Agilent Technologies 8753ET User Manual page 60

Group Delay
The phase linearity of many devices is specified in terms of group or envelope delay. The
analyzer can translate this information into a related parameter, group delay. Group delay
is the transmission time through your device under test as a function of frequency.
Mathematically, it is the derivative of the phase response which can be approximated by
the following ratio:
−∆Φ /(360 × ∆Φ)
where ∆Φ is the difference in phase at two frequencies separated by ∆F. The quantity ∆F is
commonly called the "aperture" of the measurement. The analyzer calculates group delay
from its phase response measurements.
The default aperture is the total frequency span divided by the number of points across the
display (i.e. 201 points or 0.5% of the total span in this example).
1. Continue with the same instrument settings and measurements as in the previous
procedure, "Deviation From Linear Phase."
2. To view the measurement in delay format, as shown in
DELAY
Format
3. To activate a marker to measure the group delay at a particular frequency, press:
and turn the front panel knob, or enter a value from the front panel keypad.
Marker
Figure 1-37 Group Delay Example Measurement
Group delay measurements may require a specific aperture (∆)F) or frequency spacing
between measurement points. The phase shift between two adjacent frequency points
must be less than 180°, otherwise incorrect group delay information may result.
4. To vary the effective group delay aperture from minimum aperture (no smoothing) to
approximately 1% of the frequency span, press:
SCALE DIV
Scale Ref
Measuring Electrical Length and Phase Distortion
Figure 1-37, press:
SMOOTHING ON
Avg
Making Measurements
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