confused with the LPF post-sim

[neo]

i am designing a 4 order butterworth LPF these days with the cut-off freq. of 10megaHz. but i am really confused when doing the post-simulation. u know i am a new new new new designer in this major. using the calibre RCC extraction type, i found the cut-off freq. dropped down to nearly 8megaHz, ok, i came back to my layout and modified all the skinny metal more wider i could, the GND metal for instance. but no lucky things happen to me. could u guys tell me what can i do next. what kind of parasitics that affect the cut-off freq. most and where are they. thanks a lot.

 

[Mazz]

You should look first at metal connections of you RC poles. The parasitic capacitor of this connection is in shunt with the real cap, decreasing the cut off.
You can also extract only one net (the one you suspect gives more contribution), adding some pins to the layout, and veryfy your suspects.
Mazz