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IEC - Interpolated Error Correction - HOW DOES IT WORK???

Question asked by TheGris on Sep 21, 2012
Latest reply on Sep 23, 2012 by Dr_joel
I would like to gain an understanding of interpolated error correction on the PNAs, both old (8720D/8753E) and new (N5241A/N5242A).

*_Q1 - What is the interpolation algorithm?_*

Does the interpolation use simple straight-line interpolation between calibration points?

Does the interpolation interpolate error corrected data points by (1) straight-line interpolation between other error-corrected data points, or (2) by straight-line interpolation of calibratioin points and raw data points, and then applicaiton of the interpolated cal to the interpolated data?

Or is the interpolation more complicated?

*_Q2 - Interpolation Accuracy_*

Agilent has been VERY careful to state only that when you set INTERP (IEC) on the accuracy of the error correction is not guaranteed. But... there is NOTHING that tells you how badly it is degraded.

I know from the manuals that (a) IEC is 'most accurate' when there are 67 (real) calibration points per 1 GHz in the calibration you are interpolating from; and that (b)  interpolation doesn't work if the phase shift is more than 180 degrees per 5 data points.

Are there any (reasonable) guidelines that can be used to make interpolation a useful feature?

_*Q3 - Changing "stimulus" parameters in HOLD mode and Interpolation*_

The start freq, stop freq, and number of points can be changed when you are in HOLD (no trigger, no new sweeps) and it works if IEC/interpolation is ON. This seems weird since the parameters are supposedly "stimulus" parameters that affect how a sweep is performed, not postprocessing. But it makes sense - when in HOLD, changing those parameters can be done (subsject to the obvious limitatiion that the start/stop freqs must be a subset of the actual sweep) via interpolation. Since interpolation cannot improve the accuracy of your data (you don't collect more data when interpolating) is the accuracy of an interpolated measurement the same as a non-interpolated measurement *if* you omit error correction accuracy (error introduced by cal interpolation)? 

It seems to me that the ONLY source of accuracy degradation when interpolating is the fact that the calibration points in a non-interpolated sweep line are at the exact same frequencies as the data points - and not (necessarily) when interpolating...

_*Q4 - Is there any documentation that addresses these issues? Agilent or 3rd party?*_

Thanks in advance!!!