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.
Impulse response testing
When performing impulse response testing of a system, place the impulse at the zero-phase reference position of the acquisition.
This produces a correct phase display. Because the Tek Exponential window has its zero phase reference position at the 20%
point, more of the impulse response is captured. All other window functions have their phase reference position at the 50%
position in the gated region.
There are several ways to adjust the position of the zero phase reference point with respect to your input signal:
â–
Adjust the spectral analyzer gate position
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Perform fine adjustment using the front-panel trigger level control
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Adjust the front-panel horizontal Position control
Spectral analyzer windows
In the time domain, a window is a bell-shaped function equal in length to the gate duration.
For most windows, this function tapers to zero at both ends of the gate region. Before computation of the spectral transformation,
the window is multiplied, sample by sample, times the input data in the gated region. The window function affects the shape of
the spectral analyzer response in the frequency domain.
The window functions affect the ability to resolve frequency in the output spectrum and can affect the accuracy of the magnitude
and phase measurements. The previous figure shows how the time domain record is processed.
Accurate magnitude measurements require that the input source waveform be stationary within the gated region. This means that
waveform parameters, such as frequency and amplitude, should not change significantly as a function of time within the gate
region that is input to the spectral analyzer. Also, the gate width must be greater than or equal to the period of the start frequency
of the span of the spectral analyzer. That is, there must be at least one cycle of the harmonic being measured within the gated
region.
Oscilloscope reference
DPO70000SX, MSO/DPO70000DX, MSO/DPO70000C, DPO7000C, and MSO/DPO5000B Series 725
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