BMR Characterization of Antennas through the Combined TD/FD Approach - Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications - page 147

become too high, and hence the sampling frequency too low: this represents the up-
per limit for the maximum frequency f M . In order to retrieve the frequency response
up to f M = 3 GHz, then f S should be at least 6 GHz, so as to satisfy the sampling
theorem. As a result, the corresponding T c should be lower than 0.166 ns. Consider-
ing that, for a fixed time window, the used instrument provides 4000 measurement
points in TD, the maximum time window must be shorter than 666 ns.
Indeed, the maximum time window should be definitely shorter. In fact, the used
TDR instrument performs a real-time sampling only for time windows shorter than
120 ns: beyond this limit, the instrument samples in equivalent-time. As a matter of
fact, equivalent-time sampling is inappropriate when such one-shot signals are con-
sidered. Therefore, it is advisable to use windows shorter than 100 ns. Additionally,
it is worth pointing out that, in this case, the intrinsic noise limitation is not critical,
since the steady-state condition is not reached through an oscillating and attenuating
transient.
The aforementioned considerations may be summarized as follows: if T w is too
short, then results will be less accurate at low frequencies, conversely, if T w is too
long, this will result in inaccurate results at high frequency.
On such basis, Fig. 6.11 shows that a 100 ns-long time window provides over-
all accurate results over the entire considered frequency range. The evaluation of
the rmse values (reported in Table 6.2) confirmed that a 100 ns-long time window
(for which real-time sampling is assured), provides the best results in terms of ac-
curacy. The rmse value corresponding to a 150 ns-long time window confirms the
performance degradation for higher time windows.
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