1985 ingress sources and solutions
TRANSCRIPT
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INGRESS - SOURCES and SOLUTIONS
by John w
Ward
J r .
Comcast Cablevis ion of Montgomery County,
Inc.
Signal ingress i s a problem which has
always plagued cable systems. While there
has always been poten t i a l for problems
from VHF
t e l ev i s ion
s ta t ions ,
with the
modern
cable system
encounter ing
the UHF
t e l ev i s ion band, the
suscep t ib i l i t y of
cable
systems
to ingress
in terference
i s
increasing. Immunity from
ingress
problems
can
only be achieved by
maintaining system
i n t egr i ty
a t levels
bet te r than those
required by spec i f i ca t ion .
In te r fe r ing
s ignals
leak
in to
a
cable
system
not
only
thru flaws in the cable
system
but
also
by way of consumer
equipment,
cable ready t e l ev i s ions and
VCRs.
The
increase in the
number
of these
devices
as well as other
factors
lead to a
need
to be able to ef f ic ien t ly diagnose
and
cure s ignal ingress
problems.
WHAT
IS INGRESS
Ingress , as far
as
the CATV
community
i s concerned, i s the
entrance
in to a cable
system
of
any undesired
ex ternal
radio
source . Ingress
wi l l
occasional ly
be
in
the
form of s t a t i c or e lec t r i ca l noise, but
t i s normally considered to
be
in te r fe r -
ence from a radio frequency
s ignal .
Ingress
of
such
s ignals wil l r esu l t
in in terference
to cable pic tu res .
Modern
cable
systems
have
suf f ic ient i sola t ion from
ingress , or
shielding,
to
prevent
ingress
of ca r r i e r s
in
even
the
noi s i e s t of
radio environments.
But, an awareness
of
ingress re la ted
problems i s required by the cable system
operator to
enable
repa i r
of
inev i t ab le ,
na tu ra l , flaws in the system.
Ingress i s
the
opposi te of egress ,
o r
system rad ia t ion .
The
pr inc ip le by which
both
phenomena opera te i s
the
same,
re la ted
by
the
pr inc ip le
of antenna
rec ip roc i ty ,
which
i s tha t
antennas
t ransmit
and
receive
equal ly wel l . That cable systems
do
rad ia te
energy i s an establ ished
fac t , tha t
cable
systems are suscep t ib le to ingress there-
fore fol lows. As
system
egress
levels are
reduced to within
regulat ion,
ef fec ts of
ingress
are
reduced toward acceptable
l eve l s .
St i l l ,
in order to completely over
come
the
ef fec ts of ingress
in
areas where
ex ternal
radio
s ignals
are espec ia l ly
high,
even
defec ts tha t are otherwise in s ign i f i -
cant must be found and corrected.
Inter ference due to ingress can be
c la s s i f i ed
in to
two basic
forms,
e i ther co
channel or discre te car r ie r . When there are
one or more loca l VHF TV s ta t ions loca ted
near
a
cable system
which uses a
channel
occupied
by one
of
these
VHF s t a t ions ,
there wi l l
without doubt,
sooner or l a t e r ,
be need to
cor rec t
co-channel in terference
between
the
two.
Discre te
ca r r i e r s
from
communications t ransmi t t e rs wil l cause
problems on mid-band and super-band cable
channels. As
communications t ransmi t t e rs
include everything from car phones and
personal pagers to
amateurs and the
National
Weather Service , discre te ca r r i e r
ingress
can
occur anywhere and often a t
random t imes.
Cable systems
near
the
VHF TV
t ransmi t t e rs of a large
c i t y
are the
systems
tha t wi l l l ike ly
suf fer from co
channel ingress re la ted problems. At
two
to
f ive miles
from
a fu l l
power
TV t ransmi t t e r
t i s not uncommon to have a
f ie ld
s t rength
of
35
to
40 dBmv
or
more,
very
of ten 25 dB
more
than
than
what
i s ins ide
the cable.
Levels
from TV
t ransmi t t e rs
as
far away as 30 miles may exceed
the
average
levels of a CATV plan t .
Beyond t ha t
range
the ef fec ts of ingress re la ted co-channel
in te r fe rence
become l es s not iceab le .
Co-channel
type
ingress in te r fe rence
in i t s most basic form wi l l appear on a TV
pic tu re as a st rong
beat ing
pat te rn
when
the cable channel i s not phaselocked with
the in te r fe r ing s ta t ion. I f
the
cable pro
graming i s phaselocked to
but
not
sync
locked to the unwanted car r ie r , a
wiping
of
the in te r fe r ing s ta t ions sync bar through
the background of the desired pic tu re i s
the
f i r s t
ef fec t
noticed.
I f
cable pro
graming i s both phase and sync locked to
the
local
s ta t ion, as t i s when
operat ion
i s on channel ,
the
f i r s t ef fec t wi l l be
fa int
ghosts in the pic tu re ,
e i the r of t ex t
charac ter s with the i r high energy edges, or
of
the horizontal sync bar , s tab l i zed , but
in the middle
of
the
screen. The di f ference
in
the time
t
takes the
s ignal
to a r r ive
a t the
se t
both through the
cable
and
985 NCTA Technical Papers 11
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through the a i r allows the channel to
in te r fe re with i t s e l f . I f
the
two
s ignals
ar r ived a t
the
same
time
they would mesh
perfec t ly a l l
the in terference
would be
hidden.
When opera t ing on
channel , or
with
a l te rna te
programing
phase-locked and sync
locked
to
a
local
VHF
t ransmi t t e r
the
cable signal must
be
a t l eas t 50 dB
or
more
above any ingress from the
airwave
s ignal
in order to suppress sync bar ghosting of
the
pic tu re . Without sync lock, a separa
t ion of 55 dB i s a minimum to prevent
an
annoying
wiping pat tern in the
background.
I f the cable channel
opera tes with
a l te rn
a te programing not phaselocked
to
the local
TV
s ta t ion shar ing
the
same
channel, the
cable video
car r ie r
should be 60 dB higher
than any in terference.
In
extreme cases
of
co-channel
ingress
in te r fe rence where
an
off
a i r
s ignal
i s only 40 or 45 dB down
from the cable
s igna l
st rong ghosts
or
other
dis to r t ions may be expected,
regard
l es s
of
the opera t ing
mode.
Customers
wil l
surely ca l l for service when
the
in te r fe r
ence
i s th i s
bad.
The ef fec t
of ingress due to
d i scre te
ca r r i e r in ter ference i s
s imilar to second
order beat problems in
t ha t
both wil l
appear
on
the customers
se t
as a her r ing
bone
pat tern
or wavy ser ies of
ve r t i c a l
l ines . I f several unwanted
ca r r i e r s
are
presen t as i s often the case with channels
shared
by
the
communications
bands, the
ef fec t may
be
a sof t d is tor t ion
s imilar
in
symptoms to th i rd order product
accumula
t ion. A discre te car r ie r located
near
the
color sub-carr ier of a cable channel may
cause
a
beat ing
pat tern in picture
t i n t
or
i f suf f ic ient ly s trong, may even drive
the
picture in to
black
and
white .
Picture
dis to r t ions
due to d i scre te
beats
vary
depending upon the level of the
beat
as
well
as
the
pos i t ion of
the
beat
in
the band
of
the cable
channel .
For example,
an
in te r fe r ing car r ie r 30 dB down and 10
khz.
from
the
cable video car r ie r wil l
probably not cause any noticeable
picture
dis tor t ions . Inter ference loca ted 1/2 mhz
above the video
ca r r i e r
with
a
level as
low
as 40 dB down from
it,
wil l cause st rong
beats in
the
pic tu re .
When the in terference
i s
from
a
ca r r i e r loca ted
in the middle
of
the
cable
channel, 1-1/2
mhz away from
the
video, a re jec t ion of
up
to 55 to 60 dB i s
necessary
to prevent a busy background
ef fec t .
The amount of immunity required by
the
cable
system
from
ingress beats
i s
the
same as
and can
be
compared to
FCC spec i f i
cat ions regarding
car r ie r
to second order
beat ra t ios .
12-1985
NCTA Technical Papers
OW DOES INGRESS
GET
IN
Ingress gets
in to
the
cable system
by
way of
poor
sh ie ld ing
and faul ty connec
t ions . The cable ac t s as
an
antenna and
wil l have cur ren ts from
external radio
f ie lds
induced onto
it s
shield .
Elect ron
flow, or curren t s of radio frequency
energy,
happens
only
on
the surface
of
a
conductor . Under normal condi t ions ,
the
cable signal energy flows on the ins ide
surface
of
the shield
and broadcast radio
s ignals flow on the
outer
surface of
the
shield .
A hole
in the
sh ie ld wil l
jo in
the
two
sur faces
al lowing undesired
cur
rents to flow both out and in . Unbalanced
current flow between the sh ie ld and cen ter
conductor of the cable wil l cause the un
desired s ignal to
be
added to the cable
s ignal .
Experience has
shown the most common
point
of ingress
to be
a s l igh t ly loose
connector.
The connect ion
i s normally
t igh t
enough
not
to
noticeably
ef fec t
the
cable
s igna l s
i f
not for the ingress problem.
The
connect ion
may be
jus t loose
enough to
permit a i r
molecules
to
permeate between
the
threads
and,
given time, form
a layer
of corrosion.
The
improperly made
connector
wil l also
permit
gasses to
corrode
the
aluminum
of
the
shield
i t s e l f forming
aluminum oxide, a poor e lec t r i ca l
conduc
to r . Corrosion wil l crea te a point of
r es i s t ive and/or capaci t ive nature in
the
shield
of the
cable. This
breakdown of
the
outs ide
conductor
i s the unwanted
hole
in
the
shield .
Theory
and experiment show
t ha t
a
mismatch on the inner conductor wi l l not
permit
signals
to en ter
the cable,
only
the
s ignals ins ide the cable already
wi l l
be
affected. This
can be demonstrated by cu t
t ing the
center conductor short a t a sp l i ce
in the middle
of
a sect ion
of drop. The
i sola t ion
i s
as
good as the
sh ie ld
in th i s
experiment.
I t
can
also
be demonstrated
tha t a
s ingle
crack or
hole
in the sh ie ld
60
low
band
o
high
band
--
40
..._..
B
~
30
v
20
pr inc ipa l
area l imi t
- ~
rade
A s ignal
- - ~
0
0
iles
1
2 4
8 16
32 64
FIELD STRENGTH OF TV TRANSMITTERS [3]
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not completely
around the
cable, i s
in
i t s e l f not a s igni f icant source of
ingress .
However, when
the
small cracks
are spaced
a t
regular dis tances , an
ef fec t ive amount
of
energy
i s
t rans fer red
in to the cable (as
well
as
out .
Improper
handling
or in s ta l l a t ion
of
drop
cable
can cause
periodic
cracks
in
severa l
ways.
One common way a flaw may
ar i se i s when a s tap le
gun, faul ty
i t s e l f
or improperly used, causes a severe sharp
dent
in
a
drop wire
as
the s taple
i s f i red .
Even though
the
outer
shield
i s not
ac tua l -
ly
pierced by
the
impact, a small crack
might be
crea ted .
A
ser ies of
a dozen or
so of
these,
r egu lar ly
spaced 18 to 20
inches
apar t , can reduce the shielding
of
a
drop, down from a nominal 90 dB, to
only
50
or 60 dB of i sola t ion a t
mid
band frequen
c ies . Periodic bumps and
cracks
in drop
cable
can
also be
caused
by rough
pul l ing
the wire from
boxes
and r ee l s , or f lexing
the cable sharply around
corners , although
the observed
occurrence of th i s type of
fa i lure
i s
rare .
One way the la rgest .amount
of
unwanted
s ignals
can
be t rans fer red in to the cable
i s by a t o t a l
discont inui ty
of the
shield
due
to radia l
cracks .
Faulty connect ions
are s imilar to these
radia l
cracks .
A
radia l crack a l l the way around the cable
sh ie ld wil l typica l ly
reduce the
cable
s ignals
by
about 10
to 12 dB,
implying, in
the worst case,
as low as
3dB
i sola t ion
between the the outs ide and ins ide
of
the
cable. On the o ther hand, a typ ica l bad
connector
might
reduce the i sola t ion to 40
dB, with l es s
than
1/10 of a dB
reduct ion
in
cable
s igna l s .
Compared
to
defec ts
created
by
bad
connections, the
amount of sh ie ld provided
by
the
wire i t s e l f i s of
minor importance
as
far as
ingress i s concerned. With
t runk
and feeder
l ines ,
the shielding
i s
complete
as poss ib le ,
with
more
than
110
dB of
i so -
l a t ion of ten the case. Flexible drop wires
with fo i l sh ie lds under a wire bra id , the
type used by the cable industry , typica l ly
are rated
with
85
to
100 dB sh ie ld i so -
l a t ion . [ l
The a b i l i t y
of
the cable
shield ing to physical ly withstand
handling
and
to survive the elements i s of more
importance
when
se lec t ing
drop
cable
of
t h i s
qual i ty then the
shield
f ac to r i t s e l f .
The
sh ie ld ing
factor
of
the
drop
cable
becomes s igni f icant when non standard
wire i s
used, such
as
a s i tua t ion
in which
a
house has been wired
by a
customer
using
his own
wire .
The shield for th i s
wire
can
be as low as 50 dB for
wire
with a heavy
braid, 35 dB or so with typ ica l 40 braid
sh ie ld wire . Also, aside from
the poor
shield , it i s almost impossible to make a
proper connection to these wires as the
dimensions
vary
grea t ly
from type to type
and it i s next to imposs ible to f ind a
proper
f i t t i ng . The f i t t i ng must not only
pass s ignals but
must
proper ly
sea l
the
shield
from
ingress
as
well
as survive
through time.
Other parts of the cable
system
responsible for ingress are loose amplif ier
covers
and
tap
pla tes .
Although
exper ience
i s t ha t
an
amplif ier
housing
must be open
and the amp's module cover almost off
in
order to
get a s igni f icant amount of in -
gress in to the
cable i t s e l f , ampl i f ie r
covers
must
not
be ru led out .
Tap
pla tes , however, espec ia l ly when
drops are connected to them, are cruc ia l
poin ts of shield ing breakdown in the feeder
system The r f
shield
around the edge of a
t ap pla te can only work
well
when making a
good
pressure connect ion to the
housing
pla t e .
Loose t ap pla tes , with contaminants
between
the
pla te
and
the
housing wil l
cause a discon t inu i ty
to
occur
between
the
drop shield and the sh ie ld of the feeder
cab le ,
al lowing
ingress
in to
the
drop
and
to a l es ser extent , in to the feeder i t s e l f .
Even when the tap pla te i s t i gh t , corro-
sion due to moisture i s frequent ly a prob
lem as
the
r f gasket i s located a t the
point of maximum water accumulation as a
t ap
hangs
on
the
feeder l ine . A very th in
l ayer of waterproof ing grease wil l
aid in
prevent ing t h i s
problem. A
word
of
caut ion
needed here, over
zealous
t igh ten ing of t ap
pla te screws wil l lead to s t r iped housing
th reads , clutch
type
torque drivers are
recommended.
Studies have indicated
t ha t
an
unterminated tap
port
wil l provide
grea ter
RF
i sola t ion
than
a
terminated
tap por t . [2 ]
The te rminator i t s e l f
i s
a connector
and
hence subject
to the inevi table natural
corrosion
of
the
connector
threads. As the
outer
she l l
loses
i t s
ground connection,
the
te rminator
becomes a
stub
antenna and
hence a point of ingress in to
the
cable
system. The port to
port
i sola t ion of a two
way
s p l i t t e r
i s normally about 25 dB and a
-10
d -20
Visib le Beats
B
-30
D
0
-40
w
N
-50
-60
Inv i s i b l e Beats
0
1
2 3
5
6
mhz.
from
band edge
to band
edge
CARRIER INTERFERENCE RATIO
[4)
985
NCT
Technical
Papers 13
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s tub antenna about
an
inch
and
a
half
long
wil l pick up as much as 0
dBmv of
s ignal
near a high band
VHF TV
s t a t ion . A s t rong
to moderate
in ter ference
i s
observed
on
drops connected
to adjacent tap por ts .
Por t to
por t t ransference i s
also
responsible for mysterious in terference
problems
when
temporary
disconnections of
a
neighbors
drops are made. In one
case,
when
a
neighbor
disconnected the cable drop from
his
VCR, the
center
conductor
of the drop
would contact the
metal l ic
case . The f ie ld
s t r eng th of the
local TV
s ta t ions were
about
35 dBmv
in the area . This
resul ted
in +10 dBmv
of
in terference
being back-fed
in to
the
other
customers otherwise
perfect
drop. And, due to the di rec t iona l
coupler
charac te r i s t ics of
the tap
i t s e l f ,
the r es t
of the system
was
uneffected. This happened
every night for a few hours a t a t ime,
grea t ly
reducing
the
mental s t ab i l i t y of
the serv ice personal . The
solut ion
was
simple when the reason was discovered. The
customer with
the
VCR
was
given
an
A/B
switch so he
could
switch
inputs
and still
maintain
system
i n t egr i ty .
SUBSCRIBER C USED INGRESS
Perhaps
the
most perplexing cause of
ingress problems
i s
subscr iber
owned equip
ment. I t
i s
the one
par t
of the
system over
which the
cable operator genera l ly has the
l eas t cont ro l . As the consumer becomes
more
and more
video
ac t ive , the occurrence of
ingress problems due to consumer re la ted
equipment
i s sure
to
increase .
The fac t
t ha t subscr ibers wil l loosen drops
by
simply moving converters
as wel l as
by
connect ing the i r own equipment wil l be a
sure ingress
problem
from
now
on
in
any
metropol i tan area . In a typ ica l
case
an
otherwise
perfec t ly
good
VCR
and TV se t
wi l l
be
connected with
factory
included
wiring with easy to
use push on
f i t t i ngs .
The
customer i n s t a l l s the
source
and
wi l l
suf fer the
ef fec ts
of ingress .
I ns t a l l a t i on
of qual i ty
wiring
with proper
connect ions
wil l correc t the problem. While the so lu-
t ion i s easy, it i s a serv ice c a l l never
the l ess .
Another
common occurrence
with
customer ins ta l led
equipment
leading
to
an
ingress problem
i s
the
video game
or
compu
t e r
switch
normally
supplied
with
such
equipment
When
i ns t a l l ed
before
a
cable
ready
TV or VCR , or
indeed i ns t a l l ed any
where
but
af te r a conver ter ,
they
wil l
without
doubt permit ingress . When
video
games or computers must be
connected
to
cable
ready
TV se ts it i s necessary to
i n s t a l l a well
shielded,
se l f terminat ing
C TV grade A/B switch
in
place of
the
cus
tomers switch. Using
these
and s tandard
adaptors
avai lab le a t
loca l e l ec t ron ic
dea lers , a connect ion
can
be made
tha t
wi l l
provide
a proper amount of
i so l a t ion .
In
14 1985 NCT Technical Papers
some extreme cases
it
might
be
necessary to
replace
game
switches
located af te r the
converter
i f
a local t ransmit ter i s opera t -
ing in a
channel
adjacent to the converter
output
channel . The lower
sideband
of a
broadcast
s t a t ion
extends well in to the
lower channel . Suppressed
proper ly ,
it
nevertheless has enough s t r eng th to over
come
the
poor
sh ie ld ing
of
these
inexpen
s ive,
manufacturer supplied switches.
Of
a l l the ailments created
by
subscr iber
equipment,
problems
due
to the
poor shielding of some cable
ready
TV se ts
are the only t ru ly incurable
ones.
The
amount of ingress which i s
introduced
by
cable ready
t e l ev i s ions var ies grea t ly from
model to model and no brand
can
be sa id to
be
bes t .
There
are many models of cable
ready
t e l ev i s ions tha t exhibi t excel lent
shielding while other models
of the
same
brand
don t .
I t i s also found
t ha t
problems
with
cable
ready se ts
depend
upon
the
loca t ion
of the
se t in
the
room , as
well
as
the
s t r eng th
of
the
in te r fe r ing local
t ransmi t t e r . Cable
ready
se ts are sub jec t
to the same condi t ions as the r e s t of the
cable plant and i f there i s a potent ia l for
ingress ,
it
wi l l
en ter the system through
the poorly shie lded t e l ev i s ion jus t as easy
as it would any other par t of the
di s t r ibu-
t ion
system.
I t
wil l
sometimes
be
neces
sary
to
t e l l
a
customer there
i s
nothing
t ha t
can
be done, tha t
the
se t i t s e l f i s
the problem.
Cable
converters have
the c r i t i c a l
por t ions
of
the s ignal path
ins ide
a t igh t
metal l ic box, which cons t i tu tes a
good
sh ie ld . A typ ica l t e l ev i s ion wil l
have
the
shielding open on one
s ide, or
have a c i r -
cui t
card pass through
it
with
the
shield
only
spot soldered to
the
c i r c u i t
card,
leaving gaps enough to
allow more ingress
than several loose f i t t i ngs ,
plenty enough
to cause problems.
The se t wil l
otherwise
work perfec t ly , on channels other
than
the
ones
occupied
by any communications band or
loca l t e l ev i s ion
s t a t ion .
The bes t opt ion
the cable operator has in order to correc t
a ingress problem
di r ec t ly
ins ide
the
t e l e -
vision se t , i s to place a converter before
the
cable
ready se t , al lowing
the
se t
to
operate on a c lear channel . This sometimes
upsets the customer who has paid
ex t ra for
the
cable
readiness ,
and of ten
complicates
the
ins t ruc t ions of
using
various
remotes
in
order
to
gain
sa t i s fac tory
operat ion.
Aside from placing a converter before
a cable
ready se t ,
the
only other prac t ica l
so lu t ion to th i s problem i s to
ra i se
the
s ignal
levels
in to the se t to a point where
the level
of
the in terference
becomes
in -
s igni f icant . I f
the cable
ready TV se t has
a shielding fac to r of 40 dB for
example,
about the average for the a
problem
causing
se t , and
the
f i e ld s t r eng th
of
a
local
t ransmi t t e r i s 0 dBmv,
an input
level of
-
7/26/2019 1985 Ingress Sources and Solutions
5/7
10
d rnv to
the se t wil l a provide
brute
force solut ion.
Even
th i s solut ion proves
impract ical
i f
the
se t i s loca ted only
several
miles from local TV t ransmi t t e rs ,
the input l eve l s needed to
mask
the
in ter ference
wil l
be more than
the h ighest
level the
cable
system
i s allowed to
opera te a t by law.
The degree of
the
i sola t ion provided
by cable ready se t s can eas i ly be deter
mined
by
reading the
s t rength
of
the
known
source of
in te r fe rence
di rec t ly
out
of
the
back of a
TV
se t
and comparing
th i s to
dipole readings
a t the
same loca t ion . I f
one
posi t ions
the antenna for maximum
rece ive l evels , and
does the
same with a
t e l ev i s ion rece iver ,
with
the TV se t o f f to
prevent reading RF generated by the se t
i t s e l f , a
di r ec t
est imate of the sh ie ld ing
fac tor of
the
se t
may be
made.
Cable ready
VCRs
present another
poten t i a l
source
of
ingress ,
fo r tunate ly
though they do not general ly appear to be
as
grea t of a problem as cable ready TVs.
Although
poor
VCR shielding has ocasional ly
been the
source of
ingress , the
cables
used
and other problems
with
connections are
much more bothersome
than
the VCRs
themselves.
A poorly shie lded cable ready se t
can
also present a problem to other se t s i f the
f ie ld
s t rength
of a
local
tv
t ransmit ter
i s
moderately s trong. I t i s
qui te
possible
for
a se t with 30 dB
or
l es s shield ing fac to r ,
about the
worst encountered loca ted
n
a
typical
urban environment
to
pass
ingress
a t
-10
d rnv
up a
drop
to a sp l i t t e r . There
t
wil l back-feed
down the o ther drop
leg
a t
a l evel of -35 d rnv of in terference
versus
5 d rnv of s igna l , more
than
enough
to
be
not iceab le .
FM hook-ups
create
two types
of
ingress re la ted problems.
The
f i r s t i s with
in ter ference to FM serv ices provided by
the
cable system and the second with
address
able
converters
when connected along with a
FM hookup.
The
typ ica l FM tuner wil l
work
perfec t ly
well
with l i t t l e or no
antenna
connected to
t n
an urban environment.
While the
sh ie ld ing
of the
tuner
i s gener
a l ly
very poor
t
i s
possible
to
del iver
qual i ty FM s ignals thru a cable system. I f
the s ignal from a
local
FM s ta t ion i s
del ivered unshif ted n frequency
FM
tuners
wil l general ly be unable to
d i s t in
guish
between
the two
carr iers ,
n
the
cable and off the a i r .
I f the cable opera
tor
i s careful to avoid using
FM channels
within 400
khz
of
local
FM
t ransmi t t e rs for
other ,
imported or operator generated
s ignals , problems may general ly be
avoided.
The
poss ib i l i t y
of ingress in to the
cable
system
of s ignals which could
i n t e r
f ere
with
data
car r ie r s
used by addressable
converters and horne secur i ty
systems
i s
grea t ly magnified by FM tuner
hook-ups.
With FM
t ransmit ters
often as powerful
as
TV
s ta t ions ,
combined
with a t une r s
cha r ac t e r i s t i c a l ly
poor
sh ie ld ing ,
back
feed
of
unwanted
s ignals
in to
the
cable
can
crea te serious problems. As the data car
r i e r i s
al ready
well below the operat ing
level
of a cable video car r ie r ,
the
s ide
bands of a loca l
FM
s ta t ion
can
eas i ly
d i s to r t
a data car r ie r .
Data t ransmis
s ions ,
when dis to r ted by
in te r fe rence ,
may
contain er ro rs
resu l t ing n random charac
t e r s being received
between t ransmiss ions
or worse yet ,
n
severe cases the
corrup
t ion of
a desired t ransmiss ion. To avoid
problems with data communications
n
the FM
band
you
may use a di rec t iona l coupler
between FM tap-off
device and
the
ef fec ted
equipment to prevent backfeed. The use
of
r egu lar sp l i t t e r s n place
of
FM taps
should be avoided.
TROU LESHOOTING
THE INGRESS
PRO LEM
As
the
number of
urban
homes
wired
for
cable increases
and
as older drops aproach
l i f e
expectancy
the
amount of
service
ca l l s
re la ted
to
ingress i s
bound
to
n -
crease a l so .
The
problem i s compounded by
the wider bandwidths of
modern
cable sys
tems as
they
encounter more communications
bands and even UHF t e levis ion. Increases n
the number of
communications t ransmi t t e rs
n
the spectrum and the
huge
impact
of
consumer equipment wil l lead to the
need
for
quick and
sure
means of detec t ing and
cor rec t ing
ingress
problems.
Even
today
t
i s
of ten
the
case t ha t
a cable system
n
a
urban area wil l f ind 10 to 15 and some
t imes
as much as 30
percent of i t s
service
manpower
spent finding and cor rec t ing
n -
gress re la ted problems. Routine procedures
must be
taught
to serv ice employees so
tha t
they
can
handle
the
problems ef f ic ien t ly .
I t i s often d i f f i cu l t to
dis t inguish
between f a in t in terference
and
the
symptoms
of amplif ier
dis tor t ion. However
i f the
in te r fe rence i s
st rong
enough to cause a
heavy
beat ,
t may general ly be assumed
tha t i f a amplif ier was
ernittirng
a spurious
product s trong enough to be c lear ly vis ib le
t
would
probabi l i ty
have
other
by-products
on
adjacent channels.
I f you
have
a problem
with beats
on
one channel only
t wil l
most probab i l i ty be ingress
re la ted,
a
quick
check of other
channels wil l provide
an answer. Also high s ignal levels a t a
customers se t wil l
l i ke ly
indicate
ampli
f i e r dis tor t ions as a cause of
beats ,
for
as the l evels go
up the probab i l i ty
of
di s to r t i on
increases
and problems from
ingress decrease .
985 NCTA Technical Papers 15
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7/26/2019 1985 Ingress Sources and Solutions
6/7
In an area where
t
i s
expected tha t
there
wil l
be a
suf f i c i en t
number
of
in -
gress cal ls to warrant t an area where
there
are
many local
TV s ta t ions and
e tc . ,
t
may prove
very prac t ica l to leave
the
channel of
the
s ta t ion most l ike ly to leak
in to
the system empty.
The
empty
channel
wil l
provide
a
convenient
way to
determine
the
degree
of
system
in teg r i ty .
Measure
ments
of
a local
s t a t ion
leaking in to
the
cable on a channel not occupied
by any
cable channel, when compared to the
levels
of
an
adjacent cable channel,
wil l
provide
a
good
indicat ion
of the
signal to
in te r -
ference r a t io between other local
TV
s ta t ions and cable s igna l s . I f the t e l e -
vis ion
se t or
converter
i s tuned to
the
channel
of
the
local
s t a t ion
not
on
the
cable
and
a noisy but
steady
pic tu re
i s
present , a
fa int
ghost or
beat
on another
channel wil l
almost cer ta inly be caused
by
ingress .
The
point
a t
which
the ingress f i r s t
enters a
feeder
system can also be measured
quickly
by
making
s igna l / in te r fe rence
read
ings
a t
taps , via the unused channel
method. Customers
wil l st ll
have
service
while troubleshooting i s being performed
and th i s method wil l
physically
dis tu rb the
system as l i t t l e as
possible .
This i s
desi rable
as
a
small
movement
of
a
s l igh t ly
loose connector
may
be enough to
correct
the problem
for the present ,
but t wi l l
l i ke ly
soon s t a r t misbehaving again. Also
any
dis turbance to
per fec t ly good port ions
of
the
plant
wil l
often
lead
to them
becoming l es s t i gh t , resul t ing in
more
problems.
I f
t i s necessary to maintain a
s ignal to interference
r a t io
of
50
dB or
more
between a local
t ransmi t t e r and
a
channel
used
in
a cable
system, then
the
levels on the unoccupied
cable channel
of
the local TV t ransmi t t e r
should
be -40 to
-45 dBmv
a t
the input to the
se t
or
conver
t e r . This i s
near
or
below
the lower l imi t
a normal f i e ld s t rength meter can read, so
any deflect ion of the meter scale with
at tenuat ion
ful ly
down
i s undesirable. I f
the video buzz
can
not
be
heard a t a l l , or
i f
system
generated
beats
a t the
extreme
range
of
the
meters sens i t iv i ty
are
heard
ins tead, then the service technician can be
assured the ingress problem i s not coming
from the upstream
portion
of the drop or
feeder. The t e s t becomes more
val id
as
cable
s ignal
levels
increase,
and, as
an
aside,
provide rough measurements of
system
noise.
Another
prac t ica l
t roubleshoot ing
prac t i ce
i s
to
disconnect
the sect ion of a
suspected bad drop or feeder leg and
measure the levels
of
the local t ransmit-
t e r s
di rec t ly
out of the downstream
leg.
This permits
a
di rec t
comparison
of the
s ignal
to
interference
r a t io to be
made
a t
16 1985
NCT Technical Papers
any frequency
desired when
the l eve l s of
the cable s ignals a t
tha t
point
are
known.
By disconnect ing di f fe rent sect ions of
a
drop
a t
a s p l i t t e r
and
measuring
ingress
levels
from each,
a
fas t and sure
t roub le-
shooting decis ion
can be
made. Readings
from a disconnected
sect ion
of
drop,
terminated
a t the
other end,
wil l
indicate
i f
a
drop
i s
good
or
i f
t
must be
serviced
or
possibly replaced. A drop cable
in
service should typica l ly be capable of
70
dB or more
i sola t ion
so any
detectable
levels ins ide
the drop would ind ica te the
necess i ty
for
serv ice .
s the drop i s connected to the f ie ld
s t rength meter,
f i r s t
inser t
only the cen
t e r
conductor
of
the
drop in
the
meter,
then tune to the
source
of off a i r i n t e r -
ference
and
read the level often close
to
or more than what i s
read with
a dipole a t
the same loca t ion . I f
you then
t ighten the
drop
on the meter and
turn
the at tenuator
a l l the
way down a
good
indicat ion
of drop
in tegr i ty
can be made. I f
an amplif ier
with
a
gain of
20 dB
i s
placed
between
a
good,
terminated
long
length
of
drop and
a
meter ,
in
an
area where the f ie ld s t rength
i s
40 dB or more from off a i r t r ansmi t te r s ,
t i s
jus t
barely possible to de tec t the
local
t ransmit ter above the noise f loor
indicat ing a 90 dB
shield or bet te r
for
the
drop
and the t e s t equipment
too .
The only
other
prac t ica l
way to
detect
the source of ingress
i s
to
make
use
of
the various sens i t ive
radiat ion detec
tors curren t ly avai lable from several
manu
fac tu rers .
Jus t as ingress
gets in cable ,
signals leak out and can be detected.
With
th i s equipment, and l i t t l e or no t r a in ing
beforehand,
a technician
wil l
almost
be
able to walk r ight up to
a
defect causing
ingress . Sensit ive
equipment capable of
detecting
rad ia t ion
levels
15 to
20
dB
below the FCC
rad ia t ion
threshold of
34
dBmv
i s required.
A shield factor of 60 dB
or
more
should be
maintained
in drops
when
both the f i e ld s t rength of the local
t ransmit ter
and
the s ignal level in the
drop are 5dBmv.
Standard dipoles and meters
are
d i f f i cu l t to
handle as
t roubleshoot ing
a ids , and very
often unable to de tec t fa in t
rad ia t ion from points which are , neverthe
less permiting noticeable ingress i n t e r -
ference.
In
areas of
st rong
radio i n t e r -
ference,
even
the
more
sens i t ive
equipment
i s
sometimes
incapable of
finding
fa in t
rad ia t ion from leaks permit t ing severe
ingress for example in
drops
when
cable
s ignals
close
to 0 dBmv and
the
local
t ransmi t t e rs are above 20 dBmv a t
t ha t
point .
Using
equipment
accepted
as being many
t imes
more sens i t ive than
what
i s required
to de tec t
the
FCC egress threshold , and
-
7/26/2019 1985 Ingress Sources and Solutions
7/7
capable of being ca l ib ra ted so, wi l l also
permit the t echn ic ian to determine i f a
leak
legal ly
needs to be reported or not .
Typical ly leaks
rad ia te
-40
dBmv
or
less in
about
75
to 80
of the service
problems,
while
leaks
above
-25 dBmv are
found
l es s
than 10% of
the t ime.
This
severe
a leak
wil l normally
af fec t
customers severely
and
therefore
wi l l
also
be
correc ted
very
quickly. I t i s of ten
the case ,
though,
t ha t
the s t ronges t
poin t s of
egress are points
where the l eve l in the feeder i s the g rea t -
es t hence the points where the grea tes t
immunity from ingress can be expected. So
by no
means
does a lack
of
ingress
mean
tha t a system i s t igh t and to ta l ly
within
it s
requirements .
The
repa i r act ions
taken
to correc t
ingress problems due to cable
f au l t s are
st ra ight foward
and d i rec t t igh ten it,
sp l i ce it,
or replace it. Detect ion of
~ n g r s s problems wil l
require
t ra in ing of
personnel as
to the
way to quickly d i s t i n -
guish
between
ingress
re la ted
beats
and
those of ampl i f ie r di s to r t i on .
Service
personnel
should be
able to
determine i f
customer
equipment i s a t
fau l t and
how to
bypass
these
problems.
Addit ional
t ra in ing
i s
required
as to how to use s ignal level
meters
as
a
means
of loca t ing a point of
ingress .
Specia l ized
equipment wil l make
the technician more ef f ic ien t and help
assure
FCC
compliance. The rout ine soon
wil l es tab l i sh i t s e l f i f the system
i s
suscep t ib le to
ingress problems.
NEW BUILDS IN HEAVY INGRESS
AREAS
Before
any
new build i s f i r s t turned
on, the cable opera tor should determine the
l evels
of
a l l
loca l o f f
a i r
t r ansmi t te r s
a t
var ious
locat ions throughout
the
bu i ld . I t
wil l then be possible to assign charac ter
genera tors and other
s imi la r
programing
to
channels
l i ke ly
to
have in ter ference
from
ingress . Phase
locking and
sync
locking and
45 dB of system immunity i s good
enough
for
a charac ter genera tor only, a l l tha t can
be expected i f cable
ready
TV se ts are
to
be
connected.
The
same
channel wil l be
more
usable under
the
same condi t ions in t o t a l
conver ter
build .
I t might
also
be des i rab le
to require
measurements
of
the ingress
l eve l
present
in
each
feeder
leg
as it
i s turned on.
With
the input to a sec t ion te rminated, a read
ing should be
made a t
the end of
each
leg.
Accept
no
ingress
whatsoever
and require
checks to
be made with
a spectrum
anay l izer
photo 's
inc luded.
Plan to
put
a s ign i f i can t
amount of
energy in to ingress re la ted maintenance in
an urban
bui ld
un t i l
you have
t ime to cor-
r ec t a l l cons t ruc t ion and
new drop defec t s .
Even
the best of construct ion methods and
workmanship wil l show some minor
f laws,
and
a
flaw can be very
minor
and
still be
a
s igni f icant point of ingress . But with
a
proper ly equipped
s t a f f
of t echn ic ians the
ingress
problem
can
be overcome
in
a
rou t ine fashion, with
luck.
ACKNOWLEDGEMENTS:
1: Belden Corp,CATV Coaxial Cable Catalog
#ELl0-79,
Oct.
1979, pp 15-18.
2: Reg
James,
Comcast Corp.Staff
Engineer
From
a
r epor t
July
1982
3: Based on
FCC
Rules
and Regulat ions ,Vol
I I I
p a r t
73,
ppl89-191,
1972
From Reference Data for Radio
Engineers ;
Howard w. Sams & Co., 1979, p 30-12
4:
Based
on
Je ro ld
CATV Reference
Guide
#RD-14, Apri l
1983,
p36
Specia l thanks to a l l
the
serv ice
technicians who helped gather the
data and
who
came up
with some
good
f ixes
too.
985
NCTA Technical
Papers 17