The transmit path comprises all the circuitry required to convert digital data into suitably filtered analogue I and Q signals for subsequent up-conversion and transmission. This includes digital control of the output amplitudes, digital control of the output offsets and fully programmable digital filters: default coefficients provide the RRC response required for TETRA. The receive section accepts differential analogue I and Q signals at baseband and converts these into a suitably filtered digital form for further processing and data extraction. A facility is provided for digital offset correction and the digital filters are fully programmable with default coefficients providing the RRC response required for TETRA.