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.