Patent GB-788084-A (GB)
Title: Improved phantastron circuits
IPC: H03K3/04
Filed: 1955-11-28
Abstract: 788,084. Pulse and saw-tooth generating circuits. BENDIX AVIATION CORPORATION. Nov. 28, 1955 [Dec. 13, 1954], No. 34063/55. Class 40 (6). In a phantastron circuit of the type having a cathode follower in the feed-back path to provide a low resistance re-charging path for the timing capacitor an end point is provided for the run down of the circuit that does not coincide with its bottoming point and an output pulse is provided at the end point. An impedance is provided in the plate circuit of the first cathode follower and the voltage produced across it is fed to the input of a second cathode follower and a voltage and polarity-sensitive device is connected between the cathode of the cathode follower stages. An output signal is derived from the cathode of the second cathode follower stage. Fig. 1 shows a phantastron circuit in which tube V3 is the Miller sweep tube. The trigger impulses are applied via tube V1 which also clamps the plate of tube V3 to the level determined by the voltage potentiometer connected to the cathode of tube V1. Tube V2 prevents the suppressor grid of V3 from becoming too positive and also limits the voltage variation of the suppressor grid enabling rapid transitions to occur within tube V3. The Miller capacitor C1 is connected between plate and control grid of tube V3 via cathode follower stage V4 which also provides a low resistance charging path for capacitor C1. With normal phantastron operation the plate potential of tube V3 decreases at a constant rate until it cannot drop any further, this being referred to as the bottoming point. However, in the circuit shown, the dependency on the bottoming point to terminate the run down is eliminated by in effect opening the plate to control grid feed-back circuit of tube V3. This is accomplished by setting the plate voltage of tube V6 by use of the divider network comprising resistor R2. When the voltage on the cathode of tube V4 falls to a level to cause tube V6 to conduct the voltage thereon is clamped and the feed-back to the control grid of tube V3 is interrupted. The stability of the level of the voltage gradient necessary to cause current to flow exceeds the stability of the normal plate bottoming point. Fig. 6 shows a modification of part of the circuit of Fig. 1 in which a source E2 is shown as replacing the voltage divider comprising the resistor R2 and in which the input voltage waveform is derived from the Miller sweep tube V3. When the level of the input signal is E1 (greater than voltage E2) the cathode of tube V4 is at a higher voltage than the cathode of tube V5 so that V6 does not conduct. When the level of voltage on the grid of V4 falls until its cathode voltage reaches that of the cathode cf V5, tube V6 conducts and short circuits the two cathodes. When the signal on the grid of V4 falls more rapidly the current through V4 decreases rapidly. Resultant plate voltage rises coupled through transformer T1, cathode follower tube V5 and diode tube V6 back to V4 completing a feed-back loop which rapidly cuts off the current in tube V4. A steep wave front is produced by the regenerative cut-off of the plate current while the negative overshoot, Fig. 7 (not shown), is eliminated by diode CR. The sharp regenerative rise in tube V5 causes a positive transient to cross the cathode resistor of tube V5, this is coupled to the control grid of tube V3 so that the plate rapidly bottoms. At this time normal phantastron transients occur whereby the suppressor grid regains control and the plate rapidly returns to its clamped voltage. It is this rapid return that causes the second regenerative action and provides a sharp trailing edge to the generated pulse. The phantastron is thereby re-set and is ready for the next triggering input pulse. In a modification shown in Fig. 9, the phantastron is triggered on its suppressor grid and a gas tube V10 replaces one of the resistors of the voltage divider which supplies the clamping voltage to tube V7. Slight changes in current produced by the clamping action will not cause the voltage on the cathode of the clamper tube to vary and therefore the starting points of the run down always remains the same. The transformer T1, Fig. 1, may be replaced by a resistor capacitor coupling network, Fig. 10 (not shown), the resistor being smaller than the cathode resistor of tube V4.