1985 ingress sources and solutions

7/26/2019 1985 Ingress Sources and Solutions

1/7

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


2/7

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]


3/7

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


4/7

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


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


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


7/7

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

1985 ingress sources and solutions

Documents