band-pass filter 11 since lead 18 is connected to the control winding of the saturable core reactor contained in this filter. Thus, if the ringing frequency becomes greater than f 0  a positive voltage appears on terminal 121 and consequently the current through the control winding of the reactor increases, thus decreasing the inductance of the reactor and consequently maintaining the resonant frequency at the ringing frequency.