Ft vs. operating freq in RF design

[chang830]

Hi,
We know, in RF IC design,as a rule of thumb in rf circuit design, we need the Fmax value to be at least 10 times as high as the circuit operating frequency.

Would anyone pls. tell me why?

Thanks

 

[renwl]

the larger fmax, the better Q

 

[rfsystem]

For nontuned circuits x10 is a good estimate.

For tuned circuits with inductors you can approach x2 but with loss of performance.

 

[cswang]

maximum avalible gain is more important than ft in really high frequency RF design(such as 60GHz)