Explain in detail several types of wireless transmitting circuits

FM FM transmitter, the transmitting frequency is in the analog broadcast band 88~108Mhz, using ordinary FM radio to receive, 2 kinds of audio input, MIC head to send peripheral sound, CK audio input, adjustable adjustable transmitting coil L can be changed (fine-tuned) to transmit frequency.

The figure shows the principle circuit diagram of the wireless transmitter. Q1 is a common emitter transformer coupled oscillation circuit: the load is a sub-level coil of the transformer T. After the collector output signal is coupled by T, the secondary pole is sent to the base via C1 to form a positive Feedback, vibrate. At the same time, the base is fed with a low-frequency modulation signal to amplitude-modulate the generated high-frequency oscillation

Q2 is the buffer amplifier stage, the output of Q1 is coupled to the base of Q2 (the "Q1" on the right in the figure) via C3, and L1 is the load inductance of Q2; and it is sent to the antenna for transmission via the C4/L2 series resonance circuit. R2 is grounded, that is, zero bias. Because the input signal amplitude is large and the frequency is selected by the C4/L2 resonant circuit, it is not afraid of distortion, which is more efficient.

　　 There should be a connection point at T, L1 and 9V. Since the base of this circuit does not have a DC bias, the circuit works in Class C amplification. The phase between the primary and the secondary of T is opposite, that is, when the collector current of Q1 increases, the primary induced electromotive force of T is positive and left negative, and the secondary generates positive and negative induced electromotive force on the left, and the charging current to C1 increases. When the collector current decreases, the opposite is true. The frequency is determined by the capacity of C1 and the inductance of T.

　　 Transmitting power these parameters cannot be determined now. The voltage is known, but the key is that the current is not known. The current (alternating current) is determined by the current of Q2 (the transistor behind it should be Q2), the drive of the base, and the impedance of L1.

　　 Speaking of receiving distance, it is related to receiver sensitivity, propagation environment, antenna height, and antenna gain. Generally speaking, it is also an ideal distance, and the actual distance is quite different.

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