Patent GB-785621-A (GB)
Title: Improvements to filtering network
IPC: H04B3/14
Filed: 1955-03-15
Abstract: 785,621. Pulse-transmission networks. SOC. NOUVELLE DE L'OUTILLAGE R.B.V. ET DE LA RADIO-INDUSTRIE. March 15, 1955 [March 19, 1954], No. 7559/55. Class 40(8). [Also in Group XL (c)] A filter network has a transfer characteristic : where A and a are parameters independent of the frequency # 0  = # 0 /2 # of the signal. The function G(#) may be shown to be a Fourier transform of a sinusoidal oscillation of frequency f o  modulated by a rectangular pulse of duration 2a and, on account of this fact, the network is stated to be particularly suitable for the detection of pulse-modulatd signals where random noise is present. The network may, however, be applied to the detection of pulses which do not modulate a carrier wave (# 0  = O). A network for transmission of pulses of deviation 2a having the characteristic G(#) may be realized by a number of elementary sections associated with a band-pass filter, each section comprising a delay line in parallel with a direct connection and an additive network, the delays introduced by successive sections being chosen to constitute a geometric series of decrement ¢. The largest delay introduced by the sections is made equal to a and should be equal to a multiple of half the period of # 0  where this exists. The band-pass filter may be in the input or output of the chain of sections or may be constituted by several elementary filters connected between successive sections and having different characteristics. In the embodiment of Fig. 4 the delay line comprises 40 identical LC sections and is terminated by a resistance R equal to its characteristic impedance. The direct connection 1 includes an attenuator R 11 which ensures that the amplitudes of the direct and delayed signals are equal. The signals are applied to the control grids of VI and V2 and the sum of the signals is derived from the anodes which are connected in parallel. The output signal is fed by means of a coupling capacitor C1 to the next stage 12 which is identical with the previous stage except that the delay line comprises only 20 LC sections. Stage 13 has a delay line comprising 10 sections and is connected to a band-pass filter 4 which may be reduced to a circuit tuned to the frequency # 0 . In a modification, the output from the attenuator R11 is applied directly to the delay line of the following stage, the valve V2 being omitted. In the circuit of Fig. 5 the delay line is open circuited and is connected in parallel with a resistor R equal to its characteristic impedance in the anode circuit of an amplifying valve V5. Since the pulse is reflected, the line need only have half the number of sections required for the corresponding line shown in Fig. 4. The output signal of valve V5 is applied to the valve V6 of the next stage through a capacitor 15 of high capacity compared with the capacitors of the delay line. In the circuit of Fig. 6 the delay line is terminated at either end by matched resistances R, the direct pulse and the delayed pulse being applied respectively to the grids of cathode-follower stages V10, V11. The output circuits of V10 and V11 are connected in series by balancing resistors R1, R2 and a potentiometer P is used to pick up the correct fraction of the output signal which is the sum of the outputs of V10 and V11. At very high frequencies the delay lines may comprise lengths of transmission line and waveguides may be used at microwave frequencies.