Bode diagram, why is the phase at origin 90 degrees constant?
Let us say transfer function is H(jw)=1/jw=-j/w. Its a single pole system with pole at origin.

The phase of any transfer function is given by atan(IMAG(H(jw)/REAL(H(jw))). So 

Phase(H(jw))= atan(-w/0) =-90deg for all w.

Hence, phase starts with 90deg and remains constant thereafter as maximum phase lag a pole can give is 90deg. 

The same may not be true for a pole at any other frequency. Lets say that pole is at w=a. so, transfer function is 

H(jw)=1/(1+j(w/a)).

Phase for this transfer function will be -atan(w/a)).

For w&lt;  Phase=0deg

For w>>a ==>  Phase=-90deg

For w=a  ==>  Phase=-45deg.

Hence, in this case Phase is not constant, it starts with 0 and falls to -90deg.

Hope this helps..!!