3-32
Making Time Domain Measurements
Resolution
Resolution
Two different resolution terms are used in the time domain:
• response resolution
•range resolution
Response Resolution
Time domain response resolution is defined as the ability to resolve two closely-spaced responses, or a
measure of how close two responses can be to each other and still be distinguished from each other. For
responses of equal amplitude, the response resolution is equal to the 50% (6 dB) impulse width. It is
inversely proportional to the measurement frequency span, and is also a function of the window used in the
transform. The approximate formulas for calculating the 50% impulse width are given in
Tab le 3-3. For
example, using the formula for the bandpass mode with a normal windowing function for a 50 MHz to 13.05
GHz measurement (13.0 GHz span):
With this measurement, two equal responses can be distinguished when they are separated by at least 4.53
centimeters. In a measurement with a 20 GHz span, two equal responses can be distinguished when they
are separated by at least 2.94 cm. Using the low pass mode (the low pass frequencies are slightly different)
with a minimum windowing function, you can distinguish two equal responses that are about 1.38
centimeters or more apart.
For reflection measurements, which measure the two-way time to the response, divide the response
resolution by 2. Using this example, you can distinguish two faults of equal magnitude provided they are 0.69
centimeters (electrical length) or more apart.
NOTE Remember, to determine the physical length, the relative velocity factor of the transmission
medium under test must be entered into the electrical length equation.
50 percent calculated
impulse width
0.98
13.0 GHz
-------------------------
2=
Electrical length
in air
0.151
10
9–
s
30
10
9
c
m
s
----
=
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