A radio transmitter, comprising: a feedback loop (314) which feedback loop comprises a variable attenuator and a signal analyser, means (300, 302, 304, 306, 308, 310, 312) for transmitting a signal on a predetermined radio channel, means (308) for changing attenuation level and for attenuating the feedback signal, means (316) for defining power levels on the radio channel and on the adjacent channels, means (312) for analysing the power levels to generate first and second results of analysis and for comparing the first and second results of analysis, means (318) for delaying a transmitted signal to derive a reference signal, means (312) for comparing the feedback signal and the reference signal to generate an error vector and means (320) for generating a test alarm.