is selected is then as shown in fig3 it will be appreciated that the interferers then fall outside the image region 15 and are rejected by the frequency filters 7 and 8 in order to maintain the polarities of the vlif i and q channel mixer stage 11 this mixer stage comprises first and second i channel mixers 21 and 22 and first and second q channel mixers 23 and 24 a vlif two position switch 25 synchronised with the lo two position switch 20 provides phase alternation and alternately applies the vlif lo signal from the vlif local oscillator 19 to the first i and q mixers 21 and 23 in one time slot and the phase shifted vlif lo signal from the vlif phase shifter 13 to the second i and q mixers 22 and 24 in the next time slot the i mixers 21 and 22 mix these signals with the signal from the i channel adc 9 and the q mixers 23 and 24 mix these signals with the signal from the q channel adc 10 the signals from the first i mixer 21 and the second q mixer 24 are supplied to an adder 26 which adds the.