Tektronix TDS5000B Series, Онлайн руководство пользователя

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Using the Phase Controls
You can set the vertical units to Degrees, Radians, or Group Delay in seconds. To do this, click
the Math toolbar button, click Spectral Analysis Setup, and select the Phase tab. Then select the
desired scale type from Degrees, Radians, or Group Delay.
Phase Reference Position
: 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.
For the spectral analyzer, the phase reference position is the 50% position of the gate, that is, the
middle of the gate interval of the data that is input to the spectral analyzer. This is true for all
window functions except for the Tek Exponential window. This window has a reference point at
the 20% position of the gate.
Phase Unwrap
: 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 function of frequency.
Therefore, do not use it when analyzing the harmonic content of the typical repetitive signal.
Suppression Threshold
: Random noise in the spectrum may have phase values over the entire
range. This could 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 magnitude
below this threshold is set to zero. See the following figure for an example.
Group Delay
: When the phase spectrum is a continuous function 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 feature is not useful for analysis of harmonic content of signals where the phase response is
not continuous.