Telephone Interference How to prevent it How to eliminate it by Dave LeVasseur, NØDL Dakota Division Convention August 6th, 1999 (This presentation may.

Telephone Telephone InterferenceInterference

How to prevent itHow to prevent it

How to eliminate itHow to eliminate itbyby

Dave LeVasseur, NØDLDave LeVasseur, NØDLDakota Division ConventionDakota Division Convention

August 6th, 1999August 6th, 1999

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

6-Aug-1999 TPI Presentation 2

Seminar contentsSeminar contents

DefinitionsDefinitions Overview: how telephones workOverview: how telephones work Common mode interferenceCommon mode interference Commercial filtersCommercial filters Step-by-step procedure to get rid of TPIStep-by-step procedure to get rid of TPI Building your own filtersBuilding your own filters ReferencesReferences Wrap upWrap up


DefinitionsDefinitions

On-hook: telephone in idle state; ready On-hook: telephone in idle state; ready to make or take a callto make or take a call

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

Central Office (CO): where the other end Central Office (CO): where the other end of the wires go after they leave your of the wires go after they leave your househouse

Local loop: the wires between the CO Local loop: the wires between the CO and your telephoneand your telephone



TPI: TelePhone InterferenceTPI: TelePhone Interference

Hybrid: device that separates (isolates) Hybrid: device that separates (isolates) inbound and outbound telephone signals inbound and outbound telephone signals from one another. Also known as from one another. Also known as 2-wire/4-wire converter.2-wire/4-wire converter.

Tip and Ring: the names given to the Tip and Ring: the names given to the wires in the local loop. Named for their wires in the local loop. Named for their connection to plugs used in older central connection to plugs used in older central offices.offices.



Differential mode signal:Differential mode signal:A signal which appears as a voltage A signal which appears as a voltage difference between a pair of conductors but difference between a pair of conductors but may have no reference to earth ground.may have no reference to earth ground.

V ??



Common mode signal:Common mode signal:A signal which appears as a voltage on a pair of A signal which appears as a voltage on a pair of conductors having the same phase and polarity conductors having the same phase and polarity on each conductor with respect to ground.on each conductor with respect to ground.

? VV


How Telephones workHow Telephones work

Typical telephone networkTypical telephone network

Central

Office

48V battery~100Vrms

ringing signal



Typical residential installation Typical residential installation methodsmethods

Straight cable runs:Straight cable runs:

Service Entry

Protector block

Jack

Jack

Jack



Typical residential installation Typical residential installation methodsmethods

Loop-series wiring:Loop-series wiring:

Service Entry

Protector block

Jack Jack Jack



All telephones have three separate All telephones have three separate subassemblies:subassemblies:• Speech NetworkSpeech Network• Dialing MechanismDialing Mechanism• Ringer (bell)Ringer (bell)



Speech Network contains:Speech Network contains:• Microphone (transmitter)Microphone (transmitter)• Earphone or speaker (receiver)Earphone or speaker (receiver)• Hybrid (2-wire/4-wire converter)Hybrid (2-wire/4-wire converter)


How telephones workHow telephones work

All communication occurs over All communication occurs over two wires. This requires a hybrid two wires. This requires a hybrid to separate the incoming and to separate the incoming and outgoing signals.outgoing signals.

The hybrid (2-wire to 4-wire The hybrid (2-wire to 4-wire converter) may be implemented converter) may be implemented using transformers or using transformers or operational amplifiers.operational amplifiers.



Transformer hybrid:Transformer hybrid:

Image courtesy of Midcom, Inc.



Op-amp hybrid:Op-amp hybrid:

600(Balancenetwork)

10k10k

Tx

Rx

to telephone

line



Balance

Network

Tx

Rx

2-wire port

4-wire ports

Hybrid



Balance

Network

Tx

Rx Hybrid



Balance

Network

Tx

Rx Hybrid

Sidetone



Most modern telephones rely on Most modern telephones rely on electronic rather than magnetic electronic rather than magnetic components (diodes and op-amps components (diodes and op-amps instead of transformers and instead of transformers and inductors) making them prone to inductors) making them prone to interference.interference.



Typical telephone interface circuitImage courtesy of National Semiconductor, AN-397


Common-mode Common-mode interferenceinterference

Radio Frequency Interference is Radio Frequency Interference is most likely the result of a strong most likely the result of a strong common-mode signal becoming common-mode signal becoming converted to a weaker but converted to a weaker but perceptible differential signal.perceptible differential signal.

Telephone systems use twisted Telephone systems use twisted wires to assure that any interfering wires to assure that any interfering signals are balanced on each wire.signals are balanced on each wire.



Common-mode RF becomes a differential signal by Common-mode RF becomes a differential signal by becoming unbalanced:becoming unbalanced:

V

Capacitive effects count, too.



V

Common-mode RF becomes a differential signal through Common-mode RF becomes a differential signal through rectification:rectification:



We need a special impedance: one that opposes common-We need a special impedance: one that opposes common-mode signals,mode signals,

VV

Z

Z



V

We need a special impedance: one that opposes common-We need a special impedance: one that opposes common-mode signals, but doesn’t impair differential signals.mode signals, but doesn’t impair differential signals.

Z

Z



V

Solution: Solution: The Common-Code ChokeThe Common-Code Choke



common mode current

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 mode current



Image courtesy of ARRL (RFI handbook)



This is NOT a common-mode choke:This is NOT a common-mode choke:

Image courtesy of ARRL (RFI handbook)



V

Add a couple of capacitors to reduce high-frequency differential noise:

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


Commercial FiltersCommercial Filters

Images courtesy of K-Com filters



Images courtesy of K-Com filters


Step-by-step processStep-by-step process

Make sure the problem is truly due Make sure the problem is truly due to RFIto RFI

• Verify that the interference is present Verify that the interference is present only when you’re transmitting.only when you’re transmitting.

• Run the transmitter output into a well Run the transmitter output into a well shielded dummy load - if the problem shielded dummy load - if the problem persists, the persists, the powerpower wiring may be wiring may be the culprit.the culprit.



Note that long lengths of telephone Note that long lengths of telephone wiring can act as tuned antennas...wiring can act as tuned antennas...

Telephone cable



Step 1: Check the grounding Step 1: Check the grounding pointspoints

• Verify that a ground connection of Verify that a ground connection of good integrity is available at the point good integrity is available at the point where the telephone wires enter the where the telephone wires enter the premises. premises.

If it seems the installation does not include If it seems the installation does not include a valid ground connection, contact the a valid ground connection, contact the telephone company to have it repaired or telephone company to have it repaired or installed if needed.installed if needed.



Step 1: Check the grounding pointsStep 1: Check the grounding points

• Verify that one of the wires going to Verify that one of the wires going to each jack contains a connection to each jack contains a connection to earth ground. (and if they don’t, make earth ground. (and if they don’t, make the appropriate connections so they do)the appropriate connections so they do)

• Ground all unused wires in the cable, Ground all unused wires in the cable, just for good measure.just for good measure.



Step 2: Install modular filtersStep 2: Install modular filters

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

• Check each telephone for Check each telephone for interference after installing a filter.interference after installing a filter.



Step 3: If RFI persists, break up the Step 3: If RFI persists, break up the telephone cabling by using in-line filters.telephone cabling by using in-line filters.



Make sure you’ve kept a good Make sure you’ve kept a good ground wire connection throughout.ground wire connection throughout.


Step 4: If RFI still persists, one telephone may be Step 4: If RFI still persists, one telephone may be contributingcontributing to your RFI problem to your RFI problem• Disconnect all telephones* and reconnect them one at a time until the bad telephone is found. Replace the telephone Disconnect all telephones* and reconnect them one at a time until the bad telephone is found. Replace the telephone

or improve its ability to withstand RFI using a more aggressive filtering technique.or improve its ability to withstand RFI using a more aggressive filtering technique.


*Don’t forget to disconnect fax machines, alarm systems and set-top boxes!


Building your own filtersBuilding 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 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.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 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 equivalent core height you’ll get more inductance per turn with a smaller diameter toroid.cord, but for an equivalent core height you’ll get more inductance per turn with a smaller diameter toroid.



Wind the turns “bifilar”, that is, both wires kept together (twist them Wind the turns “bifilar”, that is, both wires kept together (twist them if you like). Spiral them both in the same direction around the core.if you like). Spiral them both in the same direction around the core.

To wall jack

To telephone(keep wires short)



If you happen to know the core’s inductance factor (AIf you happen to know the core’s inductance factor (ALL) you can ) you can approximate your filter’s inductance. Try to get at least 200 µH of approximate your filter’s inductance. Try to get at least 200 µH of inductance. inductance. (inductance of most materials drops with higher frequency)(inductance of most materials drops with higher frequency)

This example has 20 bifilar turns on a toroid with an AL of 500. Always count turns on the inside of a toroid.

L = N2•AL (nanohenries)

L = (20)2•500 = 200,000 nH = 200 µH

OD:0.825”ID: 0.52”T:0.25”

Fair-Rite material #43



Add a couple of 47pF to 100 pF 1KV capacitors



Image courtesy KY-Filters



Install filters at the positions marked “X” shown here

Service Entry

Protector block

Jack

Jack

Jack

Service Entry

Protector block

Jack Jack Jack

x xx x

xx

x x x

xx

x

xxx


SourcesSources


K-ComP.O. Box 82Randolph, OH 44265Tel: (330) 325-2110Fax: (330) [email protected]

(Available at Burghardt Amateur Center in Watertown, SD)


Sources, cont.Sources, cont.

Ferrite toroids:Ferrite toroids:

Fair-Rite CorporationP.O. Box J1 Commercial RowWalkill, NY [email protected]

Amidon Inductive Components250 Briggs AvenueCosta Mesa, CA 926261-800-898-1883sales@amidon-inductive.comwww.amidon-inductive.com


ReferencesReferences

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 Radio magazine, 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


References, cont.References, cont.

Building your own filters:

John Browne, KI6KY

http://ky-filters.com/

FCC Telephone Interference Bulletin CIB-10 August 1995 www.fcc.gov/cib/Publications/phone.html

FCC Telephone Interference Survey http://www.arrl.org/tis/info/rfitelfcc.html

Thanks for your Thanks for your attention !attention !

Questions Questions ??

Telephone Interference How to prevent it How to eliminate it by Dave LeVasseur, NØDL Dakota Division Convention August 6th, 1999 (This presentation may.

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