Making Measurements Using Spectrum Analyzer Mode

Measuring Amplitude Modulation with the Fast Fourier Transform Function

Example 1: Use the manual FFT measurement to look at 60 Hz AM modulation.

1.Connect a signal generator to the spectrum analyzer’s INPUT 75 Q. Adjust the signal generator to produce an AM signal with a 60 Hz modulation frequency.

2.Press (FREQUENCY] and set the spectrum analyzer center frequency to the frequency of the modulated input signal. Press @Gi), 10 MHz. Press (FREQUENCY] again and center the signal on the spectrum analyzer display.

3.Press m), 1 MHz. Press [FREQUENCY] and center the signal on the spectrum analyzer display again.

4.Press Isw], 100 kHz. (Re-center the signal, if necessary.)

Press (SPAN) 200 kHz, re-centering the signal again if it is necessary.

5.Press VID BW AUTO HAM , 1 kHz. The video bandwidth should be about ten times greater than the highest modulation frequency of interest for the best amplitude accuracy.

6.Press BEF LVL and turn the knob to change the reference level, placing the signal peak within the top division of the screen. The signal peak must be below the reference level. The signal amplitude moves up and down because the spectrum analyzer catches the signal at different points of modulated amplitude each time it sweeps.

7.Change the amplitude scale to linear by pressing [AMPLITUDE] and SCALE LOG LIN so that LIN is underlined. The FFT will give incorrect results when the spectrum analyzer is in Log mode.

Press BBF LVL and place the signal peak within the top division of the screen.

8.Press m), 0 Hz. The spectrum analyzer now operates as a fixed-tuned receiver.

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