Re: Why do we use microstrip filter instead of simple LC fil
There are a lot of reasons. Yes, it is right that lamped L and C elements will work only far below self-resonant frequency (SRF). For best 0402 size components SRF can be as high as 6 GHz. Even so high number is not high enough to design good filter with lumped elements on microwave frequencies. The price is important, microstrip filter can be done very small and inexpensive, especially when very narrow bandwidth is not required. But there is one very important reason that probably overweight other. This is very high value of unloaded Q-factor for microstrip based resonators. On microwave frequencies it is easy to get Q about several hundreds. With special Q optimization technology I got unloaded Q about 5000 on 5 GHz. This provides very good steepness for filter slopes, excellent attenuation in stop-bands and low insertion loss. This is not widely known point. Actually band-pass filter insertion loss depends primarily on filter order and the ratio of loaded Q to unloaded. We cannot change filter order and loaded Q because these numbers are defined by the spec. But having high value for unloaded Q of filter's resonators allows to get very low insertion loss. For example, for broadband (this ensure low loaded Q) band-pass filter on 4 GHz central frequency it is possible to get insertion loss below 1 dB for 9th order Chebyshev filter with return loses about -15 dB. Definitely it is impossible for lumped LC filters.
Best regards,
RF-OM