FM generator for the range of 90 ... 110 MHz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception
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Figure 1 shows an electrical circuit diagram of an FM generator capable of operating in the "FM" range. The generator can be used together with high-quality sound-reproducing equipment.
Fig.1. Schematic diagram of the FM generator
The generator itself is made on a field tetrode VT1 type BF900. The use of a double insulated gate field effect transistor made it possible to obtain a very stable oscillator with a very low noise level in the output signal.
The desired frequency is set by the tuning capacitor C4. To ensure the required frequency deviation and small non-linear distortion, a constant bias voltage is supplied to the VD1 varicap from the variable resistor R2 engine. The low frequency input signal enters the device through the blocking capacitor C1.
During the operation to test the generator, the following results were obtained:
- output frequency, MHz ... 100;
generator load resistance, Ohm ... 50;
- high frequency output voltage amplitude, mV ... 22,5;
- RF signal frequency drift, MHz ... 0,005.
When testing the generator, a signal with a frequency of 0,5 ... 55 kHz from a stereo encoder was applied to the input. The above frequency drift was noticed after the first hour of operation, and then the frequency changed slightly.
Construction and details
On fig. 2 shows a drawing of a printed circuit board, in fig. 3 - the location of the elements of the device on it.
Ris.2.
Ris.3.
In the manufacture of the generator should be guided by the rules of high-frequency installation. The printed circuit board must be made of high-quality double-sided foil fiberglass. The foil on one side of the board is completely left and used as a "ground". In this case, it is necessary to connect the "earth" tires of both sides with through jumpers. The board itself is desirable to be placed in a shielded case with high-frequency connectors.
Under the seat of the transistor VT1, a hole is made in the printed circuit board, approximately 6 mm in diameter. From the "ground" side around the hole, it is necessary to remove the foil with a diameter of 1 ... 1,5 cm.
It is desirable to use only leadless elements (SMD components) in the device. Instead of an integral stabilizer DA1, you can use a conventional parametric stabilizer on a zener diode. However, in this case, the stability of the generator operation may deteriorate and the noise level in the output signal may increase.
Transistor VT1 type BF900 can be replaced by BF905, BF907, KP327A, etc. The inductor L1 is wound with a silver-plated copper wire with a diameter of 1 mm on a mandrel with a diameter of 8 mm and contains 4 turns. The winding pitch is 1 mm. The coil is located horizontally at a distance of 8 mm from the printed circuit board. The length of the pins L1 is 4 mm. Coils L2, L3 - broadband RF chokes, inductance 2,5 ... 6 μH.
Author: V. Kurochkin, Smorgon; Publication: radioradar.net
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