This project implements an mbed library to control a Silicon Laboratories Si570 variable-frequency oscillator (VFO) for ham radio use. The mbed is controls the frequency of a software-defined radio (SDR) receiver. You can set the frequency with a computer via a USB, RS-232, Bluetooth, or Ethernet connection. The VFO can also run stand alone using an incremental encoder and display. This project got a 'Distinctive Excellence' award in the NXP mbed Design Challenge 2011.

block diagram
S is a rotary switch with push button that is used for large frequency steps or band switching. Furthermore S is used to control menu settings. F is a rotary encoder with 360 steps per revision, used as main frequency dial.

This project is still under development, keep an eye on this site for further updates.





The Si570 XO utilizes Silicon Laboratories’ advanced DSPLL® circuitry to provide a low-jitter clock at any frequency. The device is programmed via an I2C serial interface. The Si570 uses one fixed- frequency crystal and a DSPLL clock synthesis IC to provide any-frequency operation.
The Si570 is very popular for amateur radio use. It can be used as the local oscillator in a super heterodyne receiver, or it can be the local oscillator in a direct conversion quadrature receiver for software-defined radio (SDR) such as the Softrock. In addition to its use inside a receiver, the Si570 kit can be used as a stand-alone signal source for test and measurement or as an external VFO for old shortwave receivers.



Need a brain for your project? Pic, Arduino, Teensy, there are a lot of alternatives nowadays to choose from. For this project I've chosen for the mbed Rapid Prototyping tool for Microcontrollers. There were several reasons for this choice. First of all I got one for free, to use in the NXP mbed design challenge. Secondly the mbed is very powerfull and has a lot of external interfaces on board. Third, you don't need to install cross compilers and IDE's, the compiler is 'in the cloud' on the internet. Fourth, loading software is extremely convenient, the mbed acts as a USB memory stick. When you compile your code, the binary is automatically downloaded. You copy it to the stick, press reset and voila, your code is running. Fifth: a large community is using the mbed and the list of available software libraries for all kind of interface ic's and sensors is rapidly growing.
Look for yourself at



first prototype on breadboard
This picture shows an overview photo of the first prototype. The mbed and its interfaces are on the right on a breadboard. The Si570 and Softrock radio receiver are on the left, in a case constructed from double-sided copper clad board.

breadboard with digital electronics

schematic diagram


Start of building the circuit on printed circuit raster board. For better efficiency and less heat, a TI PTN78000 switched regulator is used in stead of the 7805.

pcb back
PCB back side with GND, 5V and 3.3V lines.





The software consists of two parts:
  1. An mbed library to control the Si570 via I2C. This library can be found on the mbed website (
  2. A reference implementation of this library with universal interface for software defined radio.The frequency can be set from a connected computer via USB, RS232, Bluetooth or Ethernet. The VFO can also run stand alone using an incremental encoder and display connected to the mbed. For operation on several different bands, band filters can be selected by mbed digital outputs. Also switching between receive and transmit is done with a digital output. A preliminary version of this software can be found in the contest entry (NXP mbed Design Challenge 2011 - Contest entry.). As soon as this software is more robust it will also be placed on

Here is a screen dump of the Softrock in action. In the lower right you can see a remote control program written to interface between the ‘DSP Radio’ software and the mbed VFO. In this case the communication is via Bluetooth. The center frequency set by ‘DSP Radio’ is transmitted to the mbed and displayed. When the frequency is modified by the dial on the mbed the center frequency of ‘DSP Radio’ is set accordingly by this remote control software.