If you use a normal pitch shifter to shift an octave up, it in effect reduces the voice-box size by the same amount (50%), which is what gives you your baby-duck effect, popular among chipmunk fans.
A formant corrected shifter, such as Ultrashifter (not available on Eclipse) is an approach to get around this problem. But, the effectiveness of these is very subjective, so I will stand back and let more objective listeners give you their opinions.
