We present the design methodology and a silicon implementation of a baseband system for use in wireless sensor network applications. Starting from the RF interface, our design process began with a system level phase inspired by the platform-based design (PBD) methodology extended to the analog domain. The functional design was based on an early-late gate synchronization scheme. The PBD approach was used to explore two alternative solutions: a predominantly digital one and a predominantly analog one. To validate the functional aspect of the design, a prototype implementation based on an FPGA and a field programmable analog array (FPAA) was derived using the PBD approach. Finally, we mapped the system level description to silicon aiming at an ultra low power implementation exploiting weak inversion in a 0.13μCMOS process leading to an overall power consumption of 200μW with a 1V supply.