How Telephone Works

7/28/2019 How Telephone Works

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Telephone Interference

How to prevent it

How to eliminate itby

Dave LeVasseur, NDL

Dakota Division Convention

August 6th, 1999

(This presentation may be downloaded from www.dailypost.com/~davel)


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6-Aug-1999 TPI Presentation 3

Definitions

On-hook: telephone in idle state; ready to

make or take a call

off-hook: in the process of making or taking a

call

Central Office (CO): where the other end of the

wires go after they leave your house

Local loop: the wires between the CO and your

telephone


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Definitions

TPI: TelePhone Interference

Hybrid: device that separates (isolates)

inbound and outbound telephone signals from

one another. Also known as 2-wire/4-wire

converter.

Tip and Ring: the names given to the wires inthe local loop. Named for their connection to

plugs used in older central offices.


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Definitions

Differential mode signal:

A signal which appears as a voltage difference

between a pair of conductors but may have noreference to earth ground.

V ??


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Definitions

Common mode signal:

A signal which appears as a voltage on a pair of

conductors having the same phase and polarity oneach conductor with respect to ground.

? VV


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How Telephones work

Typical telephone network

CentralOffice

48V battery~100Vrms

ringing signal


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How Telephones work

Typical residential installation methods

Straight cable runs:

Service

EntryProtector

block

Jack

Jack

Jack


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How Telephones work

Typical residential installation methods

Loop-series wiring:

Service

EntryProtector

block

Jack Jack Jack


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How Telephones work

All telephones have three separate

subassemblies:

Speech Network

Dialing Mechanism

Ringer (bell)


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How Telephones work

Speech Network contains:

Microphone (transmitter)

Earphone or speaker (receiver)

Hybrid (2-wire/4-wire converter)


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How telephones work

All communication occurs over two

wires. This requires a hybrid to separate

the incoming and outgoing signals.

The hybrid (2-wire to 4-wire converter)

may be implemented using

transformers or operational amplifiers.


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How telephones work

Transformer hybrid:

Image courtesy of Midcom, Inc.


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How telephones work

Op-amp hybrid:

600(Balance

network)

10k10k

Tx

Rx

to

telephone

line


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How telephones work

Balance

Network

Tx

Rx

2-wire port

4-wire ports

Hybrid


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How telephones work

Balance

Network

Tx

Rx Hybrid


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How telephones work

Balance

Network

Tx

Rx Hybrid

Sidetone


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How telephones work

Most modern telephones rely on

electronic rather than magnetic

components (diodes and op-amps insteadof transformers and inductors) making

them prone to interference.


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How telephones work

Typical telephone interface circuit

Image courtesy of National Semiconductor, AN-397


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Common-mode interference

Radio Frequency Interference is most

likely the result of a strong common-mode

signal becoming converted to a weaker butperceptible differential signal.

Telephone systems use twisted wires to

assure that any interfering signals are

balanced on each wire.


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Common-mode RF becomes a differential signal by

becoming unbalanced:

V

Capacitive effects count, too.


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V

Common-mode RF becomes a differential signal

through rectification:


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We need a special impedance: one that opposes common-

mode signals,

VV

Z

Z


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V

We need a special impedance: one that opposes common-

mode signals, but doesnt impair differential signals.

Z

Z


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V

Solution: The Common-Code Choke


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common modecurrent

Magnetic flux caused by common mode current is

accumulated, producing an opposing impedance

Magnetic flux caused by

differential currents cancel

each other; impedance is not

produced.

differential modecurrent


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Image courtesy of ARRL (RFI handbook)


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This is NOT a common-mode choke:

Image courtesy of ARRL (RFI handbook)


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V

Add a couple of capacitors to reduce high-frequency

differential noise:

(Typical values range between 47pF to perhaps 100pF)


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Commercial Filters

Images courtesy of K-Com filters


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Commercial Filters

Images courtesy of K-Com filters


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Step-by-step process

Make sure the problem is truly due to RFI

Verify that the interference is present onlywhen youre transmitting.

Run the transmitter output into a wellshielded dummy load - if the problem

persists, the powerwiring may be the

culprit.


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Note that long lengths of telephone wiring can

act as tuned antennas...

Telephone

cable


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Step 1: Check the grounding points

Verify that a ground connection of goodintegrity is available at the point where thetelephone wires enter the premises.

If it seems the installation does not include a valid

ground connection, contact the telephone company

to have it repaired or installed if needed.


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Step 1: Check the grounding points

Verify that one of the wires going to eachjack contains a connection to earth ground.(and if they dont, make the appropriateconnections so they do)

Ground all unused wires in the cable, just forgood measure.


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Step 2: Install modular filters

Install modular filters on the telephone(s)exhibiting interference.

Check each telephone for interference afterinstalling a filter.


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Step 3: If RFI persists, break up the

telephone cabling by using in-line filters.


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Make sure youve kept a good groundwire connection throughout.


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Step 4: If RFI still persists, one telephone

may be contributingto your RFI problem

Disconnect all telephones* and reconnect them oneat a time until the bad telephone is found. Replace

the telephone or improve its ability to withstand RFI

using a more aggressive filtering technique.


*Dont forget to disconnect fax machines,

alarm systems and set-top boxes!


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Building your own filters

Obtain a toroidal core made out of ferrite (not

powdered iron) having a permeability between 250

and 1500. The best type of ferrite is nickel-zinc (NiZn)since this material works well at RF.

The toroid should be large enough to hold at least 20

turns of both wires. You can use a larger toroid to pass

the plug end of a telephone cord, but for an equivalentcore height youll get more inductance per turn with asmaller diameter toroid.


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Wind the turns bifilar, that is, both wires kepttogether (twist them if you like). Spiral them both in

the same direction around the core.

To wall jack

To telephone

(keep wires short)


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Add a couple of47pF to 100 pF

1KV capacitors


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Image courtesy KY-Filters


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Install filters at

the positions

marked X

shown here

Service

Entry

Protector

block

Jack

Jack

Jack

ServiceEntry

Protectorblock

Jack Jack Jack

x x

x x

x x

x x x

x

x

x

xxx


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Sources

Commercial Filters

K-Com

P.O. Box 82

Randolph, OH 44265

Tel: (330) 325-2110

Fax: (330) 325-2525

[email protected]

www.k-comfilters.com

(Available at Burghardt Amateur Center

in Watertown, SD)


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Sources, cont.

Ferrite toroids:

Fair-Rite Corporation

P.O. Box J1 Commercial Row

Walkill, NY 12589

1-800-836-0427

[email protected]

www.fair-rite.com

Amidon Inductive Components

250 Briggs AvenueCosta Mesa, CA 92626

1-800-898-1883

[email protected]

www.amidon-inductive.com


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References

QST, May, 1991, Basic Steps Toward Eliminating

Telephone RFI by Pete Krieger, WA8KZH, pages 22-25.

The ARRL RFI Book, ARRL publication No. 235, chapter 9,

Telephone RFI, 16 pages (First edition).

Ham Radiomagazine, September 1985, Understanding

Telephones by Julian Macassey, N6ARE. Also available at:

http://www.mmainteractive.com/electronics/phone/how.htm

K-Com web site: http://www.k-comfilters.com

Telecom Digest Archives: http://mirror.lcs.mit.edu/telecom-

archives


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References, cont.

Building your own filters:

John Browne, KI6KY

http://ky-filters.com/

FCC Telephone Interference Bulletin CIB-10August 1995

www.fcc.gov/cib/Publications/phone.html

FCC Telephone Interference Survey

http://www.arrl.org/tis/info/rfitelfcc.html


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Thanks for your attention !

Questions ?

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