Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Indicators and probes, consisting of an RF rectifier and a microammeter, have been used by radio amateurs for decades when tuning transmitters and antennas. The remote RF voltage monitoring devices proposed in this note allow you to monitor changes in their readings "by ear", without interrupting the observation of other devices for this. They do not require a separate power supply, since they receive it directly from the RF field of the tuned antenna.

The simplest version of an audio RF probe (Fig. 1) is a CMOS inverter oscillator based on a DD1 chip. The generator is powered by direct current coming from the rectifier of the RF voltage on the diodes VD1, VD2. The probe, on which the insulating tube is dressed, forms a very small capacitance with the antenna web, in tenths of a picofarad. However, rectification of the induced RF voltage provides a current of several tens of microamperes, which is quite sufficient for the sound generator to work. The frequency of sound when the supply voltage changes in the range of 1,5 ... 5 V changes tenfold, convincingly indicating changes in the RF voltage, its maxima and minima.

Since not all people have an absolute ear for music, another device was developed (Fig. 2), which marks the sound of the indicator at points with the same voltage. It contains two generators made on the elements of the DD1 chip. The generator on the elements DD1.1 and DD1.2 with a frequency of several hertz modulates the operation of the generator on the elements DD1.3 and DD1.4 with a frequency of several hundred hertz.

The operation of the first generator is enabled starting from some input voltage. This voltage is the smaller, the lower the current consumed by the microcircuit in the generation mode. Therefore, it is advisable to first select a microcircuit with a consumption current of less than 100 μA at Upit \u3d 60 V. In the copy used by the author, this current was 1 μA. By selecting the resistor R1.5 (within 10 ... 3 kOhm) and the number of diodes VD7 - VD3 (6 - 1,5 pcs.), You can change the parameters of the generators and the modes of their interaction over a wide range. In the author's copy of the probe, when Upit changes from 2 to 100 V, the frequency of the second (sound) generator smoothly changes from 1,5 Hz to 1,5 kHz. Then the modulating generator starts working, and the tone of the sound generator changes due to modulation, continuing to rise from 2 to 15 kHz with an increase in the supply voltage to 15 V. At a voltage of XNUMX V, the generation breaks down for both generators.

Diodes VD1.VD2 can be replaced by other germanium high-frequency ones, for example, D20, D311, GD507, GD508. Diodes VD3 - VD7 - any small silicon.

The design of the probe is shown in fig. 3. It is assembled on a printed circuit board made of double-sided foil fiberglass with a thickness of 1.5 ... 2 mm. Tracks are cut with a knife or cutter Parts are placed on the side of the conductors. From above, the device is closed by a 10 mm high casing screen made of tinned sheet 0.8 mm thick. Several holes 4 mm in diameter were drilled in the fiberglass at the place where the emitter was installed.

To avoid failure of the microcircuit when the field strength increases, it is recommended to connect a 16 ... 18 V zener diode in parallel with the rectifier capacitor.

Author: I. Vakhreev (RW4HFN)

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