Agilent Technologies 4395A - Page 393

Network
Measurement
Basics
Figure
A
-8.
Constant
Group
Delay
Note
,
however
,
that
the
phase
characteristic
typically
consists
of
both
linear
(rst
order) and
higher
order
(deviations
from
linear)
components
.
The
linear
component
can
be
attributed to
the
electrical
length
of
the
test
device
and
represents
the
average
signal
transit
time
.
The
higher
order
components
are
interpreted
as
variations
in
transit
time
for
dierent
frequencies
,
and
represent
a
source
of
signal
distortion
(
Figure
A
-9
).
Figure
A
-9.
Higher
Order
Phase
Shift
The
4395A
computes
group
delay
from
the
phase
slope
.
Phase data
is used
to nd
the
phase
deviation,
1
'
,
at
the
center
point of
a specied
frequency aperture
,1f,
to obtain
an
approximation for
the
rate
of
change
of
phase
with
frequency
(
Figure
A-10
).
This
value
,

g
,
represents the group delay in seconds assuming linear phase change over 1f.
Basic Measurement Theory A-17