Making Measurements Using Spectrum Analyzer Mode

Measuring Amplitude Modulation with the Fast Fourier Transform Function

For the best amplitude accuracy, the resolution bandwidth should be about 10 times greater than the highest modulation frequency of interest.

For the 60 Hz fundamental, a 1 kHz resolution bandwidth works well. (If harmonics are not a concern, a 100 kHz resolution bandwidth can be used and it will provide a faster update rate.)

c. Press (Bw), VID BW AUTO MAN (MAN) and use the step keys to decrease the video bandwidth until the amplitude of the measured signal drops. Then step the bandwidth up until the signal amplitude stops increasing, or until the maximum video bandwidth is reached. Use the narrowest video bandwidth that does not cause a change in the signal amplitude.

For the best amplitude accuracy, the video bandwidth should be about 10 times greater than the highest modulation frequency of interest.

For the 60 Hz fundamental, a 1 kHz video bandwidth works well

7.Press (MEAS/USER) and % AM ON OFF so that ON is underlined. The spectrum analyzer reads out the percent AM of the largest modulation frequency. An arrow indicates the signal being measured. See Figure 5-l. This measurement does not include all of the harmonics of the modulating signal.

N O T E

•The percent AM function will not run if the SIGNAL CLIPPED error message is being displayed. Increase the reference level until the error message goes away

•To return to the spectrum analyzer state prior to running the FFT function, press the FFT OFF

softkey This turns off the FFT function. Press (RECALL], INTERNAL +STATE , and 8 to recall the state from state register 8.

•When the FFT measurement is active, pressing the c-1 key will cycle between the MEASNSER and FFT menus.

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