Circuit: FM Wireless Microphone

A compact FM wireless microphone turns a simple electret capsule into a short-range transmitter tunable across the FM band. Built around two Maxim ICs, the design from July/August 2003 cleanly demonstrates audio amplification and RF generation in a single circuit.

Wireless Microphone Circuit

A nice example of how a minimal parts count can still deliver a complete RF signal chain, D. Prabakaran's circuit splits cleanly into two functional blocks: audio amplification and RF generation. The electret microphone feeds into a MAX4467 micropower op-amp, boosting the tiny audio signal to a level suitable for modulation. Biasing is handled through the MICBias pin, with R1 limiting current into the microphone element. Gain is controlled carefully to avoid distortion, which becomes noticeable above roughly 60 mV, and P1 serves as a simple but effective input level control.

"This simple transmitter can transmit speech over a short range," Prabakaran noted. "It can be used as a simple cordless microphone."

On the RF side, the MAX2606 VCO handles frequency generation. An integrated varactor eliminates the need for external tuning diodes and keeps the component count low.

"The low-noise VCO features an on-chip varactor and feedback capacitors that eliminate the need for external tuning elements. Only an external inductor (here, L1) is required to set the oscillation frequency and produce a properly operating VCO," he noted.

Using potentiometer P2, you can select a free channel by tuning over the FM band of 88 to 108 MHz. P1 works as a volume control by modulating the RF frequency. Signals above 60 mV introduce distortion, so the potentiometer attenuates from that level. "To decrease stray capacitance, minimize trace lengths by placing external components close to IC1’s pins," Prabakaran explains.

What stands out is the clear interaction between analog front-end design and RF behavior. It is clear that factors such as component placement and shielding all directly influence performance. 

The FM Wireless Mic Project

The original article, "FM Wireless Microphone” appeared in Elektor July/August 2003. Check out the article.

Editor's Note: The article first appeared in a 2003 issue of Elektor. Due to the project’s age, some associated components might not be available. Still, the circuit should inspire you to start a new design.