A little about the GPA. Scheme, description

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A little about the GPA. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes

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Electronic detuning

Most often, a varicap is used as a detuning element - a special diode, the capacitance of which changes under the action of an applied voltage. With all the ease of use, the varicap also has significant drawbacks. First of all, there is a significant temperature coefficient, which depends non-linearly on temperature and on the applied voltage, and therefore complete thermal compensation is impossible.

This means that a GPA with a varicap as the main adjustment element will always “float”, and with a varicap as a minor adjustment element it will “float” a little. Secondly, the varicap has a low quality factor, which has a bad effect on frequency stability and, thirdly, as a non-linear element, it worsens the noise characteristics of the GPA, its spectral purity, which is especially important when designing transceivers with a large dynamic range.

A little about the GPA. Scheme of electronic GPA detuning

A scheme of electronic GPA detuning is proposed, Fig. 1, which, in my opinion, passed by the attention of most radio amateurs [1]. Using this method of frequency tuning, a smaller initial frequency overshoot and less frequency drift during long-term operation are provided. At a frequency of 7 MHz, the detuning can be 250 kHz, depending on the GPA circuit. If a large detuning range is not needed, then the elements VD1, R2, C1 should be excluded, and the source of the transistor VT1 should be connected to ground.

Thermal compensation

In the process of heating, the GPA parts change their dimensions and, depending on the total temperature coefficient and its sign, the GPA frequency begins to drift up or down. Thermal compensation should be carried out with the variable capacitor half-out. Its TKE (temperature coefficient of capacity) depends on the angle of rotation of the rotor plates. The GPA housing should be heated evenly, preferably in a thermostat, controlling the temperature inside the thermostat. If there is no industrial thermostat, you can make a homemade one from a wooden box, and incandescent paws, reflectors, low-power heaters, etc. can serve as heating elements. Heating, in this case, will be less uniform.

By heating the housing of the GPA to a temperature of (40 - 50) ° and, cooling it naturally without forced ventilation, the cyclicity of the frequency change is checked. If the steady-state value of the frequency after the heating-cooling cycle differs from the initial value by 200 - 350 Hz, it is necessary to find and replace the part with a non-cyclic temperature coefficient.

Some parts have the ability to change their parameters abruptly under the influence of temperature. Most often these are capacitors, especially tubular ones - KT. Frequency flicker occurs. There is a simple way - by heating with a soldering iron, with a ceramic rod inserted instead of a tip, all the parts included in the GPA in turn, and listening to the GPA signal on a radio receiver (for example, R-326M), you can find a faulty part. When serviceable parts are heated, the frequency change occurs smoothly, without jumps and "flickering". Sometimes, flickering can occur due to mechanical instability of the installation.

By selecting thermally compensating capacitors with the required TKE, the frequency shift is achieved by no more than 10 - 20 Hz / deg when the GPA case is heated. The massive duralumin case has the greater thermal inertia, the thicker its walls and the better the stability of the GPA. Frequency control should not be performed earlier than 15-20 minutes after soldering in the GPA. Frequency stability is checked at a constant temperature in the extreme positions of the capacitor with a change in capacitance. After 15 minutes of warm-up, it should not be worse than 50-100 Hz/hour. Thermal compensation can be considered complete if, when the GPA is tuned from one end of the range to the other, the frequency drift changes sign, i.e. at the beginning it increases from warming up, at the end it decreases or vice versa. After making sure that the process occurs in this way, you can safely install the GPA in the transceiver. You should also take care of thermal compensation of the power supply.

Designing a traditional GPA

Designing a GPA is an immense topic, but the basic principles should be given. I think that it will be useful not only for beginner radio amateurs. Frequency stability is a major concern for most amateur radio designs.

1. The traditional GPA is made as an independent structure - the frame must be very rigid and preferably compact. The body is made of thick duralumin 4-6 mm thick. Wired installation should be carried out with an elongated (straight, without loops) wire 1-2 mm.

2. Installation of elements on a printed circuit board is not desirable. It is preferable to conduct it by hanging mounting on insulating racks. Ceramic fuses with pre-soldered wires may be suitable. In home-made designs, ceramic biscuit switches can be used, on the contacts of which the installation is carried out.

3. GPA should be located away from heat generating units and should not be washed by convection air ducts. If these conditions are not met, temperature control should be provided. The easiest way is "cold" temperature control. To do this, glue the GPA box on the outside with sheet (up to 10 mm) foam.

4. The frequency-setting elements of the GPA should be of the highest quality. A variable capacitor with a large air gap (1-2 nm), thick plates - preferably copper, with a porcelain axis on bearings. The inductor, if possible, is porcelain with burnt silver-plated winding. Conclusions of elements and connecting wires - the minimum length without mechanical tensions.

5. Frequency switching is provided by a ceramic switch or a remote switch on high frequency relays, for example, RPS-32 work well up to a frequency of 50MHz,

6. Frequency stability depends not so much on the circuit, but on the quality of the parts used and the workmanship. I can recommend several well-proven schemes - "Radio" No. 5-90 p. 59, "Radio Amateur" No. 9-93 p. 38.

7. After assembling and installing the GPA, it is desirable to remove mechanical stresses by heating the block to a temperature of 100-120 ° and allow it to cool naturally.

Literature

Magazine "Radio" No. 5 1989 p.96

Author: A. Kuzmenko, RV4LK; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Knots of amateur radio equipment. Generators, heterodynes.

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