Making Measurements Using Spectrum Analyzer Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
4.Use the knob to place the marker on the AM modulated signal and press
a.Save the present instrument state in state register 8.
b.Reduce the span to zoom in on the signal.
c.Set the detector mode to sample.
d.Set the scale to linear.
e.Change the span to zero span.
f.Start the continuous FFT function.
g.Set the FFT stop frequency to 10 kHz.
5.Press FFT STOP FREQ , 250 (Hz). This sets the spectrum analyzer to include the fourth harmonic of the 60 Hz modulation signal on the screen.
6.To conErm that the resolution bandwidth and video bandwidth are correct for measuring the modulation amplitude, use the following procedure:
a.Press INIKR) and use the knob to move the marker to the desired modulation signal. In this example, place the marker on the 60 Hz fundamental modulation signal.
b.Press lew) and decrease the resolution bandwidth using the (JJ key, until measured signal amplitude drops. Then press lj’jj to increase the bandwidth until the signal amplitude stops increasing and stays the same, or until the maximum resolution bandwidth is reached. Use the narrowest bandwidth that does not cause a change in the signal amplitude.
N O T E
As the resolution bandwidth is stepped down, the modulated signal must be recentered on the spectrum analyzer display If this is not done, the signal amplitude can decrease due to off tuning of the spectrum analyzer and not because of the resolution bandwidth chosen,