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Video transmitting circuit using discrete components
The circuit is illustrated in the diagram. The video signal is first amplified by a single stage and then fed into the AM modulation stage through an emitter follower. The transmission frequency is generated by an LC high-frequency oscillator, which sends the signal to the AM modulation stage. Finally, the modulated radio wave is transmitted via the antenna.
The image amplification stage is built using a triode, Tr1, forming an amplification circuit. The video signal amplitude is approximately 1V peak-to-peak at a 75Ω load. With a 3V power supply, the gain of this stage is R2/R3 = 3.3 (about 10dB), and the bandwidth is around 4MHz. VR1 is used to adjust the input signal amplitude, while VR2 adjusts the modulation bias point. D1 is a clamping diode that ensures a complete synchronization signal, even in varying brightness conditions.
The buffer stage is connected to the emitter follower using transistor Tr2. Its emitter voltage follows the changes in the image signal, causing a corresponding voltage change in Tr4, which performs the AM modulation. The high-frequency oscillation circuit is built around Tr3 as a Colpitts oscillator, with the oscillation frequency determined by capacitors C7, C8, and C9. A temperature-compensated ceramic capacitor is used for stability. Inductor L1 uses a pre-made 144MHz core inductor. The base voltage of Tr3 is approximately 1.5V, the emitter voltage is about 0.85V, and the emitter current is around 0.85mA. The output frequency is taken from the emitter.
If the AM modulation is implemented directly within the high-frequency oscillator, large variations in the operating voltage can cause frequency instability. To avoid this, a buffer stage (Tr4) is added to perform the AM modulation independently. In the resonant circuit, L2 also uses a pre-made core inductor. The collector current of this stage is 5.5mA, and the bandwidth is approximately 4.5MHz. C12 acts as a high-frequency bypass capacitor, and R11 serves as a damping resistor. The overall power consumption of the circuit is adjusted to around 25mA.
To tune the transmission frequency to the desired channel—such as channel 2—the core of L1 is adjusted. At the same time, the receiving TV should be tuned to the same channel. Then, VR1, VR2, L1, and L2 are fine-tuned to ensure the image appears clearly and reaches the best quality. This process ensures optimal performance and signal clarity.