o of the feedback arrangements 70 are determined by approximating the gains of the mixer at various frequencies and designing the feedback arrangement 70 as a band reject filter for substantially not passing or at least substantially attenuating a selected band of frequencies while substantially passing frequencies on either side of the reject band alternatively the band reject filter can be formed by the combination of the feedback arrangement 70 and the filter sub circuit 45 in this latter arrangement since h f of the filter sub circuit 45 typically provides low pass filtering the feedback arrangement 70 need provide only high pass filtering fig3 shows a graph on which gain is plotted against frequency for the entire circuit of fig2 a resulting in curve c1 and for just the feedback circuit of fig2 b resulting in curve c2 these curves c1 c2 show the transfer characteristics for the respective circuits as can readily be seen from the graph curve c1 exhibits a zero gain for 34 near zero frequencies 34 or 34 near dc 34 then steps up to a high gain over a first range f1 of frequencies which can be regarded as the frequencies of interest or in band frequencies before rolling off to a lower gain for a higher range f2 of frequencies and rolling off again to zero gain for still higher frequencies the terms 34 near zero 34 frequencies and 34 near dc 34 mean frequencies less than about half of the lowest of the frequencies of interest thus for example where the in band frequencies are 200 hz to 10 khz near zero frequencies are those below 100 hz in achieving this frequency response for the entire circuit the transfer characteristics of the feedback circuit play an important role curve.