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Using the Magnitude Controls
Vertical units can be either linear or log. You may select these choices by clicking the Math menu
button. Then select the Spectral Analysis Setup command. On the Mag tab, select the desired scale
type from Linear, dB, or dBm.
Linear: When the spectrum is linear magnitude the vertical units are the same as the source
waveform. Usually this is volts. However, it may also be watts or amperes.
dB: This sets the vertical scale of the magnitude spectrum to dB. Use the Reference Level Offset
to set which vertical position in the magnitude spectrum will be zero dB. The following equation
applies:
dB = 20 log (|X| ÷ Ref)
Where X is a complex data point in the spectrum and Ref is the Reference Level Offset value.
dBm: This selects dB as described in the above equation, but also sets the Reference Level Offset
to a value that is equivalent to 1 mW of power into 50 ohms or volts. Therefore, if the input units
are volts, then the value is set to 223.6 mV. If the input units are amperes, then the value is set to
40 mA. If the input units are watts, then the value is set to 1 mW.
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