HP 8753D, Руководство пользователя, Страница: 397 / 678

Figure 6-71. The Eects of Windowing on the Time Domain Responses of a Short Circuit
Range
In the time domain, range is dened 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.
M easur ement r ang e =
1
1F
where 1F is the spacing between frequency data points
M easur ement r ang e =
(number of points 0 1)
f r eq uency span(H z )
example:
M easur ement = 201 points
1 M H z to 2: 001 GH z
R ang e =
1
1F
or
(number of points 0 1)
f r eq uency span
=
1
(10 2 10
6
)
or
(201 0 1)
(2 2 10
9
)
= 100 2 10
09
seconds
E l ectr ical l eng th = r ang e 2 the speed of l ig ht (3 2 10
8
m=s )
= (100 2 10
09
s) 2 (3 2 10
8
m=s )
= 30 meter s
In this example , the range is 100 ns , or 30 meters electrical length. T o 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 reection measurement). The analyzer limits the
stop time to prevent the display of aliased responses .
6-130 Application and Operation Concepts