Robert A. Lawton

Optically strobed sampling oscilloscope

Describes an optically strobed sampling oscilloscope which uses optical pulses from a GaAs laser diode to strobe a Cr-doped GaAs photoconductor which serves as a sampling gate. This sampling gate has been used with a commercial sampling oscilloscope to provide a sampling measurement capability that can measure higher voltages than conventional samplers and does not exhibit strobe pulse kickout. Preliminary tests indicate a time resolution, limited by the width of the optical pulse used, for about 100 ps.

A Solid-State Reference Waveform Standard

A solid-state reference waveform filter has been developed which uses the Maxwell-Wagner capacitor effect. This filter is realized in a stripline configuration with a lossy dielectric consisting of a thick (5-¿m) layer of SiO2 on Si. The equivalent circuit of this filter is equivalent to that for previously developed filters which used a lossy liquid dielectric. A preliminary design has been completed and a filter fabricated for which the design characteristic impedance, 38 ¿, and transition duration (rise time), 300 ps, agree with measured values to within 2 and 17 percent, respectively. The temperature dependence of the filter transition duration has been estimated from the temperature dependence of the filter conductance to be about 1 percent/° C.

Electrically strobed optical waveform sampling oscilloscope

A new instrument for measuring optical waveforms is described. It is an optical sampling oscilloscope employing a GaAs photoconductive sampling gate and an electrical sampling strobe. The basic theory of operation is derived. A 100‐psec rise‐time rudimentary prototype has been built and results of measurements of a GaAs laser diode pulse are presented.

A Precision Current Comparator

A technique capable of precise comparison of currents in different parts of a network or between currents in different networks is discussed. With proper conditions, this comparison can be done with negligible perturbation of the networks under measurement. Experiments on a coaxial system at 30 MHz are discussed.

Autocorrelation and Power Measurement with Pyroelectric and Dielectric Bolometers

A pulse comparison technique is described which yields the autocorrelation function and the power spectrum of a repetitive time-domain waveform. The autocorrelation function is realized with a sliding short in a coaxial transmission line to provide time delay; a pyroelectric bolometer to provide multiplication through a square law voltage response; and a capacitor to provide integration. Problems of realization of a perfect time delay and integration limitations are considered, and it is found that noise fluctuations yield the main time resolution limitation that is equivalent to 8 ps for 15-V pulses and a 50-s integration time. The pyroelectric voltage-sensing bolometer is then compared to a pyroelectric capacitance sensing bolometer. It is shown that the capacitance sensing bolometer can handle much longer pulse durations than the voltage sensing bolometer. It is also demonstrated that the sensitivities of the two techniques are equivalent in a typical case at a capacitance sensing bolometer bridge voltage of 3 V. Measurement results of the autocorrelation function and power spectrum, using a voltage sensing pyroelectric bolometer, are given for a nominal 15-V, 500-ps time duration, whose baseband pulses have a 100-pps (pulses per second) repetition rate.

Dynamic performance of digital recorders used for monitoring high-voltage impulse tests

Frequency- and time-domain characteristics of digital transient recorders (in short, digitizers) are discussed in order to establish the requirements on digitizers used for high-voltage testing. Results of an experimental study performed on a 200-MHz 8-bit digitizer are presented and related to the design features of this instrument. The inherent design characteristics and their influence on the digitizer dynamic performance are analyzed in view of simulation of the digitizer through a computer model.

Pulse Waveform Standards for Electro-Optics

The maturing of the semiconductor industry has resulted in the need for faster methods for the measurement of pulse waveforms. The measurement requirements are fast outstripping the capability of integrated circuit testors. The fastest commercial sampling oscilloscope today has a step response transition duration (risetime) of 20 picoseconds. For the measurement of single transients, available time resolution is somewhat less. This time resolution is not adequate to support the measurement of the faster semiconductor devices being developed, such as gallium arsenide logic gates whose switching times have been estimated to be 12 picoseconds [1]. Measurement of such transitions requires a significant advance in measurement technology. A prime candidate for making the required quantum leap in time measurement resolution is the electro-optic sampler [2]. This sampler has a demonstrated resolution of 0.4 picoseconds and a theoretical resolution limit much shorter than that.

An Efficient Antialiasing Filter

The application of a solid-state reference filter as an efficient antialiasing filter is described. The analytical basis for the efficiency of this filter is described and a specific example of measuring a 1024-point waveform with an RC filter and the solid-state filter is given.

A Wide-Range CW Power Measurement Technique

An accurate power measurement technique is described, which makes possible the determination of the net power delivered to a load of arbitrary impedance over a wide power range. A standard power meter is employed to fix a reference power level. Subsequent measurements consist of dimensionless ratios that can be obtained from precision attenuators. The method is applicable to a very wide range of frequencies and was demonstrated at a frequency of 30 MHz with power measurements extending from 10-2 to 10-14 watt. Maximum uncertainties ranged from ~0.5 to 1.5 percent. This technique is applicable in the measurement of the sensitivity of very-low-level detectors, receivers, radiometers, etc.

New Standard of Electric Field Strength

A new technique is presented for establishing a standard of electric field strength using a highly conducting sphere. An analysis is made to determine the current on the sphere as a function of the electric field strength of an incident plane wave. A method of measuring that current using electronic circuitry and an optical indicator within the sphere is described, and an intercomparison is made with an independent field-strength standard. This technique is a significant improvement over previous ones in that it permits the absolute determination of field strength with a maximum uncertainty of 1 percent or less at 30 MHz and is applicable to a broad range of frequencies and field strengths.