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VHF Circuits / RF Circuits

The range of this transmitter is about 50 feet with a short whip antenna. The tone generator, is made up of a unijunction transistor, Q1, and R1, R2, R3, and C2. Transistor Q1 pulses on and off at a rate determined by the time constant of R1 and R2, together with the capacitance of C2 and the B1-emitter junction of QL Trimmer potentiometer R2 determines the frequency of the tone generated and allows a range of approximately 100 Hz to over 5 kHz.

Transistor Q2 is the rf oscillator. Its frequency is set by tuned circuits consisting of Ll, C5, C6, and the interelectrode capacitance of Q2. The values shown will give a tuning range of about 55 to 108 MHz. Capacitor C6 provides positive feedback from the emitter to the collector of Q2, for oscillation. TheA· audio tone generated by Q1 is applied to the base of Q2, causing the collector current to vary at the frequency of the tone, yielding an amplitude-modulated (AM) signaL This, in turn, varies Q2`s collector-to-emitter capacitance, which makes up part of the tuned circuit, and causes the output frequency to vary similarly, producing a frequency-modulated (FM) signal, as well. The rf signal is coupled to the antenna through capacitor C 7. Coil L1 consists of five turns of #18 bare wire, dose-wound on a piece of `/.-inch wooden doweL The length of the winding is about `I. inch. One end of capacitor C7 is soldered to the coil, one tum away from the 9-V supply end, and the other capacitor end is connected to the antenna. To adjust the vertical height and linearity of a TV set, place the tone transmitter near the set and use R2 to select the number of horizontal bars to be displayed. Once the picture is steady and the bars are sharp, adjust the set`s vertical controls, so that all the bars are of the same height and are evenly spaced. The fact that both AM and FM signals are generated makes it possible to use this circuit to check almost any receiver within the transmitter`s frequency range. A TV set`s sound section (discriminator) will reject the AM portion of the signal, but its video section will respond to it. Similarly, the TV sound section and FM receivers will respond to the FM signal produced. 

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