FMPC microphone circuit diagram made by μpc1651

Crystal Oscillator

High frequency probe, high frequency line can be customized with other specifications

Quartz Oscillator 3225 20M OSC

This circuit is designed with high gain and stable operation, ensuring excellent performance for the microphone. It is used to create an FM transmission system. The antenna is made of a 40–50 cm soft coaxial cable, allowing an effective transmission range of over 30 meters. Carefully adjust the inductor (L-pitch) and trimmer capacitor to ensure the transmission frequency covers the standard FM band of 88–108 MHz.

The wireless microphone described here uses the μPC1651 integrated circuit from NEC Corporation of Japan as its main component. This IC offers stable operation, reliable performance, and is easy to manufacture and debug. It operates within the FM band of 88–108 MHz and can be received by a regular FM radio.

Working Principle

The core of the circuit is a high-frequency oscillator built around the μPC1651 IC. The audio signal from the electret microphone's MIC is frequency-modulated through capacitor C1 and then fed into the μPC1651. The resulting FM signal is coupled to the antenna via capacitor C5 for transmission.

The μPC1651 is a high-performance UHF broadband low-noise amplifier IC with a two-stage design that ensures stability and reliability. It only has four pins: input, output, positive power, and negative power. When powered by +5V, it consumes approximately 20mA of quiescent current.

To further enhance the frequency stability of the circuit, a three-terminal voltage regulator IC, the 7805, is used to stabilize the power supply for the μPC1651.

Component Selection

When building this circuit, it's important to select the right components to ensure optimal performance. The μPC1651 should be chosen carefully, as it plays a critical role in the frequency modulation and amplification process. Capacitors like C1 and C5 are essential for coupling the audio signal and FM signal to the IC and antenna respectively. The inductor (L-pitch) and trimmer capacitor must be adjusted precisely to fine-tune the transmission frequency to match the desired FM band. Additionally, the 7805 voltage regulator helps maintain a stable power supply, which is crucial for consistent operation. Using high-quality electret microphones and good coaxial cables will also improve the overall signal quality and transmission range. By following these guidelines, you can successfully build a reliable and efficient FM wireless microphone system.