1TV_MODULATOR

Published:2009/6/25 21:56:00 Author:May | From:SeekIC

The VHF frequency is generated by a tuned Hartley oscillator circuit. Resistors R2, R3, and R4 bias the transistor, with tapped inductor L1 and trimmercapacitor C1 forming the tank circuit. Adjusting C1 determines the frequdncy. Capacitor C2 provides positive feedback from the tank circuit to the emitter at Q1. Capacitor C4 provides an RF ground for the base of Q1. Bypass capacitor C5 and resistor R1 filter out the radio frequencies generated in the tank circuit to prevent radiation from the power-supply lines. The video signal enters the parallel combination of resistors R5 and R6; this combination closely matches the 75 ohm impedance of most video cables. Resistor R6 is a small screwdrtver-adjusted potentiometer that is used to control the video input level to mixer diodes D1 and D2.



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