A Cartesian loop transmitter having isolator eliminator circuitry is presented. The isolator eliminator circuitry includes a set of low pass and wide band pass as well as narrow band pass filters for each of I- and Q-channels, root mean square detectors and dividers connected to a means for comparing. Signals from the means for comparing are received by a microprocessor which controls attenuation setting. The output level of the transmitter is adjusted by generating a small signal measuring an on-channel signal level and a small signal level and then calculating a ratio of the small signal to the on-channel signal. The attenuation setting is increased if the ratio exceeds a defined threshold.