HP 8753D - Page 272

While
only a
single ow
path is
shown,
two
identical
paths
are
available
,
corresponding
to
channel
1 and
channel
2.
When
the
channels
are
uncoupled,
each
channel
is
processed
and
controlled
independently.
Data
point denition:
A
\data
point"
or
\point"
is
a
single
piece
of
data
representing
a
measurement
at a
single
stimulus
value
.
Most
data
processing
operations
are
performed
point-by-point;
some involve
more than
one
point.
Sweep
denition:
A
\sweep"
is
a
series
of
consecutive
data
point
measurements
,
taken
over
a
sequence
of
stimulus
values
.
A
few
data
processing
operations
require
that
a
full
sweep
of
data
is
available
.
The
number
of
points
per
sweep
can
be
dened
by
the
user.
The units
of
the
stimulus
values
(such
as
power
,
frequency
,
and
time)
can
change
,
depending
on the
sweep
mode
,
although
this
does
not
generally
aect
the
data
processing
path.
Processing
Details
The
ADC
The
ADC
(analog-to-digital
converter)
converts
the
R,
A,
and
B
inputs
(already
down-converted
to
a
xed
low
frequency
IF)
into
digital
words
.
(The
A
UX
INPUT
connector
on
the
rear
panel
is
a
fourth
input.)
The
ADC
switches
rapidly
between
these
inputs
,
so
they
are
converted
nearly
simultaneously
.
IF
Detection
This
detection
occurs
in
the
digital
lter
,
which
performs
the
discrete F
ourier transform
(DFT)
on
the
digital
words
.
The
samples
are
converted
into
complex
number
pairs (real
plus
imaginary
,
R+jX).
The
complex
numbers represent
both
the
magnitude
and
phase
of
the
IF
signal.
If
the
A
UX
INPUT
is selected,
the
imaginary
part
of
the
pair
is
set
to
zero
.
The
DFT
lter
shape
can
be
altered
by
changing the
IF
bandwidth,
which
is
a
highly
eective
technique
for
noise
reduction.
Ratio
Calculations
These
calculations are
performed
if
the
selected
measurement
is
a
ratio
of
two
inputs
(for
example
, A/R
or B/R).
This
is
a
complex
divide
operation.
If
the
selected
measurement
is
absolute
(such as
Aor
B),
no
calculations
are
performed.
The
R,
A,
and
B
values
are
also
split
into
channel data
at this
point.
Sampler/IF
Correction
The
next digital
processing
technique
used
is
sampler/IF
correction.
This
process
digitally
corrects
for frequency
response
errors
(both
magnitude
and
phase
,
primarily
sampler
rollo)
in
the analog down-conversion path.
Sweep-To-Sweep A
veraging
Averaging is another noise reduction technique
. This
calculation involves taking the complex
exponential average of several consecutive sweeps
. This technique cannot
be used with
single-input measurements
.
Application and Operation Concepts 6-5