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272

Reference Level: This sets the vertical position

of the displayed spectrum. Its value is the

magnitude at the top of the display screen.

When this control is adjusted, the spectral

waveform along with its zero reference marker

move vertically on the screen as shown. This

control does not change the spectral data.

Reference Level Offset: This changes the value

of Ref in the equation for dB shown above.

Unlike the Reference Level control, this control

actually changes the output data values in the

spectrum. Zero dB is shown on the display

screen by the marker associated with the

spectral waveform. Adjusting the Reference

Level Offset causes the spectral waveform to

move vertically with respect to the waveform

reference marker as shown. This moves the

waveform without changing the Reference

Level control setting. Sometimes it is beneficial

to adjust this control so that the peak of a

fundamental is at zero dB. Then you can

measure other harmonics in terms of how many

dB they are away from the fundamental. Click

the dBm button to preset this level to the

equivalent of 1 mW into 50 ohms.

Real and Imaginary Magnitudes: You can display the linear magnitude of the real data or the

imaginary data in the spectrum. This is useful if you process the spectrum off line and transform it

back into a time domain trace. You can save the real and the imaginary spectrum into reference

memory, and export the waveforms directly into Mathcad, MATLAB, or Excel documents to

update in real time.

To turn on a real or imaginary spectrum, click the Math menu button, select Define/Edit

Expression Editor, and select the Freq tab. Click either the Real or the Imag buttons and enter an

expression. Click the appropriate channel button, and click Apply.

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