Environmental Engineering Reference
In-Depth Information
0.53
w
i
thout steel plate
with steel plate at 1m-distance
0.52
0.51
0.50
0.49
0.48
8
9
10
11
12
13
14
15
time (ns)
Fig. 6.8
Comparison between TDR waveforms of the antenna, with and without the steel
plate placed at a distance of 1 m from the AUT
Therefore, referring to the TDR waveform reported in Fig. 6.3(c), it appears that
the upper limit for the time windowing in order to ensure a SNR
>
1 is approximately
T
w
,
UP
<
35 ns. SNR measurements, directly available on the used instrument, con-
firmed these theoretical considerations.
Indeed, longer time windows can be used, provided that the influence of noise
is reduced through an appropriate post-processing of the acquired TDR waveform.
In particular, a time domain filter can be applied before the FD-transformation. The
feasibility of this approach was confirmed by the results achieved applying a 5th
order-moving average filter (available within the MATLAB filtering toolbox) to the
TD data of a 100 ns-long time window. Fig. 6.9 shows the comparison between the
S
11
,
ref
(
f
)
curve and the
S
11
(
f
)
curves obtained with and without the application of
the filter.
As expected, the
S
11
(
curve obtained from the filtered TDR waveforms pro-
vides a more truthful picture of the AUT response. This is confirmed by the evalua-
tion of the
rmse
between the magnitudes of
S
11
(
f
)
f
)
(with and without the application
of the filter) and of
S
11
,
ref
(
f
)
:the
rmse
is 0.01 for the filtered data and 0.04 for the
non-filtered data.
These results suggest that the choice of a long time window, such as the consid-
ered 100 ns-long time window, in conjunction with an appropriate time-domain fil-
tering, can efficiently circumvent the noise limitation, and can simultaneously guar-
antee a better frequency resolution. Additional experimental results (not reported
here for the sake of brevity), confirmed that a long-time windowing, together with
the aforementioned filtering procedure, is definitely the most appropriate choice
when the frequency response of the device must be retrieved also at the lowest
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