VHF FM transmitter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The three-stage transistor VHF FM transmitter is designed to operate in the range of 144-146 MHz. Its output power is about 1,2 watts into a 60 ohm load. The master oscillator with quartz frequency stabilization (see figure) is made on a transistor T1, connected according to a common base circuit. A D1 varicap is connected in series with the quartz resonator, with the help of which frequency modulation is carried out.

Preliminary constant mixing is supplied to the varicap from the divider R8R9. The master oscillator uses quartz resonators at frequencies of 72-73 MHz (the third mechanical harmonic). A frequency doubler is assembled on transistor T2. The output amplification stage is made on the transistor T3. connected according to the scheme with a common emitter. The inductor L5 together with the coupling capacitor C12 and the capacitance of the collector junction form an output P-filter. Inductor Dr4, included in the base circuit of the output transistor, is shunted with a low-resistance resistor R7 in order to prevent self-excitation of the final stage at the resonant frequency of the inductor.

(click to enlarge)

Before setting up the transmitter, a bias of about 8 V is applied to the varicap from the R9R3 divider. The transmitter output is loaded onto the equivalent of the antenna. In the master oscillator, it is selected from which turn of the inductor L1 it is better to make a tap. The number of turns before the tap, counting from the grounded terminal, should be minimal, but sufficient for stable operation of the master oscillator. The doubler and final stage are adjusted to the maximum output power.

In the transmitter, amplitude modulation can also be applied by including a transistor power amplifier (with an output power of about 0,5 W) into the collector circuit of the output stage through an appropriate transformer. Since, during amplitude modulation, the voltage at the collector of the output transistor can exceed the voltage of the power source by 2 or more times (during overmodulation), in order to avoid failure of the output transistor after the modulator (for example, in parallel with C10), it is necessary to turn on the zener diode with a stabilization voltage less than the maximum allowable voltage at the collector of the output transistor. The winding data of all contour coils are given in the table.

All chokes are made on ferrite cores and have an inductance of about 1 μH.

The transmitter can use transistors GT313 (T1), KT603 (T2), KT606 (T3), varicap KV102 or D902 (D1).

Literature

1. "Old Man" (Switzerland). 1972, No. 9.

Publication: N. Bolshakov, rf.atnn.ru

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