Multi-function micro frequency modifier, FM Transmitter
Keywords: multi-function micro frequency modulverter
Multi-function micro frequency modifier
Author: Liu Chunhe Yao Xiaoliang
Performance output power does not exceed 5-8MW, the launch range can be around 300 meters, with a common FM radio receiving, the most sensitiveity and clarity of the test, the most challenging part of the circuit design is just With a 3V power supply and a half-wave antenna, there is such emission capabilities. The circuit consumption is less than 5mA, and two dry batteries can be used for 80 to 100 hours.
The circuit is very stable under normal operation, and the frequency drift is small. After 8 hours of test, it is still necessary to adjust the receiver. The only thing affecting the output frequency is the condition of the battery. When the battery is aging, the frequency is slightly changed.
The frequency of the transmitter is near 82.8MHz, and the radio interference of this band is less.
working principle
The circuit is shown below. Composed of one level audio amplification and one-stage RF oscillator.
There is a FET in the actual microphone in the elective microphone, and FET amplifies the capacitance of the front-tube-induced film, which is the reason why the electret microphone is very sensitive.
The audio amplification is carried out by the transistor Q1, and the gain is about 20 to 50. The amplified signal is sent to the base of the oscillation level Q2. Q2 is approximately 88 MHz, which is adjusted by oscillation coils (a total of 5 laps) and capacitance C4, which is also determined in Q2, 18PF adjustable capacitor C5 and bias elements (such as R5 and R4).
After the power is turned on, the capacitor C3 is gradually charged by R4, while the capacitor C5 is charged by the oscillation coil and R5, but faster; 47PF capacitor C4. It also charges (although the two ends are only very small), while the coil generates a magnetic field. .
When the base voltage gradually increases, the transistor Q2 is turned on and effectively puts the inner resistance and connected to both sides of the 18PF capacitor.
The base voltage continues to rise, and the capacitor C5 attempts to prevent the movement of the emitter potential, and when the energy of the capacitor is exhausted no longer prevents the movement of the emitter potential, the voltage decreases, the transistor is turned off, the current flowing into the coil Also stop, magnetic field declaration. And generate a reverse voltage, the collector potential is over 3V from the original 2.9V, and charging the C4 in the opposite direction, this voltage is also charged while charging the C5 and increases the voltage drop on R5, so that the transistor enters a deeper turn-off state.
As the lug-based anti-potential energy is consumed, the emitter potential of Q2 drops, and the transistor begins to turn, the current flows into the coil reverses the voltage on the coil, forming the concentrate potential drop, and transmits to the emitter by C5 To make Q2 saturation, the cycle begins to repeat. The Q2 is oscillated to generate an AC signal of 88 MHz. The audio signal from the predetermined amplification is collected into Q2 base, changing the oscillation frequency, generating the desired FM signal.
Our other product:
