Audio Circuit DiagramsOscillators Circuit DiagramsRadio - Wireless

FM Audio Transmitter Schematic Circuit Diagram

Project Concept: Designing a Simple VHF FM Transmitter

When embarking on this project, the author envisioned creating a straightforward VHF FM transmitter. The objective was to develop a device capable of playing audio files from an MP3 player or computer on a standard VHF FM radio. The primary challenge was to devise a circuit that avoided the need for homemade coils, a common complexity in other FM transmitter designs.

Circuit Innovation: Coil-Free Design for Simplified Operation

The key innovation of this project was to eliminate the requirement for coils that typically need to be wound at home. By doing so, the circuit’s complexity was significantly reduced, making it more accessible to enthusiasts. This streamlined design choice ensured that users could set up the transmitter without the hassle of winding coils, enabling them to enjoy their favorite music throughout their homes.

Versatility in Usage: Seamless Integration in Cars

Beyond home use, this transmitter provided a unique advantage when incorporated into car audio systems. Unlike traditional setups, there was no need for a separate input to the car stereo to play music files from an MP3 player. This seamless integration simplified the user experience, making it effortless to enjoy personalized music while driving.

Technological Foundation: Maxim Integrated Products’ MAX2606 IC

To maintain simplicity and compactness, the project employed a chip from Maxim Integrated Products, specifically the MAX2606 [1]. This integrated circuit (IC) belonged to the MAX2605-MAX2609 series and was purpose-built for low-noise RF applications with a fixed frequency. By utilizing this specialized IC, the project achieved its goal of creating an efficient, user-friendly VHF FM transmitter for various applications.

FM Audio Transmitter Schematic Circuit Diagram 1

FM Audio Transmitter Schematic Circuit Diagram 2

VCO Circuit Design: Integration of Colpitts Oscillator

The core of the IC operates on a Colpitts oscillator circuit, utilizing a Voltage Controlled Oscillator (VCO). The chip incorporates a variable-capacitance (varicap) diode and feedback capacitors, requiring only an external inductor to set the central oscillator frequency. Fine-tuning is achieved by adjusting the voltage to the varicap. The inductor, with a relatively low Q factor (35 to 40), is sufficient according to Maxim’s specifications.

Power Supply and Interference Prevention: USB Connection Strategy

The circuit operates on a supply voltage between 2.7 and 5.5 V, making it compatible with a USB power source. A common-mode choke, integrated in series with the USB connections, prevents interference between the circuit and the PC supply. This thoughtful design choice ensures stable power delivery and reduces potential disruptions.

Audio Input and Frequency Control: User-Friendly Interface

The stereo signal, input via K1, is combined through resistors R1 and R2. It then passes through volume control P1 to the Tune input of IC1, where it modulates the carrier wave. The audio signal bandwidth is restricted using filter R6/C7. Frequency adjustment across the entire VHF FM broadcast band is facilitated by P2, connected to the 5 V supply voltage. P1, despite its counterintuitive functioning (clockwise reducing volume), simplifies the board layout.

Compact PCB Design and Antenna Setup: Practical Implementation

The PCB, designed in the Elektor Labs, employs 0805 SMD resistors and capacitors. Remarkably compact at 41.2 x 17.9 mm, the board’s size resembles a dongle. The antenna, a nearly straight copper track at the board’s edge, provides an effective range of about 6 meters (18 feet). The inclusion of a 5-way SIL header allows versatile inputs for a 3.5 mm jack plug, P1 input, and supply voltage, enabling independent power sources like batteries.

Considerations and Legal Implications: Adherence to Regulations

Careful attention is needed during soldering, particularly for filter choke L2, where close connections exist. It’s crucial to avoid shorting the USB supply. Additionally, the circuit’s deviation and audio bandwidth vary with P1 settings, necessitating adjustments to R1 and R2 if needed. It’s important to note that the use of a VHF FM transmitter, including low-power variants, is subject to radio regulations and may not be legal in all countries. Compliance with local laws and regulations is essential.


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