Forward Transform Measurements

This is an example of a measurement using the Fourier transform in the forward direction, from the time domain to the frequency domain (see Figure 6-77):

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(a) CW Time

(b) Transform to Frequency Domain

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Figure 6-77. Ampltier Gain Measurement

Interpreting the forward transform vertical axis. With the log magnitude format selected, the vertical axis displays dE9. This format simulates a spectrum analyzer display of power versus frequency.

Interpreting the forward transform horizontal axis. In a frequency domain transform of a CW time measurement, the horizontal axis is measured in units of frequency. The center frequency is the offset of the CW frequency. For example, with a center frequency of 0 Hz, the CW frequency (250 MHz in the example) is in the center of the display. If the center frequency entered is a positive value, the CW frequency shifts to the right half of the display; a negative value shifts it to the left half of the display. The span value entered with the transform on is the total frequency span shown on the display. (Alternatively, the frequency display values can be entered as start and stop.)

Demodulating the results of the forward transform. The forward transform can separate the effects of the CW frequency modulation amplitude and phase components. For example, if a test device modulates the transmission response (&I) with a 500 Hz AM signal, you can see the effects of that modulation as shown in Figure 6-78. To simulate this effect, apply a 500 Hz sine wave to the analyzer rear panel EXT AM input.

Application and Operation Conwpts 6-143

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HP 8753E manual I i iiiii t, Forward Transform Measurements