 Very wide stopbands can be achieved by cascading several waffle @-@ iron filter units together . Each unit is designed for a stopband in different , but overlapping , ranges . The filter with the highest frequency stopband has the smallest , and greatest number , of teeth . The units are connected together with λ / 4 impedance transformer sections of waveguide in order of progressively higher frequency operation . Since the impedance transformers are operating at different frequencies the ones connected to the units with smallest teeth are shorter than those connected to units with larger teeth . Matthaei gives an example of a three unit waffle @-@ iron filter designed to stop all harmonics of the passband from the second to the tenth – a combined stopband of 2 @.@ 2 GHz to 13 @.@ 7 GHz with a rejection of 60 dB . 
