Modulator for AM radio station. Scheme, description

Modulator for AM radio station. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Modulators

I developed this modulator for a portable radio station, but its successful characteristics allow it to be used in fairly powerful stationary AM transmitters. When changing the supply voltage, pay attention to the microphone's DC mode, or use a different type of microphone, such as "Pine".

The K140UD20A microcircuit, unfortunately, does not allow working at lower values of the supply voltage, at least this quality of work did not suit me. On the other hand, an increase in the supply voltage leads to an increase in the power dissipated in the output transistors, so it is better to replace them with more powerful ones such as KT819, KT818. The absence of an initial bias on the terminal transistors does not have a strong effect on the quality of the speech signal, since the output transformer effectively suppresses higher harmonics.

Fig.1 (click to enlarge)

The transformer is best done on an iron core with a cross section of at least 200 square millimeters, the primary winding contains at least 500 turns of PEL 0.3 wire. The secondary is wound with a thicker wire, the number of turns is calculated using a coefficient of 0.65 (transformer efficiency), to obtain the required modulation voltage amplitude. On the primary winding in my modulator, up to 20 volts from peak to peak was obtained, with a 20 millivolt signal from the microphone ... You can use a ferrite core, but you have to press high frequencies, equalizing the through frequency response in the desired frequency band.

The printed circuit board was not developed, for reasons of space saving. My humble experience has shown that sometimes curved volumetric wiring takes up less space than the most sophisticated printed circuit board. In this case, the microcircuit, electrolytes and transistors were glued to the transformer in convenient places, and everything else was "hanging" on them. The only exception was the microphone. It is advisable to shield the entire modulator (just in case).

Author: Dmitry Malakhov; Publication: N. Bolshakov, rf.atnn.ru

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