Oscilloscope Reference Spectral math phase controls
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 t he 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 next 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.
DSA/DPO70000D, MSO/DPO/DSA70000C, DPO7000C, and MSO/DPO5000 Series 767
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