Yes, bendwith limit must be set to 20Mhz but it has nothing to do with ripple freq.
SMPS generally work in the kHz range (About 100kHZ), which corresponds to us(microsec) range, not ms(milisec.) range. ms range is freq from main power.
PARD will be the frequency of the SMPS circuit, not Hz(ms) range.
If the ripple is measured at the output capacitor there is no need to add up capacitors. And it is best way to measure ripple.
https://www.youtube.com/watch?v=eVB_n4t57Yc
https://www.youtube.com/watch?v=pKXPqApOYfk