bode from time domain data

Hi,

Last week I did experiments on our plant. The goal is to create a bode for this plant. This plant is powered by simulink, so therefore I put the following signals on the plant: White noise(using band limited white noise block) White noise with max freq of 102 Hz(using quadscope) Sine starting at 1hz increasing to 20Hz with steps of 1Hz each 30 secs(using a sine, and manually editing the frequency). The data of those experiments is saved in time domain. The input is read after the input block and saved to the workspace, and the output is read from the loopback and saved to the workspace.

I therefore have for each experiment 2 the input(amplitude with time) and output(amplitude and time).

I then tried creating the bode with the System Identification Toolbox. I did this by creating 2 arrays containing just the amplitude of the input and output. I loaded those as a Time-Domain signal with a samping interva of 1/4096(as there are 4096 points per sec measured). When I click on 'Time Plot' The input and output signals are displayed correctly as I expect them(and how I have plotted them in the workspace).

Then when I click 'Frequency function' a bode appears, and I expect this to be the bode of the system. However this bode starts at 16Hz~100.5rad/s while I actually want it for 0.1 to 20 Hz.

I also tried using Transform Data->Transform to: Frequency domain data/Frequency function, however as I expect the responses from the 3 different signals to be the same, this is not the case for either transformation.

Can anyone help me out, and tell me what I am doing wrong. Thanks in Advance,