Designing lower frequency (200 MHz to 1 GHz), miniature microstrip bandpass filters that achieve sufficient rejection on closely situated out of band frequencies are difficult. Combline bandpass filters can solve this problem. By Anurag Bhargavan
Typically, the selection of filters that can be used in 200 MHz to 1 GHz band is very limited. This selection becomes even narrower if a sharp rejection profile is required. Filters that can be designed in this frequency range are typically lumped element filters, SAW filters, helical filters, interdigital filters and combline filters.
If sharp rejection is needed in this frequency range the lumped element filter is not the design of choice because of the component's "Q" limitation.
Helical filters are the best choice since they provide excellent rejection profiles for this range. However, the drawback to helical filters is that they suffer from a footprint that is too large and are difficult to assemble and tune.
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