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Section
lV
which
goes
to
a high
(1)
state.
The
X
current
sinl
remains
off
because
Q',
T'and \{r'are
all
high.
The
Y
current
sink
remains
off
boausc
I'ls
-
0 artd
T'
=
I
.
.l'.16. With a
phase
difference
of
-
120
degrees
applied to
the inputs, Q
goes
low,
B
channel
is
iilverted
and the
phase
differencc applicd
to
the
phase
dctectors
is
+60
degrees.
The rcsulting
phase
detector
output
is -
1.2
Vdc. [n
this
case,
the
Xcurent sinkiscutoff
bcause
Q'=
l(Q=0),
M,
=
0
and
T' =
I
(T
=
0).
Thc
Y
currcnt sink
is
cut off
because
Ms=0
and
T'=
I
(T=0).
The
rcsult
is e
correct
pluse
readilg with
no
olTsets
requircd. The
Pltase
Control
Logic
operates
in
the
sanre
nranner
for ali
negativc
phase
differences
greater
tharl
'90
dcgrccs
ard
less
than -192
degrees.
447.
lf the
phase readiDg exceeds
-
192
degrecs
(
1.92
Vdc). M'
goes
high and a
set
comnland
is
applied to
the
"Nl"
storage
element.
Both
current
sinks are
the
gated
on
(xs
outlined
in
Paragraph
.1-.14)
artd
the
phase
reading
changes
liorn
" 192
degrees
to
+ 168
degrees.
This
is
sirnilar
to
the
+
191
degree
conditior
whcre
both
cufient
sinks
ar!'
gated
off a|d
the
reading
is
converted
to -
168
degrees.
Lhvirg the switcl
ng
poilts
at 119]
degrees
rathcr
tharl
t
180
degrees
ptovides
a
t ll
dcgrce overrange
capability
wllich
prcvents the
Phase
Feeding
from ouillating
betrveen
t
180
degrees
duc
to
the
ovsrsl)oot
in the Output
Filtcr
(A9
or
Al0).
4-48.
Eflects
of
Harmonic
0istortion
on
Phase.
4-49.
In
the
3575A,
phase
difference
is nreasured
between
the
axis
crossing
points
of
the two input
signals.
If
an
applied
signal coDtains harmonics
oI
the
fundamental
frequency,
the
axis
crossing
poirlts
rvill
clunge
dependitrg
on
the
magnitude.
pluse
and order
of
the
haruronics.
llennonics
that are in
phase
with the
Iundanrerttal fie-
quency
do not
change
the axis
crossing
points
and,
therefore,
do not
affoct the
accuracy
of
plusc
readings.
llannonics
that
are
out of
phasc
with
the
fundairental,
howwer, do
change
the
axis
crossilg
points and.
ir
sorne
cascs,
producc
crrors
in the
phase
reading.
4-50.
Odd harmodcs that llre
out oI
phase
with
the
fundarneltal tend
to
produce
a
syrnmetrical
wavefonn
tllat
is
completely shifted in pluse.
This t),pe
of
error
is
dilTicult
to
eiiDinate
without
the benefit
of
selqrtive
filtering.
Since
the 3575A is a
broadband inslrumelrt. high
levcls
of
odd.
out-ol-phasc harmortics can produce
phase
offsets.
Figtrre
3-7
(Section
[Il)
is
a
graph
whicir
shows
the
worsL-cuse
error
produced
by
odd,
out-of-phase
harmonils.
'fhc
worst-case
condition
occurs when the harmonics
are
shifted
g0
degrees
with
respect
to
the
fu|damental.
As
indicated
in
the graph.
t})e
error
is
relatively
insignificant (0.57
degrees)
rvltcn
the
total odd
lurmo
c
distortion
is
nrore
than.10
dB bekrrv the
fundarnental.
+51.
Eyen harmonics
that are
out
oI
pllasc
with
the
fundanental
change
the
irx
is
crossiirg
points but.
unlikc
odd
harmonics,
producc
an
unsymmctricai rvaveform.
Ihis
makes
it
possible
to
cancel
the
effei:ts
oI
evcn
harnronic
4.E
Model
3575A
distortion in
the
3575A by
the
use
of two
phase
detectors
which
operate
180
degrccs
out
of
phase.
This technique
is
llustrated
in
Figures
4-8 and
4-9.
In
Figure 4-8, the
phase
detector
outputs,
P1 and
P2, are
controlled by
transitions
that
occur
at
the
a-xis
crossing
points
of
the
applied
signals,
A
and B. Thc
Pl
phase
detector
is gated
on by
a positivc
transition
on A
ald off
by
a
negative
transition on
B;Pl
is
gated
on by
a
negative
transition on
A and
off
by
a
positive
transitior
on B.
The
phase
detector outputs
are
summed
(and
invertcd)
to
produce
a sirgle
pulse
train
((D)
with
an
average
valuc
that
is proportional
to
the
phase difference
between
A
and B. The
two
i,rput
signals
in
Figure.l-8
are
irl
phase
and
the
average
value
of
the
output
pulse
train
is
equal
to
the
peak
voltagc introduced by
Pl
or
P2
which.
irr
this
casc,
is
-
1.8
V.
.1-52. Figure 4-9
is
identical
to
Figure 4-8 except the
signal
applied
to
channel
B contains
a
l0
% second
hartnonic
tlut
is out
of
phase
with the
fundamental.
The
combined
avcrage
output (rf
the
two
phase
detectors is
still
equal ro
-
1.8
V
because
the
distorted
signal
on
B
produces
equal
and
opposite errors
on
Pl
and
P2
thereby cancelling
the
effccts
of
the distortion.
4-53.
Error Correction
Scheme.
4-54.
The
3575A
uses
logic
circuitry
(A5lC4
through
IC6.
Schernati.
No. 2)
in
a unique error correction
scheme
to
a
.e.
Fiqure
4-8.
A
and B
ln
Phase
-
No Distortion.
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