Honored Contributor
Honored Contributorjust an additional note here :
--- Quote Start --- a large area under the FPGA clear for a "mini" ground plane --- Quote End --- such a smal copper area can't be called plane. it is just a solid fill If you assume the thickness between the two copper sides then there won't be a low Z. Just calculate the Z for 50um and 100um distance to see the effect and now imagine a pcb with normaly 1500um ... also with an eye on impedance, the area underneath a qfp is too smal that it will have an effect on the lower frequencies for decoupling. here the caps will do the work. but again even the position of these caps and their value needs to be carefully calculated that you won't get a high Z for certain frequencies. at those frequencies the pcb will radiate like a good antennae there are some (not free) tools out there, that display the impedance Z of the pcb over the pcb area as a 3D diagramm and you can see how Z changes due to the position of the caps (and the values) and if that is not enough ... the shape of these planes have a not to neglect effect okay ... i am in a lucky postion to have such a tool and it helped me to pass all approvals with the first pcb design without any housing or shielding at all.
