theoretical design of overkill fm transmitter

Thread Starter


Joined Aug 19, 2023
Alright, if we're going for an "overkill" design, then we'd aim for the absolute best in terms of performance, quality, and features. Here's a design outline with overkill in mind:

1. **Audio Input & Processing**:
- **Codec**: Use a high-quality audio codec, such as the AKM AK4499 or similar, which offers top-tier audio conversion.
- **Pre-processing**: Implement digital audio processing (like DSP) for EQ, compression, limiting, and other functions. Brands like Analog Devices have some of the best DSP chips.

2. **Modulator**:
- **Chip**: The **Si4713** from Silicon Labs, as it offers advanced features like RDS/RBDS, integrated digital pre-emphasis, and superior audio fidelity.
- **Crystal Oscillator**: Use a high-stability, low-phase-noise crystal oscillator to ensure the stability of the FM frequency.

3. **RF Amplification Chain**:
- **Driver Amplifier**: Start with a small amplifier to boost the Si4713's output. Mini-Circuits offers a wide range of MMIC amplifiers suitable for this task.
- **Power Amplifier**: Use the MRF151G by NXP or a similar high-quality RF power MOSFET for the final amplification stage.
- **Feedback Loop**: Integrate a feedback mechanism to ensure linearity and reduce distortion.

4. **RF Filtering & Protection**:
- **Band-pass Filter**: A high-Q cavity filter post-amplification to ensure only the desired FM frequencies are transmitted.
- **Isolator**: Install a circulator or isolator after the final amplification stage to protect the amplifier from reflections and ensure maximum power is transmitted to the antenna.
- **SWR Monitor**: High-quality SWR monitoring to protect against impedance mismatches and potential damage to the power amplifier.

5. **Power Supply**:
- **Custom Linear Power Supply**: Aim for ultra-low noise and ripple. A benchtop-grade linear power supply, possibly with multiple taps for the different voltage requirements of the components.
- **Backup Power**: Consider integrating a battery backup or uninterruptible power supply (UPS) system.

6. **Control & Monitoring**:
- **High-end Microcontroller or FPGA**: A top-tier microcontroller or FPGA for system control, monitoring, and potential DSP tasks.
- **Touch Screen Interface**: An interface for real-time monitoring and control. Display parameters like output power, SWR, voltage, temperature, and more.
- **Remote Control**: Implement a secure remote control mechanism, possibly via Ethernet or Wi-Fi, allowing for adjustments and monitoring from a distance.

7. **Enclosure & Thermal Management**:
- **Machined Aluminum Chassis**: Custom-machined with integrated heat sinks and ventilation channels.
- **Active Cooling**: High-quality, quiet fans, like those from Noctua, with variable speed control based on temperature sensors placed around critical components.

8. **Antenna System**:
- **High-gain Yagi or Circular Polarized Antenna**: Professionally tuned for the FM band.
- **Motorized Rotator**: Allows for antenna direction adjustments, increasing flexibility in directing the transmission.

9. **Connectivity**:
- **Top-of-the-line RF connectors**: Use Type-N or 7/16 DIN from brands like Amphenol or Huber+Suhner.
- **High-quality Audio Inputs**: Balanced XLR or TRS inputs for the audio. Implement gold-plated connectors for all connections.

10. **Safety & Compliance**:
- **Redundant Safety Systems**: Multiple interlocks, temperature cut-offs, and SWR protection mechanisms.
- **Compliance**: Ensure all parts and the final design comply with FCC or local regulations.

11. **PCB Design**:
- **High-frequency PCB Material**: Something like Rogers RO4350B.
- **Gold Finish**: Use a gold (ENIG or ENEPIG) finish for the PCB.

Such an overkill design would be an absolute beast in terms of performance and features. But remember, the design process would be complex, requiring multiple iterations and testing phases. Having an experienced RF engineer on board would be critical. The costs would be significantly high, but if "overkill" is the goal, this would definitely achieve it!

Audio Input & Processing:
Codec: The AKM AK4499 is an excellent choice. Alternatively, ESS Sabre DACs are also held in high regard for their audio quality.
Pre-processing: Consider using SHARC DSPs from Analog Devices, known for their audio processing capabilities.

For the Si4713, ensure the latest firmware is flashed, and the I2C communication with your microcontroller or FPGA is robust.

RF Amplification Chain:
Use RF-specific PCB layouts and ensure adequate shielding between stages to prevent interference and oscillation.

Power Supply:
Ensure adequate protection circuits (like over-voltage and over-current protection). Linear Technology (now part of Analog Devices) offers some high-performance power management ICs.

Control & Monitoring:
Microcontroller/FPGA: Look into Xilinx or Altera (now part of Intel) FPGAs for more intensive tasks. For microcontrollers, the STM32 series from STMicroelectronics is versatile and powerful.

Enclosure & Thermal Management:
Consider adding a thermal interface material between components and heatsinks to improve heat dissipation.

Antenna System:
Ensure the antenna system is grounded properly to prevent potential lightning strikes.

Given the high-quality audio path, consider also implementing a digital audio input like SPDIF or even AES/EBU.

Safety & Compliance:
For safety, consider adding fuses or circuit breakers where appropriate.

PCB Design:

For PCB layout, especially in RF sections, consider using ground planes and vias liberally to ensure a solid ground and reduce interference.


Joined Mar 31, 2012
nah this is just a theoretical design i wanted to share. :)
To what end???

Whatever you throw out as the "absolute best performance" someone else could add a dozen features that would make it better.

You also assume things that might be incompatible with what the user wants -- you talk of a high-gain antenna to achieve high directionality, but what if the user is specifically looking for omnidirectional broadcast?

"Best performance" only has meaning within the context of the customer's needs -- which includes cost and timeline.

Thread Starter


Joined Aug 19, 2023
Anybody who is a mad scientist enough to wanna try it, that's why it's theoretical. :) no ill will, as opinion and theory are hand in hand in this case. just wanted to get the possibility out there. anybody could improve it, change it entirely, just putting it into the world. thanks, feel free to delete or lock the thread.


Joined Jan 27, 2019
The RF Amplification and Antenna sections have regulatory implications. “Trying this out” is the same as violating the law in the US, and all the other jurisdictions I am aware of.


Joined Aug 21, 2017
I had once a young guy prone to transmitting. He organized the 25 Watt machinery on the FM and transmitted there music and bit a talk about standups some 12 hours every day. He was working in first kilometer out the Riga capital. Until the commercial radio channels was issued claim about him and Radio Inspection squeezed his duck-neck. Sqeezed once, second time, and third time means imprizonment. Then he sold out all to one russian criminal authority who made this channel legal, thus it works until today in russian occupants human-hating language. So... some people feel magnetized by microphone connected to antenna.