We present a Software Defined Radio (SDR) based IEEE 802.15.4 transceiver testbed for GNURadio. Our testbed is Open Source and fully interoperable with off-the-shelf TelosB sensor motes and the Contiki sensor mote operating system, from the physical layer to the network stack. The testbed can be setup and configured easily via a graphical user interface and applications can interface with the SDR using TCP sockets. Furthermore, the SDR is able to log traffic in PCAP format to investigate networks with common software like Wireshark. We believe the main applications of the transceiver to be twofold. First, the communication stack has a modular, layered structure, which allows for rapid prototyping, is educational, and is easy to grasp, lowering the steep learning curve that SDRs typically have. Secondly, the integration of a network stack in GNURadio pushes interoperability from the physical to the network and application layer and thus enables the investigations of higher layer metrics with SDRs.