This simple FM wireless microphone transmitter can transmit speech over a short range. It can be used as a simple cordless microphone. The circuit uses two integrated circuits from Maxim. IC1 a MAX4467, is an amplifier raising the microphone signal to a level suitable for frequency modulation (FM). IC2 is a voltage-controlled oscillator (VCO) with integrated varactor (a.k.a. varicap diode). It is this small that can be integrated in a wireless handheld microphone.
The wireless microphone nominal frequency of oscillation is set by inductor L1. The inductor value 390 nH provides an oscillation frequency of about 100 MHz. For best performance, L1 should be a high-Q component. L1 may consist of 4 turns of silver-plated wire wound around a 10-mm drill bit, and stretched to a length of about 1.5 cm. The wire diameter can be anything between 26 SWG (0.5 mm) and 20 SWG (1 mm). No core is used.
The MAX4467 is a micropower opamp for low voltage operation and providing 200-kHz gain bandwidth at a supply current of just 24 μA. When used with an electret microphone, some form of DC bias for the microphone capsule is necessary. The MAX4467 has the ability to turn off the bias to the microphone when the device is in shutdown mode. This can save several hundred microamps of supply current, which can be significant in low power applications particularly for battery powered applications like cordless microphones. The MICBias pin provides a switched version of Vcc to the bias components.
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Resistor R1 resistor limits the current to the microphone element. The output impedance of the MAX4467 is low and well suited to driving cables over distances up to 50 m. The MAX2606 intermediate-frequency (IF) voltage-controlled oscillators (VCO) has been designed specifically for portable wireless communication systems. The IC comes in a tiny 6-pin SOT23 package.
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. To minimize the effects of parasitic elements, which degrade circuit performance, place L1 and C5 close to their respective pins. Specifically, place C5 directly across pins 2 (GND) and 3 (TUNE).
Potentiometer P2 then lets you select a free channel by tuning over the FM band of 88 MHz to 108 MHz. FM microphone’s output power is about –21dBm (approx. 10 μW) into 50 Ω. P1 serves as a volume control by modulating the RF frequency. Signals above 60 mV introduce distortion, so the pot attenuates from that level. To decrease stray capacitance, minimize trace lengths by placing external components close to IC1’s pins.
Using a wire antenna of about 75 cm the transmitter should have a range of about 35 m. Try to keep all leads as short as possible to prevent stray capacitance. The FM wireless microphone transmitter operates on a single supply voltage in the range 4.5 V to 5.5 V from any standard battery source. The transmitter must be housed in a metal case, with shielding installed between the two stages (AF and RF). Try to keep all leads as short as possible to prevent stray capacitance.