Transfer function related S-parameters like S21 or S12 are not really defined for a sampling system which is nonlinear. One input frequency generates many output frequencies in terms of heterodyne products and the system is also not time invariant [small signal AC analysis is only valid for a linear (or weakly nonlinear) time-invariant system]. We measure reflection related S-parameters (S11 and S22) in steady state track mode for all T/H products, although these are not significantly affected by which mode the device is in. The sampling transfer function, describing the frequency response of the T/H sampling process, is provided in the data sheet to describe the transfer function the user can expect from input to the sampling point in the device, including any sampling aperture effects that would limit bandwidth (which are negligible compared to input amplifier bandwidth, due to very narrow sampling aperture).
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