System Identification with frfit()

[John Ryan]

Bonjour,

I have measured the phase and magnitude response of a system and I am trying
to identify a model for it with frfit(). This works well up to about 10Hz
but then can be 20dB different in gain, 150degrees in phase. Best fit is
with order 3 or 4.

My data is a log frequency sweep from 0.7Hz to 60Hz. It is a little noisy
(+/- 0.5dB, +/-3deg). The response has a 'peak' in the phase between 7 and
20Hz and increasing gain over this region.

I tried separating the model. First I divided the measured data by the
modeled response (to extract just the data that does not fit the model) and
then frfit() to the remaining data. This did not help as the fit to the
remaining data was just as bad.

Can anybody suggest how to analyse this system?

Thanks for any help,

Regards, John

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