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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING GIR with LED indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Rarely found in an amateur radio laboratory, a heterodyne resonance indicator can be used to assess the resonant frequency of a high-frequency oscillatory circuit or the parameters of its components - capacitance or inductance. The design proposed by the author has small dimensions and, in comparison with the GIR with a magnetoelectric indicator, is more convenient in operation. To determine the resonant frequency of an oscillatory circuit in a given range or to measure small values of inductance or capacitance, a simple heterodyne resonance indicator (GIR) with light indication can be used. Its scheme is shown in Fig. 1.
The RF generator is assembled on a high-frequency transistor KT316A with a resonant circuit according to the capacitive three-point circuit. The operating frequency range is 110 ... 170 MHz. The generator frequency is tuned by changing the voltage across the varicap VD2 with a variable resistor R2. When an unloaded generator is operating, the voltage rectified by the VD3 diode closes the field-effect transistor VT2, the current through it is small and the LED does not light up. If the coil L1 of the generator is placed in close proximity to the coil of the oscillatory circuit, then when the GIR is tuned to resonance with the external oscillatory circuit, the losses introduced by this circuit increase so much that the closing voltage at the gate VT2 noticeably decreases. The LED turns on to indicate that the tuning frequency of the associated circuits matches. The frequency range of the generator can be changed within certain limits by the appropriate choice of the inductance of the L1 coil or by using another varicap. However, it should be borne in mind that with an increase in the number of turns of the coil, the intrinsic (interturn) capacitance also increases, which limits the range of tuning of the generator. Power for GIR can be used from a battery of galvanic cells with a voltage of 9 V or another external power supply. A special power switch is not provided in the device.
To increase the sensitivity of the GIR, it is desirable to select a field-effect transistor VT2 (KP303B) with a minimum cutoff voltage. A tinned tin case from the Krona battery was used as the case. To install a variable resistor R2 in the center of the upper (according to Fig. 2) part of the case, a hole is drilled and a cut is made with scissors from the edge to this hole. After installing the resistor, this slot is sealed. To rotate the axis of the variable resistor, a suitable plastic gear wheel was used, on which it is convenient to apply a digital scale for the GIR tuning frequency. The HL1 LED is installed next to the wheel so that it acts as a risk for counting the tuning frequency. To improve the reading accuracy, the indicator case can be turned with a needle file to give it a triangular shape (like LEDs of the KIPM06, KIPM07 series, which can also be used in this design). Almost all parts are mounted on a small board installed inside the case. Elements VD1, VD4, R1, R2 and LED HL1 are mounted directly in the housing. Coil L1 consists of four turns of PEL 0,45 wire wound on a mandrel with a diameter of 3 mm. This coil is soldered from the outside of the board (case) so that the distance between the coil and the GIR case is about 15 mm. The device is installed in the following order. A battery connector board with a soldered VD4 diode is installed in the case, which is fixed by soldering pieces of copper wire. Then a variable resistor is installed with elements R1 and VD1 soldered to it. An LED is glued into the housing. The board is installed in place after setting up, and the corresponding leads are soldered. When setting up the device by selecting the resistor R4 in the bias circuit VT1, stable generation is achieved over the entire frequency range. Further, at the lowest (according to the scheme) position of the variable resistor engine, by selecting the resistor R6 in the gate circuit of the KPZ0ZB field-effect transistor, the minimum brightness of the LED is reached. Calibration is best done using an exemplary frequency meter or oscillatory circuits with a known resonant frequency. The frequency value is scratched out with an awl on the plastic wheel of a variable resistor. Before the measurement, a battery or other power source with a voltage of 9 V is connected to the GIR terminal block. The L1 coil is brought closer to the circuit under test and the wheel is rotated until the HL1 indicator lights up, opposite which the resonance frequency is read. The operability of the GIR can be checked by introducing a metal object into the coil L1. In this case, the energy consumption of the circuit also increases, which will immediately be indicated by the lighting of the HL1 indicator. To determine the inductance of the coil, a capacitor with a known capacitance is soldered to it in parallel, forming a "trial" circuit. The coil of the device is brought closer to the coil under test, and by rotating the wheel the tuning indicator is ignited, after which the resonance frequency is determined on the scale. The inductance of the tested coil Lx is found from the known values of the resonance frequency F and the capacitance of the capacitor C according to the formula Lx \u25330d 2 / (C-FXNUMX), where L is the coil inductance in μH; C is the capacitance of the exemplary capacitor in pF; F - frequency in MHz. The capacitance of a capacitor is evaluated in a similar way. An oscillatory circuit is assembled from the test capacitance Cx and the exemplary inductance L and its resonant frequency F is determined using the device. The capacitance is calculated by the formula Cx \u25330d 2 / (LFXNUMX). GIR can be used especially effectively to determine the inductance of coils in fractions of a microhenry. For example, a coil of eight turns of 0,45 mm PEL copper wire wound on the threaded part of an M3 screw used as a mandrel has an inductance of 0,1 μH. Author: V. Gorbatykh, Ulan-Ude
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