Improved GIR. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Everyone who has dealt with a heterodyne resonance indicator knows that working with it is a rather painstaking task, because. during the measurement process, it is necessary to manipulate not only the frequency adjustment knob, but also the sensitivity control of the device, and in some designs [1] also the mode knob.

This is due to the fact that in almost all oscillators tunable in a wide frequency range [1, 2], the RF voltage amplitude also varies over a wide range. In order not to miss the moment of resonance, the tuning knob must be rotated as slowly as possible and carefully observe the readings of the dial indicator.

Working with the GIR is greatly simplified and accelerated if it is supplemented with a device that fixes the moment of resonance with some kind of light indicator.

On fig. 1 shows a diagram of a GIR with an LED resonance indicator. Its operation is illustrated by the graphs in Fig. 2 and fig. 3. The higher the rotation speed of the tuning capacitor rotor, the steeper the front of the change in the RF voltage on the circuit (line A1 in the graphs of Fig. 2 and Fig. 3).

The task is to fix a sharp decrease in the level of HF voltage. It is solved by using a differential amplifier, which, in the general case, does not respond to the absolute value of the parameter, but to its change in any direction.

The GIR master oscillator is assembled on a transistor VT1 according to the scheme described in [3]. The differential amplifier is assembled on transistors VT3, VT4, VT5. When tuning in the range in the direction of decreasing capacitance or, which is the same, in the direction of increasing the RF voltage (shown by the arrow in Fig. 2 and Fig. 3), the rectified voltage of negative polarity at the VT3 gate increases smoothly.

On the drain VT3 and the left plate of the capacitor C7, the positive polarity voltage also increases smoothly. Transistors VT4 and VT5 are locked. At the moment of resonance, the voltage at the VT3 gate changes sharply towards a positive potential, there is a sharp drop in the drain potential of VT3. Capacitor C7 "transfers" this potential drop to the base VT4. As a result, VT4 and VT5 open and the HL1 LED flashes brightly. Flash duration depends on the C7R7 charge time constant.

On the transistor VT2, a DC amplifier for the measuring device is assembled

Q - quality factor in arb. units
U - high-frequency voltage in arb. units
a is the angle of rotation of the condenser rotor C, deg.
C is the capacitance of the capacitor.
t is the time of rotation of the capacitor rotor, arb. units
v.1 - moment of resonance.

RA. Resistor R5 sets the required sensitivity of the device. With the help of the R4VD4 chain, an additional positive bias is applied to the source VT2. With resistor R3, the arrow of the device is set to any place on the scale that is most convenient for observing the moment-resonance.

Working with the device is very simple. The investigated oscillatory circuit is connected with the GIR circuit. The tuning knob quickly moves the capacitor from the maximum capacitance position to the other extreme position. If there was no LED flash, there is no resonance in this subrange.

If a flash of the LED was observed, setting the tuning knob approximately to the position at which there was resonance, set the maximum sensitivity of the measuring device with resistor R5, set the arrow to the middle of the scale with resistor R3 and, slowly turning the GIR tuning knob, determine the moment of resonance in the traditional way. For a more accurate determination of the moment of resonance, a "stretching" tuning capacitor with an air dielectric C5 with a capacity of 2 ... 15 pF is used, the handle of which is displayed on the front panel of the GIR. The value of the resonance frequency is read on the scale of the frequency meter.

The values ​​of L, C* are given in the table. Radio amateurs themselves can calculate the values ​​​​of L, C * and winding data L based on the selected cutoff frequencies of the subbands, the available variable capacitor and frames for the inductors. The method for calculating L, C* has been repeatedly cited in the technical literature, for example [4,5].

When repeating the GIR according to this scheme, it must be taken into account that a periodic disruption of oscillations (relaxation) can be observed in the low-frequency range due to the high quality factor of the circuit and the large POS. You can get rid of this either by including a 47 - 200 Ohm resistor in the tap from the coil, or by making a tap not from the middle of the coil, but closer to the "ground" end. It should also be noted that the LED will flash every time the capacitor rotor rotates rapidly in the direction of increasing capacitance, because. in this case, the RF voltage on the circuit decreases.

Literature

1. Transistor GIR // Radio. - 1971. - N 5. - S. 55.
2. Borisov V. GIR // Radio. - 1974. - N3. - S. 53.
3. Gavrikov V, Prakhin P. Amplitude-stable local oscillator // Radio. - 1984. - N 2. - S. 22.
4. Biryukov S. On the calculation of oscillatory circuits of generators // Radio. - 1992. - N11-S. 23.
5. Malinin PM Handbook of radio amateur-designer. - M.: Energy, 1978.

Author: V. Demyanov, Kremenchug; Publication: N. Bolshakov, rf.atnn.ru

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