HP 8753D - Page 302

Group
Delay
Principles
F
or
many
networks
,
the
amount
of
insertion
phase
is not
as important
as the
linearity of
the
phase
shift
over
a
range
of
frequencies
.
The analyzer
can measure
this linearity
and express
it
in
two
dierent
ways:
directly
,
as
deviation from
linear phase
,or
as group
delay,
a derived
value
.
Group
delay
is
the
measurement
of
signal
transmission
time
through
atest
device.
It is
dened
as
the
derivative
of
the
phase
characteristic
with
respect to
frequency.
Since the
derivative
is
basically
the
instantaneous
slope
(or
rate
of
change of
phase with
respect to
frequency),
a
perfectly
linear
phase
shift
results
in
a
constant
slope,
and therefore
a constant
group
delay
(see
Figure
6-19
).
Figure
6-19.
Constant
Group
Delay
Note
,
however
,
that
the
phase
characteristic
typically
consists
of
both
linear 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
(see Figure
6-20
).
Figure 6-20. Higher Order Phase Shift
The analyzer computes group delay from the phase slope. Phase data is used to nd the phase
change,1  , over a specied frequency aperture, 1f, to obtain an approximation for the rate of
change of phase with frequency (see Figure 6-21). This value, 
g
, represents the group delay in
seconds assuming linear phase change over 1f. It is important that 1  be  180

, or errors will
Application and Operation Concepts 6-35