Variable Repetition Research Articles

Variable width pulse generator using avalanche transistors and charge-storage diodes

A variable width pulse generator is described in which a delay-line-formed avalanche transistor pulser is used to turn off a charge-storage diode. Pulse width is controlled by a variable d.c. bias current flowing through the charge-storage diode and is continuously variable from 3–30 ns. The rise time is determined by the avalanche transistor and the fall time by the charge-storage diode. Output is 0–8 V into 50 ohm with a rise time of one ns and a fall time of 2 ns. Two similar circuits are combined to form a double pulser with variable repetition rate 100 kc/s-2 Mc/s and with variable pulse spacing of 10–200 ns.

Silicon-controlled rectifier D-C to D-C power converters

The operation of a d-c chopper circuit using a silicon-controlled rectifier (SCR) is analyzed. A description is given of a saturating reactor (SR) and capacitor which is used to extend the conducting interval of the controlled rectifier and then turn it off. Some useful variations of the basic circuit also are presented. Emphasis is placed upon the waveforms produced, and typical oscillograms are shown. The specific circuits analyzed employ a variable repetition frequency to control load voltage, but the modifications necessary for operation at a fixed frequency with variable “on” time are also mentioned.

Non-linear resistance of thermal and non-thermal origin in bismuth and reheated pyrolytic graphite

The resistance of bismuth and reheated pyrolytic graphite at liquid helium temperature in a magnetic field of 27 kOe and 43 kOe, respectively, was measured using current pulses of variable duration and repetition rate. Nonlinearity in the current-voltage plots was observed at pulse repetition rates of 281 to 6000 cps with 20 mu s pulses for bismuth, and pulse repetition rate of 108 cps with 5 mu s pulses for pyrolytic graphite. (L.B.S.)

High Current Low Impedance Pulse Generator

Design parameters are given for a pulse generator useful for driving injection luminescent diodes, photoflash tubes, and other loads of impedance 3Ω or less. Rectangular output pulses are provided with a choice of 5- or 50-μsec duration. The maximum pulse current of 200 A is supplied at variable repetition rates of up to 20 pulses/sec for the wider pulse and 200 pulses/sec for the 5-μsec pulses.

Sound Sources and Probes for the Measurement of Pulsed Acoustical Waves in Water

Acoustical pulses of variable length and repetition rate are generated by an electronic system composed of instruments which are commercially available, except for a gate unit. The system is designed to permit acoustic transmission and reflection experiments in a controlled water medium within the confines of a laboratory-size wooden tank. Pulses of sine wave between 60 kc and 1 Mc are used. An acoustical source constructed of a barium-titanate spherical cap generates an acoustical pressure field as high as 2.1×104 μbar (86.7 db re 1 μbar at 1 m) at 230 kc at a distance of 1 m which is sensed by miniature hydrophones with lead titanate-zirconate disk elements as small as 0.063 by 0.009 in. The ceramic is enclosed by metal to overcome problems encountered previously with waterproof coatings incapable of keeping water from the ceramic for long periods of immersion. Pressure sensitivity of the probe hydrophone with preamplifier (49.6-db gain) is 10.3×10−6 v/μbar (−99.8 db re 1 v/μbar) at 230 kc.

System for Oscillographic Recording of Selected Pulses from a Series of Randomly Arriving Impulses

When velocity logging oil wells, a sonde is raised from the bottom of the well borehole at a slow, but variable and unpredictable rate. This sonde transmits pulsed acoustic energy at a variable pulse repetition rate into the earth surrounding the borehole and, in addition, detects the refracted acoustic pulses after they have traveled a few feet through the earth surrounding the borehole. In order to ascertain certain characteristics of the earth, it is desirable to record photographically those pulses occurring, not at specified time intervals, but at specified depth intervals. This is accomplished by converting the sonde depth to electrical gate pulses occurring at the desired depth intervals. These gate pulses are used to open a gate which permits only the first acoustic pulse following a gate pulse to pass through the gate and be photographed on a triggered oscilloscope. The equipment is fully automatic in that the camera film is automatically advanced between recorded pulses and precautions are automatically taken to prevent double exposures. Similar instrumentation should be applicable to related problems involving the photographing of system responses as a function of some measurable parameter which varies slowly but unpredictably with time.

A Tone Burst Generator

A tone burst generator is described for the production of electronically keyed oscillations free of transients. The unit consists of an oscillator, a keying circuit, and a negative pulse generator with variable duration and repetition rate. The advantages of the tone burst method in testing the frequency and transient response of amplifiers and transducers are pointed out. Some applications including reverberation time measurements are also mentioned. The almost instant rise and decay of the tone bursts was achieved by providing practically zero impedance path to ground for the keying pulses.

A Laboratory Oscilloscope

The laboratory oscilloscope is designed primarily for use in the nuclear physics laboratory but is a versatile instrument well suited to general circuit research. It contains a sweep generator with writing speeds ranging from 8 inches per microsecond to 1 inch per millisecond. The sweep is initiated by a voltage amplitude discriminator which may be triggered by: (a) an internal trigger generator operating at variable repetition rates, (b) a jitter-free delayable trigger which may be generated at any time from minus 2 to plus 400 microseconds with respect to a fixed trigger from the master generator, (c) from any arbitrary signal greater than 5 volts, and (d), any input to the signal amplifier larger than 0.2 volt. A high fidelity delay line is incorporated in the amplifier circuit so that the sweep may be initiated by the input signal to the amplifier before the signal appears at the scope plates. A 1-volt input to the amplifier will give a 1½-inch screen deflection with a minimum rise time of 0.15 microsecond. The trigger generator, which starts the delay circuits, may be triggered externally by pulses of 15-volts amplitude or more. Sufficient voltage is applied to the type-5CP11 scope tube to permit photography of transients with writing speeds of 3 inches per microsecond using a camera with an f:2 lens opening and Ansco Fluorapid film or Eastman 103–0 plates.