Limitations of a Finite Measurement Span
Curve fitting is performed over a finite portion of the theoretical frequency
response. Unexpected out-of-span poles and zeros may be required to provide
an accurate fit inside the measurement span. For example, a typical op-amp
has an open-loop response that begins to roll-off between 10 kHz and
100 kHz due to a single pole. Closing the loop pushes this roll-off farther out
(for example, to 150 kHz). Thus, an active filter may be designed to have two
poles, but is actually affected by another pole farther out. This may be
accounted for in a curve fit, but it depends upon the max order curve-fit error
tolerance (determined by the measurement variance) and the size of the effect.
Agilent 35607A
Curve Fit Option 1D3 Operator's Guide
16-16
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