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Electronic detuning

Most often as part of detuning applied to the varicap is a special diode, the capacitance of which changes under the action of the applied voltage. With all the ease of use, the variable capacitor has significant shortcomings. First of all, there is a significant temperature coefficient, non-linear depending on the temperature and on the applied voltage, therefore, impossible full compensation.

This means that GPA with varactors as the main element of the restructuring will always "sail", and with the varicap as part of a small adjustment will be a little to "swim." Secondly, the variable capacitor has a low quality factor, which is bad for the stability of the frequency and, thirdly, as a nonlinear element that degrades the noise performance of the GPA, its spectral purity, which is especially important in the design of transceivers with a large dynamic range.

The scheme of the electronic detuning GPA, Fig.1, which, in my opinion, went unnoticed by the majority of radio Amateurs [1]. Using this method of frequency tuning is provided by a smaller initial run frequency and lower her care after prolonged use. On the 7MHz frequency detuning can be 250 kHz, depending on the GPA scheme. If you don't need a large range of detuning, we should eliminate the elements VD1, R2, C1, and the source of transistor VT1 should be connected to ground.

Temperature compensation

In the process of heating the parts of the GPA change their sizes and, depending on the total temperature coefficient and its sign GPA frequency starts to drift up or down. Temperature compensation should be extracted half the variable capacitor. From the angle of rotation of the rotary plate determines his TKE (temperature coefficient of capacitance). Heat the case of the GPA should be uniform, preferably in a thermostat, controlling the temperature inside the thermostat. If there is no industrial thermostat, you can make homemade wooden box and the heating elements can serve as paws bulbs, reflectors, low-power heaters and Heating etc., thus, will be less uniform.

Heating the body of the GPA to the temperature (40 - 50)° and cooling naturally without forced ventilation, checks the cyclicity of change of frequency. If the steady-state frequency value after the heating-cooling cycle differs from the original 200 - 350 Hz, you need to find and replace the item with no cyclic temperature coefficient.

Some items possess the property under the influence of temperature leap to change the parameters. Most often it is the capacitors, particularly, tubular - CT. There is a "flicker" frequency. There is a simple method - heating soldering iron, inserted in the place of the sting ceramic rod, all parts included in the GPA, and listening to the signals of the GPA on the radio (for example, P-M) you can find the defective part. When heated, causing part of the frequency change occurs smoothly, without jumps and "blink". Sometimes, flickering may occur due to mechanical instability of the installation.

The selection of the temperature compensating capacitors with the desired TKE achieve the frequency drift of no more than 10 to 20 Hz/deg during heating of the housing of the GPA. Massive dural body has the greater thermal inertia, the thicker its walls and the better the stability of the GPA. Monitoring frequency should not be performed earlier than 15-20 minutes after soldering in the GPA. Frequency stability test at a constant temperature in the extreme positions of the variable capacitor capacity. After 15 minutes of heating it should not be worse than 50-100 Hz/hour. Temperature compensation can be considered finished, when rebuilding the GPA from one end of the band in other frequency drift changes sign, i.e. at the beginning of it from warming up increases, decreases at the end or Vice versa. Making sure that the process occurs in this way, we can safely set the GPA in the transceiver. Also consider the temperature compensation of the power source.

Designing traditional GPA

Design GPA - theme is endless, but the basic principles should be cited. I think that it will be useful not only for novice hobbyists. Frequency stability is a serious problem for most Amateur radio designs.

1. Traditional GPA runs as an independent design - the frame should be very rigid and preferably compact. The casing is made of thick aluminum with a thickness of 4-6 mm. wiring should be elongated (straight, no loops) wire 1-2 mm.

2. The mounting elements on the PCB are not desirable. Preferably to a hinged mounting on insulating supports. May be suitable ceramic fuses with pre-vydannimi wires. In the self-made designs you can apply ceramic disk switches, contacts installation.

3. GPA should be placed away from heat-generating components and should not be washed convection air ducts. If these conditions are not met, it is necessary to provide temperature control. Easiest "cold" temperature control. For this GPA box outside to glue the sheet (10 mm) foam.

4. Castorocauda elements of the GPA should be as high quality. Variable capacitor with a large air gap (1-2), thick plates, preferably copper, with a porcelain axis on bearings. The coil, if possible, with porcelain has gold plated winding. Conclusions elements and connecting wires - minimum length without mechanical tension.

5. The frequency switching is provided by disk ceramic switch or the remote switch high frequency relay, for example, RPS-32 work well up to a frequency of 50 MHz,

6. The frequency stability depends not so much from the scheme as on the quality of parts used or workmanship. I can recommend a few, well-established schemes - "Radio" No. 5-90 page 59, "Radio Amateur" No. 9-93 p. 38.

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

Literature

  • The Radio magazine No. 5 of 1989 page 96
  • Author: A. Kuzmenko, RV4LK; Publication: N. Bolshakov, rf.atnn.ru