Table of Contents
Advertisement
Nlodel
-15
75
A
mininlize
rhr'
efir'cts
of
noise
on
phase readings.
The
follorving
porcgr:phs
briefly
describe
how
noise
can intro-
ducr'errors
lnd ho*
these
errors
are
corrected.
-1'55.
Becuuse
the
-1575r\
is a
broadband,
wide
range
instrurnL'nt.
ir
is
cap3blc
of
respolding
to
high frequency
noise
thal
.rn
lalse
trigger"
the J-K
flip
flops
in
the
Phase
Detector.
Fds.,
trisgering
is
where
the J-K
flip
flops
are
irnproperl)
set"
or
"reseti!
by
noise
transitions that
occur
near
the &\is.rossing
points
of
the applicd
signals.
This
is
Iustr3tcd
ii1
Figures
-1-10
and
4-l
1.
Figure
.l-10
shows
the
outputs
of
the
t*'o
phasc detectors
Pl
and
P2
produced
by
noise-Iree signals.
A
and B.
Figure
4-11
shows
the
sane
phase
relationship as
Figure
.+-10
however
the
signal
on
channel
A
contains
a
nutnber
of
positive
and
negatiye
transitions produced by
noise.
The
J-K
flip
flops
respond
to
these
transilions
causing
a
very significant error
(designated
by
shaded
areas)
in
the
phase
reading.
Note that P2
is
initiall)
set
high
b!'
a
negative noise
trarsition
on
A
(same
as
A')
and
is
not
reset
until
B
soes
high 250
degrees
later.
Also note
thar all lour
sigruls.
A.
B,
Pl
and
P2
are high
imrnediatel)
iillori
ing
thc
noise
pulses
on
the leading
edge
of
A.
During nonrlal
phase n'teasurements these
four
signals
should
never
be high
at the
sanre
tine.
This
cotditiorl
is,
thereforc. an
crror
stare
rvhich
can
be defined
by
the logic
equation
.\.B
P1.Pl
=
I
and
lirther
detected and corrected
Section
IV
Figure
4-10. A
and B
Clean -
No
Eror.
NO SY
CLEAN
Figure
4-l
l.
Large
Error
Produced
byNoise.
using
logic circuitry.
This method
of
error
correction
is
used
in
the
35754
and
is accornplished
by
rneans
of
logic
ctcuitry
(A5lC4 through IC6)
on
the
Phase Detector
Assembly.
The
four prirnary
signats
(A,
B,
Pl
and P2)
and
thet
opposites
(A',
B',
Pl
'and
P2')
are
applied to the crror
correction circuits. The
error correction
circuits
sense
any
of
the
eight error
states
listed
in
Table
4-3 and
"set"
or
"reset"
the
appropriate
flip
flop
to correct the error.
Table
4-3.
Erlor
Correction
Equations.
Equation
1
2
3
4
5
6
7
8
A.B.P
1.P
2
A"B"P
1
.P
2
A.B.P 1"P 2'
A'-B'.P
l"P
2
A'.8.P
1"P 2
A.B"P
1.P
2'
A.B"P
1'.P 2
A"B.P
1.P
2'
Reset
P
2
low
Set P
t
high
Ser
P
2 high
Reset
P
2
low
Set
P
i
high
Sei
P
2 high
.+-56.
Since
error
states
must be
present
for a
short time
before
they
can
be
detected and corrected,
they
can
still
affect the
accuracy
of
phase
readings.
Due
to
the
effor
coIIection
scheme,
however, worst-case
errors are
limited to
Figure
4-9.
Even
Harmonics
Cancelled.
1-9
Advertisement
Table of Contents
