Simple FM Long Range Transmitter

This article gives the FM transmitter circuit schematics with necessary explanation. The main component used here is the VMR6512 IC which is a highly integrated FM audio signal (Hi-Fi Audio FM) transmitter chip. You can use this as FM radio transmitter for different applications such as FM transmitter for car, digital FM transmitter, Hi-Fi wireless headphone, Conference Broadcasting System, accessories of audio-visual entertainment equipment and radio station in campus. This wireless audio transmitter chip integrates superior digital signal processor (DSP), matching network, frequency synthesizer and RF power amplifier. So it can grasp FM audio modulation without any external components. You need a pre amplifier section in order to transmit our voice directly as FM. Refer my condenser mic pre amplifier circuit.

Circuit diagram 

Components required

1. VMR6512 Hi-Fi Audio FM transmitter module
2. Push button switch x 3 
3. Antenna
4. Audio amplifier

Constructional details

• Audio amplifier circuit is already posted in my previous article. Construct your audio amplifier first.  

• Then you can directly connect the output of audio amplifier to the VMR6512 Hi-Fi Audio FM transmitter input (Audio in).

PCB layout of FM transmitter

Features of VMR6512 Hi-Fi Audio FM transmitter

• Broadcasts sound without any external components.

• Audio frequency processed by DSP, providing high quality sound.

• Using frequency synthetic technology, high stability of oscillation frequency and output power adjustments.

• Frequency range is 88.0MHz to 108.0MHz, low-end can be extended to 76.0MHz upon request.

• It can input analog audio or digital audio and possesses external UART interface and can be easily controlled by external CPU or PC machines.

• It provides frequency setting UP / DOWN input and can be used independently.

Pin descriptions of VMR6512 IC

1. RST- Reset the module with high PWL 
2. RXD - Control serial input RX 
3. TXD - Control serial output TX 
4. DIN - Digital audio data input 
5. DFS - Digital audio frame synchronized signal input 
6. DCLK- Digital audio clock signal input 
7. RIN - Analog audio input on the right 
8. LIN - Analog audio input on the left 
9. GND - Ground 
10. RFOUT - RF output 
11. GND - Ground 
12. NC - No connection 
13. NC - No connection 
14. DOWN - Frequency reduction input, each low PWL impulse output frequency decreases 0.1MHz, continuous low PWL repeats every 0.3 seconds. 
15. UP - Frequency increasing input, each low PWL impulse output frequency increases 0.1MHz, continuous low frequency every 0.5 seconds. 
16. D/A - Audio input digit / analog selection 
17. NC - No connection 
18. VCC - Power input (2.7-3 .3 V)

Functions of the Pins as per the Manufactures

Reset
Reset pin set to high PWL will reset the controller, DSP and frequency synthesizer. After reset, the communication frequency will be100.0MHz and the power will be 115dBuV. The system begins working 160ms once the Reset signal becomes lower.

Analog audio input Rin, Lin
Rin and Lin is the analog audio input pin of VMR6512 module. There is a capacitor in the pin. So the other exterior components are not needed. Audio input impedance is approximately 56 kΩ.

Digital audio input interface
Digital audio input interface consists of SCLK, DIN and FS. It can be set to I2S, DSP and Left Justified three different formats through commanding and connected without a glitch with almost all the DSP.

UP / DOWN pin
UP and DOWN pins are used to modify the operating frequency exclusive of the external controller. Each low PWL impulse larger than 0.05 seconds of UP / DOWN pin will make output RF power raise or decrease 0.1MHz. If you stay low PWL, then the operating frequency will change continuously every 0.3 seconds.

RF output
As the unit has internal matching networks, RF output signal pin should be directly connected to the line without any components. It is proposed that antenna should use 1/4 wavelength wire or rod antenna.

Pin out of VMR6512 Hi-Fi Audio FM transmitter module