bode plot for servo motor

hi help,

I am new to Bode plot. i understand what it means, but if i apply to real application, i messed up.

for servo system(servo driver to drive motor with typical A,B,C incremental encoder),

what i get from bode plot is a Gain vs frequency graph.

for example  -20db at 200Hz.

i get it means , at 200Hz, where the motor rotate at certain rpm. the  feedback amplitude/ input amplitude < 1. 

but how can feedback varies in Amplitude if it is ABC encoder?  

isn't it only vary in frequency? 

what is the use of amplitude in this case?