HP 8753D - Page 402

F
orward Transform
Measurements
This
is an
example
of
a
measurement
using
the
F
ourier
transform
in
the
forward direction,
from
the time
domain
to
the
frequency
domain
(see
Figure
6-76
):
Figure
6-76.
Amplier
Gain
Measurement
Interpreting
the
forward
transform
vertical
axis
.
With
the
log
magnitude
format
selected,
the
vertical
axis
displays
dB
.
This
format
simulates
a
spectrum
analyzer
display
of
power versus
frequency
.
Interpreting
the
forward
transform
horizontal
axis
.
In
a frequency
domain
transform
of
a
CW
time
measurement,
the
horizontal
axis
is
measured in
units
of
frequency
.
The
center
frequency
is
the
oset
of
the
CW
frequency
.F
or example
,
with
a
center
frequency
of
0
Hz,
the
CW
frequency
(250
MHz
in
the
example)
is
in the
center of
the
display
.
If
the
center
frequency
entered
is
a
positive
value
,
the
CW
frequency shifts
to the
right
half
of
the
display;
a
negative
value shifts
it to
the left
half
of
the
display
.
The
span
value
entered
with
the
transform
on
is
the total
frequency span
shown on
the
display
.
(Alternatively
,
the
frequency
display
values
can
be entered
as start
and stop
.)
Demodulating the
results of
the forward
transform. The
forward
transform
can
separate
the
eects
of
the
CW
frequency
modulation
amplitude
and
phase
components
.
F
or
example
,
if
atest
device modulates
the
transmission
response
(S
21
) with
a500
Hz
AM
signal,
you
can
see
the
eects
of that
modulation
as
shown
in
Figure
6-77
.
T
o
simulate
this
eect, apply
a
500
Hz
sine wave to the analyzer rear panel EXT AM input.
Application and Operation Concepts 6-135