Europaisches Patentamt 094 765 • J) European Patent Office Publication number: 0 B1 Office europeen des brevets EUROPEAN PATENT SPECIFICATION Date of publication of patent specification: 03.05.89 Intel.4: H 03 K 3/335, H 03 K 3/33 Application number: 83302521.6 Date of filing: 05.05.83 (54) Short pulse generator. Priority: 17.05.82 US 378558 Proprietor: SPERRY CORPORATION 1290, Avenue of the Americas New York, N.Y. 10019 (US) (§) Date of publication of application : 23.1 1.83 Bulletin 83/47 Inventor: Rao, Basrur Rama 60 Allen Street Publication of the grant of the patent: Lexington Massachusetts 02173 (US) 03.05.89 Bulletin 89/18 Representative: Singleton, Jeffrey Designated Contracting States: ERIC POTTER & CLARKSON 27 South Street DEFRGBITNL Reading Berkshire, RG1 4QU (GB) References cited: US-A-3 832568 US-A-3 997 843 ELECTRONIC DESIGN, vol. 16, no. 6, 14th March 1968, pages 202-209, Rochelle Park, US; GQ W.B. MITCHELL: "Avalanche transistors give fast pulses" ID <0 O> O from the of the mention of the grant of the European patent, any person may o Note: Within nine months publication give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1 ) European patent convention). LU Courier Press, Leamington Spa, England. EP 0 094 765 B1 Description US — A — 3,997,843 discloses a monocycle pulse generator employing an open delay line as a The present invention pertains to the field of pulse shaper. short pulse generators. The present invention is defined in the Early prior art short pulse generators utilised a 5 appended claims and a solid state generator for mercury relay switch to discharge a transmission providing a high voltage, subnanosecond rise line which had been charged to a high voltage time pulse of substantially nanosecond duration through a long time constant. This technique is in accordance with the principles of the present capable of providing subnanosecond pulses with invention may comprise a step recovery diode rise times of the order of 100 picoseconds at peak m fired by the discharge of three capacitors. The pulse voltages in the order of several hundred three capacitors are charged in parallel and then, volts. These switches, however, cannot be by means of fast switching avalanche transistors, operated at very high duty cycles because of are connected in series for discharge to obtain a mechanical limitations of a vibrating reed used voltage that is substantially equal to the sum of therein for the switching operation. Additionally, 15 the capacitor voltages when charged. The series' the mechanical contacts tend to deteriorate, coupled capacitors are then coupled via an output resulting in jittery and noisy pulses. Since the avalanche transistor to a differentiator, causing & deterioration of the mechanical contacts is pro- doublet pulse to be coupled to the step recovery portional to the number of times the contacts are diode. This doublet pulse triggers the step re- being opened and closed, the life of the switch is 20 covery diode stack to provide a fast rise time, high inversely proportional to the duty cycle. amplitude negative pulse of substantially nano- Another technique employed in the early prior second duration. art utilised Hertzian (spark gap) generators. These A pulse generator in accordance with the devices can supply pulses with amplitudes of the present invention will now be described in greater order of 1,000 volts and rise times of the order of 25 detail, by way of example, with reference to the 100 picoseconds at repetition frequencies above accompanying drawings, in which: — 200 Hertz. Hertzian generators, however, have a Figure 1 is a schematic diagram of a driver lifetime limited by the width of the spark gap circuit for the pulse generator according to the which determines the width of the generated present invention, pulse. 30 Figure 2 is a schematic diagram of the pulse One obvious solution to the mechanical and generator, responsive to the driver circuit of lifetime limitations of the mercury and Hertzian Figure 1, generators is to utilise solid state devices which Figure 3 is an equivalent circuit for the pulse are rapidly switchable and extremely reliable. It is generator of Figure 2 that is valid during the well known that tunnel diodes, avalanche 35 charging interval, and transistors (see Electronic Design, Volume 16, No. Figure 4 is an equivalent circuit for the pulse 6, 14th March 1968 at pages 202 to 209), and step generator of Figure 2 that is valid during the recovery diodes can be used to generate a series impulse interval. of impulse functions at a repetition rate sub- Referring to Figure 1, a driver 10, that may be stantially equal to the frequency of a driving 40 utilised for a short pulse generator in accordance function. Nanosecond pulses with rise times of with the present invention, comprises charging the order of 25 picoseconds are achievable with circuit 11, a switching circuit 12, a pulse shaping tunnel diodes. These generators, however, are circuit 13, and three capacitors C1f C2, and C3. The low amplitude devices achieving levels only of driver 10 further comprises transistors 11a, 11b, the order of 0.5 volts. Short pulses with ampli- 45 and 11c coupled in parallel to respective diodes tudes as high as 1,000 volts have been generated 11d, 11e,and 11f. Each transistor 11a, 11band 11c with a series stack of avalanche transistors. The may be of the type known in the art as MPS U10, rise and fall times of these generators is of the whilst each of the diodes 1 1 d, 1 1e and 1 1f may be order of 6 nanoseconds, which for many applica- of the type known in the art as 1N914. The tions is too slow. A significant improvement in the so junction of the cathode of the diode and the base pulse rise time has been achieved with the of the transistors for the parallel pairs 1 1 a — 1 1 d, combination of avalanche transistors and step 11b — 11e, and 11c — 1 1f are coupled via resistors recovery diodes. These devices exhibit rise times R-ia, Rib- and R1c, respectively, and a terminal 14 to of the order of 100 picoseconds, but provide peak a power supply (not shown), whilst the junction of amplitudes only of the order of 25 volts. A device, 55 the emitter of the transistor and the anode of the comprising a multiplicity of serially coupled step diode of these transistor-diode pairs are respec- recovery diodes, capable of providing pulse tively coupled to the capacitors C,, C2, and C3, and widths of 200 picoseconds and pulse amplitudes the collectors of the transistors 11a, 11b, and 11c in excess of 180 volts is disclosed in US — A — are respectively coupled to the terminal 14 via 3,832,568. The high pulse amplitudes of this 6o resistors R2a, R2b and R2o. Capacitors C1f C2, and C3 device, however, are achieved only when the are charged in parallel from the terminal 14 by pulse generator is operating into a high current flowing through the corresponding impedance resonant load. Additionally pulse transistors. The charging period is relatively rapid repetition rate of the device is limited to fre- since the current required for charging the quencies of the order of 40 kHz. 55 capacitors is effectively multiplied by the (3 of the EP 0 094 765 B1 transistors. Current continues to flow through the shorted end arriving at the input end after a time transistors 11a, 11b and 11c charging the capaci- lapse of 2 l2/v2, where v2 is the propagation tors Cu C2 and C3 until the leakage current velocity along the transmission line comprising through the diode, of each transistor-diode the pulse shaper 13. At the time the signal returns parallel combination, and the corresponding 5 to the input end of the pulse shaper 13, the avalanche transistor of the switch 12, as for transistor 1 2c is cut off causing the reflected pulse example transistor 12a for the transistor diode from the pulse shaper 13 to be coupled to the parallel combination 11a— 11 d, back biases the output terminal 23, thereby establishing a doublet base of the transistor. When this condition is pulse 24 thereat. The doublet pulse 24 will have a reached, the transistors 11a, 11b, and 11c are 10 positive peak amplitude h, substantially equal to turned off. This operation charges the capacitors 120V, a negative peak amplitude h2 substantially C1f C2, and C3 in parallel to the supply at terminal equal to 65V, and a total width t2 substantially 14, which may be 230 volts. equal to four nanoseconds, one cycle of a 250 During the charging period and the completion MHz wave, when, in addition to the parameters thereof, the avalanche transistors 12a, 12b and is previously stated, the circuit components have 12c are substantially at cut off, by virtue of a -6V the following values: supply coupled to the gate thereof from terminals 16a, 16b, and 16c, via resistors R3a, R3b, and R3c, Ria,b,c=150,000 ohms respectively, only leakage current flowing there- R2a^b,o=680 ohms through. A positive trigger pulse, from a series of 20 R3a,b!o=in'°no ohms positive trigger pulses 17 of width Tt and height A, R4a'b'=5,000 ohms coupled via a terminal 18 and diodes 21 and 22 to R5=2,000 ohms the base of the transistor 12a causes the latter to Cii2=150 picofarads avalanche, thereby placing the capacitor C, in A=4 volts series with the capacitor C2 and establishing a 25 t, = 15 microseconds voltage level at the anode of the diode 1 1 e that is I2=15.3 centimeters the sum of the voltages across the capacitors C, and C2.