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DIY: Easily build a digital wireless FM transmitter

Posted by Gizmo Guy on 15th Apr 2021

"FM transmitter" is one of the most popular devices among electronics hobbyists, professionals, and even non-technical personnel. This article will learn how to build a simple, stable and digitally controllable FM transmitter.

For the FM transmitter design, I chose the VMR6512 module, which is really like a complete radio frequency (RF) module on the chip! All necessary circuits for basic FM transmitters, such as inductors and trimmers, have been removed.

According to the VMR6512 specification sheet: "VMR6512 is a highly integrated FM audio signal transmitter module. It integrates an advanced digital signal processor (DSP), frequency synthesizer, RF power amplifier (RF PA) and matching network. . Therefore, it can achieve FM audio modulation without any external components. It can also use digital pre-emphasis, digital filtering, automatic gain control, and digital frequency control technology to make VMR6512 broadcast-quality sound."

Its operating frequency range is between 88.0MHz~108.0MHz. Figure 1 shows a schematic diagram of this transmitter.

Figure 1: Schematic diagram of FM transmitter.

LED D1 shows that the power supply (3.3V) is correctly connected to the circuit. Capacitors C1 and C2 reduce power supply noise (0805 package). Three tactile switches (buttons) are also used in this design: SW1 resets the module, SW3 increases the frequency (+0.1MHz), and SW2 is used to decrease the frequency (-0.1MHz).

Figure 2 below is a view of the top layer of a circuit board (PCB), and Figure 3 shows a view of the bottom layer of the PCB. Figure 4 shows the 3D view of the PCB after assembly.

Figure 2: PCB top-level view.

Figure 3: PCB bottom view.

Figure 4: 3D view of PCB after assembly.

I use the SMA connector for P2 (antenna connection), so it can be used to connect to the antenna, or to connect the output to the RF amplifier. It is worth noting that before you increase the transmitter power, please check the regulations and broadcasting regulations of your country/region.