Tektronix TDS5054, User Manual

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Creating and Using Math Waveforms
TDS5000 Series User Manual
3- 157
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
Ꮛ
| X |
Ref
Ꮠ
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
Ω
.
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
A. If the input units are
watts, then the value is set to 1 mW.
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 (see Figure 3--45). This control does not change the
spectral data.
0 dB
20 dB 15 dB 10 dB
Figure 3- 45: Effects of adjusting the reference level
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