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Figure 6-72. The Effects of Windowing on the Time Domain Responses of a Short Circuit
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In the time domain, range is defined as the length in time that a measurement can be made
without encountering a repetition of the response, called aliasing. A time domain response
repeats at regular intervals because the frequency domain data is taken at discrete frequency
points, rather than continuously over the frequency band.
Measurement range =
-&
where
AF’
is the spacing between frequency data points
(number of points
-
1)
example:
Measurement
range
= frequency
span(Hz)
Measurement = 201 points
1 MHz to 2.001
GHz
Range
=
&
or
(number of points
-
1)
frequency span
(201-
1)
=
(10 xl
106)
Or
(2 x
109)
=
100 x
lo-’
seconds
Electrical length = range x the speed of light (3
x
l@m/s)
= (100 x
lo-’
6)
x (3 x
l$
m/s)
= 30 meters
In this example, the range is 100
ns,
or 30 meters electrical length.
To
prevent the time domain
responses from overlapping, the test device must be 30 meters or less in electrical length for a
transmission measurement (15 meters for a reflection measurement). The analyzer limits the
stop time to prevent the display of
aliased
responses.
6.136
Application and Operation Concepts