Patent GB-756657-A (GB)
Title: Circuits for clipping or attenuating signal amplitude excursions exceeding a predetermined level in signal transmission lines
IPC: H04N5/14
Filed: 1954-11-16
Abstract: 756,657. Amplitude limiting circuits. WESTERN ELECTRIC CO., Inc. Nov. 16, 1954 [Nov. 18, 1953], No. 33151/54. Class 40 (4). [Also in Group XL (b)] An amplitude limiting circuit for connecting in shunt to a television transmission line consists of an amplifier having a shunt connected negative feed-back path which is operative, only when the signal amplitude exceeds a predetermined level, to reduce the input impedance of the amplifier, thereby placing a low resistance shunt across the transmission line. In the basic circuit, Fig. 3, signals received in antiphase over the transmission conductors feed the valve pair V1 and V2 to produce an output to condenser C1 as described in Specification 541,986. " Longitudinal " or in-phase currents produce no output. Valves V3 and V4 receive bias from potentiometer R2 . . . R4 such that V4 is always conducting but V3 is normally cut off. The normal working bias of V3 is additionally determined by the sync-pulse peaks through the action of condenser C1 and the clamping diode V5. When " whiter-thanwhite " signals are received these are sufficient to cause V3 to conduct and V4 to produce a signal in antiphase through the action of the common cathode resistor R7. The signals from V3, V4 are fed back in shunt to the inputs of V2 and V3 to provide the necessary negative feed-back. In a practical embodiment, Fig. 5, the first stage V1, V2 is given additional gain by replacing the common cathode resistor by a valve V6 fed from the output of V2. Thus a signal incoming to V2 causes the cathode of V1 to move in phase therewith and thus out of phase with the signal on the grid of V1, thereby doubling the potential across the grid and cathode of V1. Each valve is provided with stabilizing negative feed-back, V2 by direct coupling of the grid and anode, and V1 by coupling of the screen-grid and grid, thereby avoiding the connection of stray capacitance to the output of V1. The output from V1 is fed to cathode follower V7 whose low input impedance minimizes the effect of stray capacity Co. This capacity is also compensated by a negative feed-back circuit 29 in the cathode lead of valve V1. The output of V7 feeds the stage V3, V4 whose outputs are coupled to the input of V1, V2 to produce negative feed-back as previously described. The grid - to - anode capacity C<1>o of V3 produces an unwanted output to one of the transmission leads via valve V4 and a compensating circuit 32 is provided which if connected to the other transmission lead could produce a compensating in-phase current in the other transmission lead. It is, however, demonstrated that the whole of this " longitudinal " current can now be eliminated by connection instead of a peaking coil 38 shunted by a resistance 37 in the anode circuit of V7. Condenser C2 and filter 30, 31 prevent video signals reaching valve V4. To prevent undesired oscillation at very high frequencies, an anti-resonant negative feed-back circuit 34 is introduced in the cathode lead of V3. The shunt capacitance of the circuit on the transmission line is compensated by light loading coils 42.