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This is a very simple, inexpensive and fun project that will bring a lot of fun to the tester or hobbyist. This simple transmitter can transmit voice over short range. It can be used as a simple wireless microphone. The circuit uses two of Maxim's integrated circuits. IC1, MAX4467, is an amplifier that boosts the microphone signal to a level suitable for frequency modulation (FM). IC2 is a voltage controlled oscillator (VCO) with a built-in variator (aka varicap diode). Its nominal frequency of oscillation is set by the inductance L1.
The inductor value 390 nH provides an oscillation frequency of about 100MHz. For best performance, L1 should be a high Q component. The L1 can be made from 4 turns of silver wire wrapped around a 10mm drill bit and spans about 1.5cm. The diameter of the wire can range from 26 SWG (0.5 mm) to 20 SWG (1 mm). No kernel is used. The MAX4467 is a micro-power operational amplifier for low voltage operation and provides a gain bandwidth of 200 kHz with only 24 µA supply current. When used with electret microphones, some form of DC polarization is required for the microphone housing.
The MAX4467 has the ability to disable microphone offset when the device is in shutdown mode. This can save hundreds of microamps of current, which can be important in low power applications, especially for battery-powered applications such as wireless microphones. The MICBias pin provides a converted version of Vcc for the polarizing components. Resistor R1 limits the current to the microphone element. The output impedance of the MAX4467 is low and is well suited for cable transmission at distances up to 50 m.
The intermediate frequency (IF) voltage controlled oscillator (VCO) MAX2606 is designed specifically for mobile wireless communication systems. The IC comes in the form of a small 6-pin SOT23 package. The low-noise VCO features an on-chip variator and feedback capacitor that eliminates the need for external tuning elements. Only an external inductor (here L1) is needed to correct the oscillation frequency and produce a properly functioning VCO. To minimize the effect of parasitic elements on circuit performance, place L1 and C5 near their respective pins.
Specifically, place C5 directly between pins 2 (GND) and 3 (TUNE). Potentiometer P2 is then used to select an empty channel by tuning the FM band from 88 MHz to 108 MHz. Output power is approximately –21 dBm (approx. 10 µW) to 50 Ω. P1 plays the role of volume control by adjusting the RF frequency. Signals above 60 mV cause distortion, so the pot will degrade from this level. To reduce stray capacitance, minimize trace length by placing external components near the pins of IC1. Using a wire antenna about 75 cm, the transmitter should have a range of about 35 m.
Try to keep all cables as short as possible to avoid stray capacitance. The transmitter operates on a single voltage source between 4.5V and 5.5V from any standard battery source. The transmitter shall be housed in a metal enclosure, with a shield installed between the two floors (AF and RF). Try to keep all cables as short as possible to avoid stray capacitance.