User`s guide

Identifying F requency-Response Models
This section supports the estimation procedures “How to Estim ate
Frequency-Response Models in the GUI” on page 3-3 and “H ow to Estimate
Frequency-Response Models at the Command Line” on page 3-5.
What Is Frequency Resolution?
Frequency resolution is the size of the smallest frequency f or which details
in the frequency response and the spectrum can be resolved by the estimate.
A resolution of 0.1 rad/s means that the frequency response variations at
frequency intervals at or below 0.1 rad/s are no t resolved.
Note Finer resolution results in greater uncertainty in the m odel estimate.
Specifying the frequency resolution for etfe and spa is different than for
spafdr.
Frequency Resolution for etfe and spa
For etfe and spa, the frequency resolution is approximately equal to the
following value:
2π
M
radians
sampling interval
⎛
⎝
⎜
⎞
⎠
⎟
M is a s calar integer that sets th e size of t he lag window.
AlargevalueofM gives good resolution, but results in mode uncertain
estimates.
The default value of M for
spa is good for systems without sharp resonances.
For
etfe, the default val ue of M gives the maximum resolution.
Frequency Resolution f or spafdr
In case of etfe and spa, the frequency response is defined over a uniform
frequency range, 0-F
s
/2 radians per second, where F
s
is the sampling
frequency—equal to twice the N yquist frequency. In contrast,
spafdr lets
3-7