Its a nice tool. I've one doubt and hope you will clarify it. We know that t = RC. Can I select any value of R and C so that the equation is satisfied. For eg: if I want t = 20ns which one is better..?

Option (A): R = 200 Ohm and C = 100 pF 

Option (B): R = 1K Ohm and C = 20 nF

Is there any rule to select the value of Capacitor..?

Basically, I'm delaying the continous square wave signal through an RC circuit and also want to know what should be the maximum frequency of the input signal to the RC to have 20ns..?

Many Thanks...!