6 MEASUREMENT DATA ANALYSIS
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6.5 Time Domain Gating
Time domain gating is a function, which mathematically removes the unwanted
responses in time domain. The function performs time domain transformation and
applies reverse transformation back to frequency domain to the user-defined span in
time domain. The function allows the user to remove spurious effects of the fixture
devices from the frequency response, if the useful signal and spurious signal are
separable in time domain.
Note Use time domain function for viewing the layout of useful
and spurious responses. Then enable time domain gating
and set the gate span to remove as much of spurious
response as possible. After that disable the time domain
function and view the response without spurious effects
in frequency domain.
The function involves two types of time domain gating:
§bandpass – removes the response outside the gate span,
§notch – removes the response inside the gate span.
The rectangular window shape in frequency domain leads to spurious sidelobes due to
sharp signal changes at the limits of the window. The following gate shapes are offered
to reduce the sidelobes:
§maximum;
§wide;
§normal;
§minimum.
The minimum window has the shape close to rectangular. The maximum window has
more smoothed shape. From minimum to maximum window shape, the sidelobe level
increases and the gate resolution reduces. The choice of the window shape is always a
trade-off between the gate resolution and the level of spurious sidelobes. The
parameters of different window shapes are represented in table 6.7.
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