Operating Basics
TDS5000B Series Quick Start User Manual
77
Using the Magnitude Controls
Vertical units can be either linear or log. When the spectrum is linear magnitude, the vertical units are the same
as the source waveform. When the vertical scale of the magnitude spectrum is set to dB, use the Reference
Level Offset to set which vertical position in the magnitude spectrum is zero dB. Setting the vertical scale to
dBm sets the Reference Level Offset to a value that is equivalent to 1 mW of power into 50 Ω.
The value of the Reference Level is the magnitude at the top of the display screen. Reference Level does not
change the spectral data but Reference Level Offset does. Adjusting the Reference Level Offset causes the
spectral waveform to move vertically with respect to the waveform reference marker. This moves the waveform
without changing the Reference Level control setting.
Using the Phase Controls
You can set the vertical units to Degrees, Radians, or Group Delay in seconds. Phase is a relative measurement
that must have a time domain reference point. The phase value is specified with respect to this phase reference
position.
The spectral analyzer produces phase values from -π to π radians or -180 to 180 degrees. However, when you
perform impulse response testing and the phase is continuous, then phase values outside these ranges may
occur. The spectral analyzer then wraps the data with discontinuities in the display from +180 to -180 degrees.
Phase unwrap will display the correct result by unwrapping the phase. Phase unwrap is only valid when the
phase spectrum is a continuous funct ion of frequency. Therefore, do not use it when analyzing the harmonic
content of the typical repe titive signal.
Random noise in the spectrum may have phase values over the entire ra nge. This c ould make the phase display
unusable. However, you can set the suppression threshold control to a level in dB. The phase of any complex
spectral points with a ma gnitude below this threshold is se t to zero.
When the phase spectrum is a cont inuous funct ion of frequency, group delay may be computed. This is true of
impulse response testing where an impulse is fed into the system, and the spectrum of the response of the
system output is computed.
Group delay measures how well a system passes a signal in terms of phase distortion. Group delay is the
derivative of the phase with respect to frequency. This f eature is not useful for analysis of harmonic content of
signals where the phase response is not continuous.
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