Agilent Technologies 4395A - Page 394

Network
Measurement
Basics
Figure
A
-10.
Rate
of
Phase
Change
V
ersus
Frequency
When
deviations
from
linear
phase
are
present,
changing
the
frequency
step
can
result
in
dierent
values
for
group
delay
.
Note
that
in
this
case
the
computed
slope
varies
as
the
aperture
1f
is
increased
(
Figure
A
-11
).
A
wider
aperture
results
in
loss
of
the
ne
grain
variations
in
group
delay
.
This
loss
of
detail
is
the
reason
that
in
any
comparison
of
group
delay
data it
is
important
to
know
the
aperture
used
to
make
the
measurement.
Figure A
-11. V
ariations in Frequency Aperture
In determining the group delay aperture
, there is a tradeo between resolution of ne detail
and the eects of noise
. Noise can be reduced by increasing the aperture
, but this will tend to
smooth out the ne detail. More detail will become visible as the aperture is decreased, but the
noise will also increase
, possibly to the point of obscuring the detail. A good practice is to use a
smaller aperture to assure that small variations are not missed, then increase the aperture to
smooth the trace.
A-18 Basic Measurement Theory