Hi Felix,
In MATLAB, when estimating a transfer function using tfest(), you can specify various options through tfestOptions() to tailor the estimation process to your system’s characteristics.You may approach this issue by using the WeightingFilter option in tfestOptions() to give more weight to the frequency range where the system has not been excited sufficiently. This can help improve the accuracy of the estimated transfer function in the lower frequency range.
For more information on tfestOptions, please refer to the MathWorks documentation -
Hope this helps.