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Section
III
d.
Remembering
tnat
channel
A
is tile
reference
channel, connect the
signals
to
be
mea$.rred
to
the 3575A
Input
colnectors.
e.
Allow
time
for
the reading
to
stabilize.
f.
Observe
the
relative
amplitude reading
in
dB.
A
negatiye reading indicates
that
B
is
lower than
A;a
positive
reading means
that
B
is
geater than A.
g.
When making relative
measurements
ensrre
that the
input
levels
are
within the limits
of
the
voltage
range
settings being
used.
3-48.
Phase
Measrements
349.
With the
DISPLAY switch
in the
PHASE position,
the
35754
measures
the
phase
difference
between
the
two
input
signals
in
degrees.
To
measure
phase, proceed
as
follows:
a.
Set
the DISPLAY switch
to
the
PHASE
position.
b.
Set
the
PHASE
REFERENCE
switch
to
+
A
or
as
outlined in
Paragraph
3-34.
c.
Set
the
Voltage
Range
and
FREQUENCY RANGE
switches
as
outlined inPangrapts
3-24
and 3.27
.
d.
Connect
the
signals
to
be
measured
to
the 35754
Input
connectors.
3-8
Model
3575
A
e.
Allow
time
for
the reading
to
stabflize.
f.
Obsewe
the
phase
reading. Since
A
is
the
reference
channel,
a
negatiye
reading indicates
that B
lags
A;
a
positive reading indicates
that
B
leads
A,
3-50.
Effects
of
Harmonic Distortion
on
Phase
Read-
ings.
In the
3575A,
phase
difference
is measured between
the zero
crossing
points of the
applied signals.
If
an
applied
signal
contains
harmonics
of
the
fundamental frequency,
the zero
crossings
may be
shifted
with
respect
to
a
pure
sine
wave.
If
the
instrument
responds
to
the
false
crossings
introduced
by
the
distortion,
an eroneous offset will
appear
in
tlle
phase
reading.
3-51.
The
amount
of
enor
introduced
by
harmonic
distortion
depends
on
the
magnitude,
phase
and
order
of
the
harmonics. Harmonics
that
are
in
phase
with
the
fundamental
(such
as
in
a
square wave)
do not
cbange
the
zelo
crossing
points and do not affect the
phase
readi€.
In
the
35754,
the effects
of
even
harmonics
are cancelled
b1
two
phase
detecton that
operate
180
degrees
out
ofphase.
For
this
reason,
wen
harmonics,
regardless
of
their
phase,
do not affect the
phase
reading.
This
leaves
only
odd
harmonics
that
are
out
of
phase
with
the fundamental.
3-52. The amount
of
eror
introduced
by
odd
harmoaic
distortion
again depends
on the magnitude and
phase
of
the
harmonics.
The largest
error
occurs
when
the
odd harmon-
bs
are
90
degrees
out
of
phase
with
the fundamental.
In
Figure
3-6.
dBV
to
Volts Conve6ion.
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