art predistortion alignment can be for example a model sbg 1 manufactured by wandel amp golterman it provides intermediate frequency test tones illustrated in fig2 a the test tones consist of fixed frequency tones 36 and 37 and a swept frequency tone 38 nonlinearities in the system to which the tones are applied result in intermodulation distortion in the form of a tone such as tone 39 sweeping in a direction opposite to tone 38 the test tones are applied by way of predistortion circuit 42 to transmitter 30 the radio frequency output of transmitter 30 is transmitted by antenna 44 34 over the hop 34 from station a to station b the radio frequency signal acquired by antenna 46 is applied to receiver 31 at the received power level the intermediate frequency output of receiver 31 is filtered by reject filter 48 the effect of filter 48 is shown in fig2 b fixed frequency tones 36 and 37 are attenuated as much as possible in the absence of tones 36 and 37 sweeping tone 38 will not give rise to substantial intermodulation distortion in the signal path which follows the output of reject filter 48 is applied by way of predistortion circuit 50 to a second transmitter 52 at station b the radio frequency output of transmitter 52 is transmitted on antenna 54 over the hop to antenna 56 of station a the signal from antenna 56 is applied to receiver 58 and the intermediate frequency output of receiver 58 is applied to the input of test.