Presentation on theme: "Telephone Interference How to prevent it How to eliminate it by Dave LeVasseur, NØDL Dakota Division Convention August 6th, 1999 (This presentation may."— Presentation transcript:
Telephone Interference How to prevent it How to eliminate it by Dave LeVasseur, NØDL Dakota Division Convention August 6th, 1999 (This presentation may be downloaded from
6-Aug-1999TPI Presentation2 Seminar contents b Definitions b Overview: how telephones work b Common mode interference b Commercial filters b Step-by-step procedure to get rid of TPI b Building your own filters b References b Wrap up
6-Aug-1999TPI Presentation3 Definitions b On-hook: telephone in idle state; ready to make or take a call b off-hook: in the process of making or taking a call b Central Office (CO): where the other end of the wires go after they leave your house b Local loop: the wires between the CO and your telephone
6-Aug-1999TPI Presentation4 Definitions b TPI: TelePhone Interference b Hybrid: device that separates (isolates) inbound and outbound telephone signals from one another. Also known as 2-wire/4-wire converter. b Tip and Ring: the names given to the wires in the local loop. Named for their connection to plugs used in older central offices.
6-Aug-1999TPI Presentation5 Definitions b Differential mode signal: A signal which appears as a voltage difference between a pair of conductors but may have no reference to earth ground. V ??
6-Aug-1999TPI Presentation6 Definitions b Common mode signal: A signal which appears as a voltage on a pair of conductors having the same phase and polarity on each conductor with respect to ground. ? VV
6-Aug-1999TPI Presentation7 How Telephones work Typical telephone network Central Office 48V battery ~100Vrms ringing signal
6-Aug-1999TPI Presentation8 How Telephones work b Typical residential installation methods Straight cable runs: Service Entry Protector block Jack
6-Aug-1999TPI Presentation9 How Telephones work b Typical residential installation methods Loop-series wiring: Service Entry Protector block Jack
6-Aug-1999TPI Presentation10 How Telephones work b All telephones have three separate subassemblies: Speech NetworkSpeech Network Dialing MechanismDialing Mechanism Ringer (bell)Ringer (bell)
6-Aug-1999TPI Presentation11 How Telephones work b 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)
6-Aug-1999TPI Presentation12 How telephones work b All communication occurs over two wires. This requires a hybrid to separate the incoming and outgoing signals. b The hybrid (2-wire to 4-wire converter) may be implemented using transformers or operational amplifiers.
6-Aug-1999TPI Presentation13 How telephones work Transformer hybrid: Image courtesy of Midcom, Inc.
6-Aug-1999TPI Presentation14 How telephones work Op-amp hybrid: 600 (Balance network) 10k Tx Rx to telephone line
6-Aug-1999TPI Presentation15 How telephones work Balance Network Tx Rx 2-wire port 4-wire ports Hybrid
6-Aug-1999TPI Presentation16 How telephones work Balance Network Tx RxHybrid
6-Aug-1999TPI Presentation17 How telephones work Balance Network Tx RxHybrid Sidetone
6-Aug-1999TPI Presentation18 How telephones work b Most modern telephones rely on electronic rather than magnetic components (diodes and op-amps instead of transformers and inductors) making them prone to interference.
6-Aug-1999TPI Presentation19 How telephones work Typical telephone interface circuit Image courtesy of National Semiconductor, AN-397
6-Aug-1999TPI Presentation20 Common-mode interference b Radio Frequency Interference is most likely the result of a strong common-mode signal becoming converted to a weaker but perceptible differential signal. b Telephone systems use twisted wires to assure that any interfering signals are balanced on each wire.
6-Aug-1999TPI Presentation21 Common-mode interference Common-mode RF becomes a differential signal by becoming unbalanced: V Capacitive effects count, too.
6-Aug-1999TPI Presentation22 Common-mode interference V Common-mode RF becomes a differential signal through rectification:
6-Aug-1999TPI Presentation23 Common-mode interference We need a special impedance: one that opposes common- mode signals, VV Z Z
6-Aug-1999TPI Presentation24 Common-mode interference V We need a special impedance: one that opposes common- mode signals, but doesnt impair differential signals. Z Z
6-Aug-1999TPI Presentation25 Common-mode interference V Solution: The Common-Code Choke
6-Aug-1999TPI Presentation26 Common-mode interference 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
6-Aug-1999TPI Presentation28 Common-mode interference This is NOT a common-mode choke: Image courtesy of ARRL (RFI handbook)
6-Aug-1999TPI Presentation29 Common-mode interference V Add a couple of capacitors to reduce high-frequency differential noise: (Typical values range between 47pF to perhaps 100pF)
6-Aug-1999TPI Presentation30 Commercial Filters Images courtesy of K-Com filters
6-Aug-1999TPI Presentation31 Commercial Filters Images courtesy of K-Com filters
6-Aug-1999TPI Presentation32 Step-by-step process b Make sure the problem is truly due to RFI Verify that the interference is present only when youre transmitting.Verify that the interference is present only when youre transmitting. Run the transmitter output into a well shielded dummy load - if the problem persists, the power wiring may be the culprit.Run the transmitter output into a well shielded dummy load - if the problem persists, the power wiring may be the culprit.
6-Aug-1999TPI Presentation33 Step-by-step process b Note that long lengths of telephone wiring can act as tuned antennas... Telephone cable
6-Aug-1999TPI Presentation34 Step-by-step process b Step 1: Check the grounding points Verify that a ground connection of good integrity is available at the point where the telephone wires enter the premises.Verify that a ground connection of good integrity is available at the point where the telephone 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. If it seems the installation does not include a valid ground connection, contact the telephone company to have it repaired or installed if needed.
6-Aug-1999TPI Presentation35 Step-by-step process b Step 1: Check the grounding points Verify that one of the wires going to each jack contains a connection to earth ground. (and if they dont, make the appropriate connections so they do)Verify that one of the wires going to each jack contains a connection to earth ground. (and if they dont, make the appropriate connections so they do) Ground all unused wires in the cable, just for good measure.Ground all unused wires in the cable, just for good measure.
6-Aug-1999TPI Presentation36 Step-by-step process b Step 2: Install modular filters Install modular filters on the telephone(s) exhibiting interference.Install modular filters on the telephone(s) exhibiting interference. Check each telephone for interference after installing a filter.Check each telephone for interference after installing a filter.
6-Aug-1999TPI Presentation37 Step-by-step process b Step 3: If RFI persists, break up the telephone cabling by using in-line filters.
6-Aug-1999TPI Presentation38 Step-by-step process b Make sure youve kept a good ground wire connection throughout.
6-Aug-1999TPI Presentation39 b Step 4: If RFI still persists, one telephone may be contributing to your RFI problem 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.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. Step-by-step process *Dont forget to disconnect fax machines, alarm systems and set-top boxes!
6-Aug-1999TPI Presentation40 Building your own filters b Obtain a toroidal core made out of ferrite (not powdered iron) having a permeability between 250 and The best type of ferrite is nickel-zinc (NiZn) since this material works well at RF. b 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 youll get more inductance per turn with a smaller diameter toroid.
6-Aug-1999TPI Presentation41 Building your own filters b 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. To wall jack To telephone (keep wires short)
6-Aug-1999TPI Presentation42 Building your own filters b If you happen to know the cores inductance factor (A L ) you can approximate your filters inductance. Try to get at least 200 µH of inductance. (inductance of most materials drops with higher frequency) This example has 20 bifilar turns on a toroid with an A L of 500. Always count turns on the inside of a toroid. L = N 2 A L (nanohenries) L = (20) = 200,000 nH = 200 µH OD:0.825 ID: 0.52 T:0.25 Fair-Rite material #43
6-Aug-1999TPI Presentation43 Building your own filters Add a couple of 47pF to 100 pF 1KV capacitors
6-Aug-1999TPI Presentation44 Building your own filters Image courtesy KY-Filters
6-Aug-1999TPI Presentation45 Building your own filters Install filters at the positions marked X shown here Service Entry Protector block Jack Service Entry Protector block Jack xx x x x x xxx x x x xx x
6-Aug-1999TPI Presentation46 Sources b Commercial Filters K-Com P.O. Box 82 Randolph, OH Tel: (330) Fax: (330) (Available at Burghardt Amateur Center in Watertown, SD)
6-Aug-1999TPI Presentation47 Sources, cont. b Ferrite toroids: Fair-Rite Corporation P.O. Box J 1 Commercial Row Walkill, NY Amidon Inductive Components 250 Briggs Avenue Costa Mesa, CA
6-Aug-1999TPI Presentation48 References b b QST, May, 1991, Basic Steps Toward Eliminating Telephone RFI by Pete Krieger, WA8KZH, pages b b The ARRL RFI Book, ARRL publication No. 235, chapter 9, Telephone RFI, 16 pages (First edition). b b Ham Radio magazine, September 1985, Understanding Telephones by Julian Macassey, N6ARE. Also available at: b b K-Com web site: b b Telecom Digest Archives: archives
6-Aug-1999TPI Presentation49 References, cont. b b Building your own filters: John Browne, KI6KY b FCC Telephone Interference Bulletin CIB-10 August b FCC Telephone Interference Survey