FMPC microphone circuit diagram made by μpc1651

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This circuit is designed with a high gain and stable operation, ensuring excellent performance for the microphone. It's ideal for forming an FM transmission system. The antenna uses a 40–50 cm soft coaxial cable, allowing an effective transmission range of over 30 meters. By carefully adjusting the L-pitch and trimming capacitor, the transmission frequency can be set within the 88–108 MHz range.

The wireless microphone in this example uses the μPC1651 integrated circuit from NEC Corporation in Japan as its main component. This IC offers stable operation, reliable performance, and ease of production and debugging. The operating frequency can be selected within the FM band (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 C1 and then fed into the μPC1651. The resulting FM signal is coupled to the antenna via C5 for transmission.

The μPC1651 is a high-performance UHF broadband low-noise amplifier IC featuring a two-stage amplifier that ensures stability and reliability. It only requires four pins: input, output, positive power, and negative power. Under a +5V power supply, the quiescent current is approximately 20mA.

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

Component Selection

When building this circuit, it's important to choose high-quality components to ensure optimal performance. For instance, the capacitors should be selected for their stability and low leakage, while the resistors must be precise to maintain proper biasing. The antenna should be made of flexible coaxial cable to allow easy adjustment and placement. Additionally, the use of a trimmer capacitor allows fine-tuning of the frequency to match the desired FM band. Proper PCB layout and shielding are also essential to reduce noise and interference, ensuring a clean and strong signal transmission.