To increase the time domain measurement range,first increase the number of points, but remember that as the number of points increases, the sweep speed decreases. Decreasing the frequency span also increases range, but reduces resolution.

Resolution

Two different resolution terms are used in the time domain:

nresponse resolution

nrange 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 dD) 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 Table 6-12. 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):

0.98

50% calculated impulse width = 13.0 (GHz) x ‘2

= 0.151 nanoseconds

Electrical length (in air) = (0.151 x lo-’ s) x (30 x 10’ cm/s)

= 4.53 centimeters

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 the example above, 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.

For example, a cable with a teflon dielectric (0.7 relative velocity factor), measured under the conditions stated above, has a fault location measurement response resolution of

0.45centimeters. This is the maximum fault location response resolution. Factors such as reduced frequency span, greater frequency domain data windowing, and a large discontinuity shadowing the response of a smaller discontinuity, all act to degrade the effective response resolution.

Figure 6-73 illustrates the effects of response resolution. The solid line shows the actual reflection measurement of two approximately equal discontinuities (the input and output of an SMA barrel). The dashed line shows the approximate effect of each discontinuity, if they could be measured separately.

Application and Operation Concepts 6-139

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HP 8753E manual Resolution