Software defined radio (SDR) technologies have been long promoted for use in the professional mobile radio (PMR) application domain, as typified by the TETRA standard. The need to retain and enhance compatibility with a proliferation of legacy systems provided the incentive, and the typically very narrowband nature of these communications schemes made SDR appear feasible. In practice the evolution of these standards has moved faster than the development of viable SDR implementations. In recent years, use of PMR radio systems has expanded beyond voice to include data. There is now increasing demand for high speed data access to support applications such as the transmission of highresolution images (eg maps for emergency services), and video clips. This has resulted in an evolution in the TETRA standard, from the original voiceonly service, to an enhanced data version (TEDS) and finally to a proposed new approach which implements a WiMAX subchannel within the TETRA band. This paper will analyse the effect that this evolution in the standards will have on any potential SDR implementation.