EE Product News

Reference Design Sends FM Radio To PC Via USB


Reference Design Sends FM Radio To PC Via USB
Pairing a Si4701 FM tuner and C8051F321 USB microcontroller (MCU), the company's USB FM reference design board plugs into any Windows PC supporting USB connectivity and plays high-quality FM broadcasts through the computer's speakers. The design contains the Windows-based FM tuner application, complete source code, and engineering documentation, allowing users to easily design, demonstrate, and evaluate high-quality products. Reportedly the market's smallest, the USB MCU integrates a transceiver and clock recovery, eliminating the need for resistors, a crystal, regulator, EEPROM, or other external components. Digital resources include a pipelined 25 MIPS 8051 CPU with 16 KB of flash memory, UART, plus SMBus and SPI interfaces. The FM tuner packs selectivity filtering, automatic gain control, a frequency synthesizer, and audio processing in a single CMOS device. It requires one supply-bypass capacitor and consumes less than 20 mm2 worth of board space. Other tuner features include a digital pre-processor for the European Radio Data System and the US Radio Broadcast Data System. Documentation, Gerber files, and related development tools are available at the company's website.
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Key Components

  1. The Si4701 is the industry's first FM radio tuner IC to leverage digital integration and 100% CMOS process Technology.
  2. The C8051F321 is an 8051-based mixed-signal microcontroller that delivers up to 25 MIPS throughput with a 25 MHz clock. It comes with a built-in a/d converter and USB. In this design, the 10-bit ADC is samples both audio channels at a sample rate is 192 kHz. It switches between the right and left channel during successive conversions using an onchip ADC multiplexer. The audio sample rate for stereo samples is consequently 96 kHz, and each audio sample is 4 bytes in size. These are sent to the host in signed two's complement format.
  3. The MC33202 is a dual package op-amp from ON Semiconductor. This is used to create a single-pole low-pass filter exhibiting a corner frequency at 23,405 Hz and a gain of 3.
  4. The firmware begins by initializing all necessary device peripherals and the Si4701 FM tuner. Once this is complete, the microcontroller operates in one of two modes, command or audio. In command mode, the device can accept control commands from the USB host for operations such as seeking, tuning, or saving presets. In audio mode, the microcontroller sends ADC samples and a small amount of status information up to the host. The MCU enters audio mode once the host has started polling for audio data, and exits when it receives any command from the host.
  5. When the Windows software GUI is first launched, it looks for a USB FM radio device. When the device is found, it is opened up as a USB Audio Device for audio streaming and as a HID device for FM Tuner control. In addition, the default WAV output device is opened to playback the audio stream.

On the Web
The software, firmware, schematic, and layout source for this design are located in The latest version of these files can be found at:


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