Encyclopedia of radio electronics and electrical engineering / radio reception

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The proposed RF generator is an attempt to replace the bulky industrial G4-18A with a smaller and more reliable device.

Usually, when repairing and adjusting HF equipment, it is necessary to "lay" the HF bands using LC circuits, check the passage of the signal through the RF and IF paths, tune individual circuits into resonance, etc. Sensitivity, selectivity, dynamic range and other important parameters of HF devices are determined by circuit design, so it is not necessary for a home laboratory to have a multifunctional and expensive RF generator. If the generator has a fairly stable frequency with a "pure sine wave", then it is suitable for a radio amateur. Of course, we believe that the arsenal of the laboratory also includes a frequency meter, an RF voltmeter and a tester. Unfortunately, most of the HF RF generator circuits I tried produced a very distorted sine wave, which could not be improved without unjustifiably complicating the circuit.

The RF generator, assembled according to the scheme shown in Fig. 1, proved to be very good (it turned out to be an almost pure sine wave in the entire HF range). The scheme from [1] was taken as a basis. In my circuit, instead of adjusting the circuits with a varicap, KPI is used, and the indicator part of the circuit is not used.

Fig. 1

In this design, a variable capacitor of the KPV-150 type and a small-sized range switch PM (11P1N) are used. With this KPI (10 ... 150 pF) and inductors L2 ... L5, a section of the HF band of 1,7 ... 30 MHz is blocked. In the course of work on the design, three more contours (L1, L6 and L7) were added to the upper and lower sections of the range. In experiments with KPIs up to 250 pF, the entire HF range was covered by three circuits.

The RF generator is assembled on a printed circuit board made of foil fiberglass with a thickness of 2 mm and dimensions of 50x80 mm (Fig. 2). Tracks and mounting patches are cut with a knife and cutter. The foil around the parts is not removed, but is used instead of the "ground". In the figure of the printed circuit board, for clarity, these sections of the foil are conventionally not shown. Of course, you can also make a printed circuit board, shown in [1].

Fig. 2

The entire design of the generator, together with the power supply (a separate board with a 9 V voltage regulator according to any scheme) is placed on an duralumin chassis and placed in a metal case of suitable dimensions. I used a 130x150x90mm cassette from old equipment. The front panel displays the range switch knob, the KPI tuning knob, a small-sized RF connector (50 Ohm) and an LED power-on indicator. If necessary, you can install an output level regulator (variable resistor 430...510 Ohm) and an attenuator with an additional connector, as well as a graduated scale.

Unified sectional frames of the MW and LW ranges from outdated radio receivers were used as coil frames for the circuits. The number of turns of each coil depends on the capacity of the KPI used and is initially taken "with a margin". When adjusting ("laying" the ranges) of the generator, part of the turns is unwound. The control is carried out by frequency meter.

The inductor L7 has a ferrite core M600-3 (HN) Sh2,8x14. Screens are not installed on the coils of the circuits. The winding data of the coils, the boundaries of the subranges and the output levels of the RF generator are shown in the table.

In the generator circuit, in addition to the indicated transistors, field-effect KP303E (G), KP307 and bipolar RF transistors BF324, 25S9015, VS557, etc. can be used. It is desirable to use blocking capacities imported small-sized.

Coupling capacitor C5 with a capacity of 4,7 ... 6,8 pF - type KM, KT, KA with low RF losses. It is highly desirable to use high-quality KPIs (on ball bearings) as KPIs, but they are in short supply. More accessible are adjusting KPIs of the KPV type with a maximum capacitance of 80 ... 150 pF, but they break easily and have a noticeable "hysteresis" when rotating forward and backward.

Nevertheless, with rigid installation, high-quality parts and warming up the generator for 10 ... 15 minutes, it is possible to achieve a frequency “drift” of no more than 500 Hz per hour at frequencies of 20 ... 30 MHz (at a stable room temperature).

The signal shape and the output level of the manufactured RF generator were checked using an S1-64A oscilloscope.

At the final stage of adjustment, all inductors (except for L1, which is soldered at one end to the body) are fixed with glue near the range switch and KPI.

Literature

1. Shortwave GIR - Radio, 2006, No. 11, p.72.

Author: A. Perutsky, Bendery, Moldova.

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