be 250 khz higher and the second frequency f 2 should then also be 250 khz higher for the position of the frequency band b 3 to remain the same the lowpass filter 633 should then have comparatively steeper slopes because the frequency distance relative to the frequency f 2 would be reduced the complexity of the filter 633 could also be reduced if f 2 were slightly increased for the same position of the band b 2 this would make the requirements imposed on the low pass filter 635 more severe because the sampling frequency of the 15 bit analog to digital converter is limited the frequencies f 1 and f 2 could still be freely chosen however the complexity of the filters 631 to 635 would then be influenced as appears from the above considerations therefore the frequencies f 1 and f 2 are chosen so that the relative slope steepnesses of all filters are the same relative slope steepness is to be understood to mean herein the quotient of the difference of the highest useful frequency and the lowest mirror frequency divided by the central frequency f 1 f 2 or f t 2 for the filters 631 633 and 635 this quotient amounts to approximately 0 108 for suppression of the undesirable frequencies by for example 20 db these filters would have to be at least of the 22 nd order if the complexity of one of these filters were.