what is input groupdelay

hi
any 1 tell me what is mean by input group delay,or coupling bandwidth in cavity filter filter ?
and how to calculate the that parameter?

τ = - dΦ / dω

τ group delay

Φ phase

ω angular frequency


see Matthaei, Young, and Jones for more details. I have seen it both with and without the negative sign. I am incertain about the convention. HFSS on-line help has additional information if you can get to the documentation.