Transient Technologies LLC

Drift Step Recovery Diode Transmitter for High Power GPR Design

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Some elements of the modern ground penetrating radar (GPR) determine its performance factor, resolution and depth of sounding. There are impulse transmitter, ultra-wide-band receiver as well as transmitting and receiver antennas. Improvement of the GPR's parameters is usually achieved by modernization of receiving circuits, antenna design, decreasing of input noises and using of complex computational algorithms for on-line and post-processing. As active element for impulse generation are widely used step recovery diodes (SRDs) or avalanche transistors. However such devices can not generate nanosecond pulses up to some hundreds volts on the antenna terminal.

This work is devoted to application of drift step recovery diodes (DSRDs) in GPR transmitter design. Current drive circuit based on charge DSRD model has been computed and optimized. Investigation results for pulse generator characterized by peak power up to 5 kW and rise times as small as 2 nanosecond (ns) are reported. Application abilities of commercial power rectifier diodes in the DSRD mode are shown. Transmitter based on DSRD can operate with low impedance antennas, high repetition rate and efficiency.

Key words: drift step recovery diode, ground penetration radar, nanosecond pulse generation.

Ground penetrating radar (GPR) can be divided into some functional elements. There are transmitter, receiver, antennas set. control and computational unit and display. Transmitter is the most important component of the GPR and along with receiver and antennas it determines attainable performance factor, vertical resolution and depth of sounding.

Heart of the transmitter is subnanosecond pulse generator. As an active element of the scheme is usually utilized either avalanche diode (AD) or transistor (AT) or step-recovery diode (SRD). These devices are able to form subnanosecond pulses with high repetition rate and shape stability.

However, their peak power range don't exceed one kilowatt level and these widely used active elements are not suitable for creation of high power GPR that is destined for operation in difficult environments. For sharpening of kilovolts pulses with rise times on the order of a few nanosecond are capable a new active elements - drift step recovery diodes (DSRDs).

The main goal of this work is theoretical consideration and experimental investigation of DSRD application possibilities as an active element of subnanosecond generator for high power GPR design.

Properties of The Drift Step Recovery Diodes
Effect of high power nanosecond impulse generation by drift step-recovery diodes (DSRDs) lias been discovered by Russian inventors in 1931 (Grekhov et al.. 1931). In traditional SRD charge is stored in the diode by means of a nearly steady-state forward current flow. That is the forward bias exists continuously for times compared to or longer than the hole and electron lifetimes in the active region. Conversely high power DSRD uses a short forward bias pulse to introduce stored charge to the device. Since the pulse width is considerably less than the carrier lifetimes, the charge is concentrated near the junctions, which is desirable for a sharp reverse step recovery. These structures have been shown to be capable of operating at much higher power levels than conventional SRD structures. The DSRD have been found to be useful primarily above one kilovolts and offer lifetimes only somewhat better than conventional SRD. Brylevsky et al. (1988) thanks DSRD achieved peak powers more than 1.6 megawatt on 10-Ohm loading with two-nanosecond rise time.

The step-recovery effect in the DSRD can be observed only by satisfaction of specific conditions. Because charge carrier mobility in the drift diodes are low therefore current at the straight direction through the p-n junction not constant but briefly. Moreover straight time transition for the diodes with a long lifetime of the charge carriers lias to be as short as possible. If diode has a short lifetime charge

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