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. The base electrode simultaneously sends 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 phases between the primary and secondary of T are 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 can not be determined by these parameters 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.
When it comes to 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.
The basic principles of the two circuits are the same. Both circuits are controlled by 0 and 1 signals to achieve high-frequency pulse transmission. The operating frequency is determined by the crystal oscillator, and the advantages and disadvantages cannot be distinguished from the stability. The circuit in Figure 2 is simpler and easier to debug. In Figure 1, a frequency selection network is added between the triode collector and the power supply, and a frequency selection network is also added between the triode collector and the antenna. The output frequency is more pure, but debugging is more troublesome. . Another difference is that the base of the upper triode in Figure 2 introduces AC negative feedback through the resistance. In fact, this has no obvious effect on the stability of the circuit. It is better to keep the high-frequency capacitance to ground on the power supply, but it is not shown in Figure 2.
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