Pulse amplifier of the short-range radar system. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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To measure the speed of moving objects, such as cars, near-range radar systems based on the Doppler effect are widely used [1]. Microwave oscillation generators of these systems are most often performed on Gunn diodes operating in a continuous mode. The capabilities of such short-range radar systems can be expanded by switching the generators to a pulsed mode of operation. In this case, it becomes possible, in addition to measuring the speed of objects, to also determine the distance to them.

(click to enlarge)

In accordance with the passport data for Gunn diodes [2]. for their excitation, positive polarity pulse generators with an amplitude of 5 ... 6 V are required at an output current of 1.5 ... 2 A. Standard pulse signal generators, as a rule, operate on a standard load of 50 Ohms and have an output voltage of 1 V.

Figure 1 shows an amplifier circuit that allows you to increase the output parameters of a standard pulse signal generator to the required values. The amplifier contains an input resistive voltage divider, two amplification stages, a stable current generator, and a control output.

(click to enlarge)

The input voltage divider is made on resistors R1...R3. It ensures the matching of the amplifier with the output impedance of the generator and the stabilization of the depth of the overall negative feedback covering the amplifier. In both stages of the amplifier, built on transistors VT2 and VT4. active collector thermal stabilization of quiescent currents is applied (3). The quiescent currents of the transistors themselves were chosen based on the undistorted amplification of pulses with a duty cycle varying from 10 to infinity. For transistor VT2, the quiescent current is 70 mA, for transistor VT4 - 300 mA. The currents are set by selecting the resistances R5 and R12.

In the process of starting the generator on the Gunn diode, its resistance changes. To reduce the effect of changing load resistance on the characteristics of the amplifier, its output stage is made according to a common collector circuit, and the amplifier itself is covered by a common negative feedback through the R7-C8 chain. As a result, the output impedance of the amplifier does not exceed 0,4 ohms.

A change in the temperature of the Gunn diode crystal leads to a change in the instantaneous generation frequency [4]. To reduce this factor, a stable current generator based on a VT5 transistor is installed in the amplifier, which provides heating of the diode in the periods between trigger pulses. The generator current is adjusted using the potentiometer R18 within 0.1 ... 0.5 A. The amplifier has a control output for recording the amplitude of the pulses applied to the Gunn diode. Diode VD1 is installed to protect the amplifier transistors from breakdown if the power polarity is reversed. Diode VD2 is necessary to restore the constant component at the output of the amplifier.

The amplifier is assembled on a printed circuit board measuring 80x75 mm from double-sided foil fiberglass with a thickness of 2 ... 3 mm. The drawing of the board is shown in Fig. 2, Fig. 3 shows the location of the elements.

The dotted line in Fig. 3 indicates the places of metallization of the ends, which is necessary to eliminate parasitic resonances and ground the necessary sections of the printed circuit board. This can be done with metal foil. Transistors VT2. VT4 and VT5 are attached to the base using thermally conductive paste. The inductors are glued to the printed circuit board using dielectric spacers made, for example, from non-foil fiberglass.

Setting up the amplifier begins with setting the given quiescent currents of transistors VT2 and VT4 with resistors R5 and R12. Then, as a load equivalent, a resistor with a resistance of 4 ... 6 ohms is connected to the output of the amplifier. A negative pulse with an amplitude of 0,1 ... 0,2 V is applied to the input of the amplifier and the required gain is set by changing the resistance R7. It should be borne in mind that when the resistance R7 is less than 100 ohms, an overshoot appears on the leading edge of the pulse. This is due to the signal delay in the common feedback loop. The selection of resistances R19 and R20 sets the limits for regulating the current given by the generator to VT5.

Literature

1. Bakulev P.A., Stenin V.M. Methods and devices for selection of moving chains. - M.: Radio and communication, 1986.
2. Semiconductor devices. Microwave diodes: Reference book /B.A.Nalivaiko et al./. - Tomsk MGP RASKO. 1992.
3. Titov A. A. Calculation of the active collector thermal stabilization circuit and its use in amplifiers with automatic control of the consumed current. - Electronic equipment. Ser. microwave technology. 2001. No. 2. P.26.
4. Tsarapkin D.P. Microwave generators based on Gunn diodes. - M.: Radio and communication, 1982.

Authors: A.Titov, V.Pushkarev, Tomsk

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