Telephone Interference

1. 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 www.dailypost.com/~davel)

2. Seminar contents • Definitions • Overview: how telephones work • Common mode interference • Commercial filters • Step-by-step procedure to get rid of TPI • Building your own filters • References • Wrap up 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 TPI Presentation

4. 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 in the local loop. Named for their connection to plugs used in older central offices. TPI Presentation

5. Definitions • 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 ? ? TPI Presentation

6. Definitions • 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. ? V V TPI Presentation

7. How Telephones work Typical telephone network Central Office ~100Vrms ringing signal 48V battery TPI Presentation

8. How Telephones work • Typical residential installation methods Straight cable runs: Jack Service Entry Protector block Jack Jack TPI Presentation

9. How Telephones work • Typical residential installation methods Loop-series wiring: Service Entry Protector block Jack Jack Jack TPI Presentation

10. How Telephones work • All telephones have three separate subassemblies: • Speech Network • Dialing Mechanism • Ringer (bell) TPI Presentation

11. How Telephones work • Speech Network contains: • Microphone (transmitter) • Earphone or speaker (receiver) • Hybrid (2-wire/4-wire converter) TPI Presentation

12. 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. TPI Presentation

13. How telephones work Transformer hybrid: Image courtesy of Midcom, Inc. TPI Presentation

14. How telephones work Op-amp hybrid: Tx 600 (Balance network) 10k 10k Rx to telephone line TPI Presentation

15. How telephones work Rx Hybrid 2-wire port Balance Network Tx 4-wire ports TPI Presentation

16. How telephones work Rx Hybrid Balance Network Tx TPI Presentation

17. Sidetone How telephones work Rx Hybrid Balance Network Tx TPI Presentation

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

19. How telephones work Typical telephone interface circuit Image courtesy of National Semiconductor, AN-397 TPI Presentation

20. Common-mode interference • Radio Frequency Interference is most likely the result of a strong common-mode signal becoming converted to a weaker but perceptible differential signal. • Telephone systems use twisted wires to assure that any interfering signals are balanced on each wire. TPI Presentation

21. V Capacitive effects count, too. Common-mode interference Common-mode RF becomes a differential signal by becoming unbalanced: TPI Presentation

22. V Common-mode interference Common-mode RF becomes a differential signal through rectification: TPI Presentation

23. Z V V Z Common-mode interference We need a special impedance: one that opposes common-mode signals, TPI Presentation

24. Common-mode interference We need a special impedance: one that opposes common-mode signals, but doesn’t impair differential signals. Z V Z TPI Presentation

25. Common-mode interference Solution: The Common-Code Choke V TPI Presentation

26. Common-mode interference Magnetic flux caused by common mode current is accumulated, producing an opposing impedance differential mode current common mode current Magnetic flux caused by differential currents cancel each other; impedance is not produced. TPI Presentation

27. Common-mode interference Image courtesy of ARRL (RFI handbook) TPI Presentation

28. Common-mode interference This is NOT a common-mode choke: Image courtesy of ARRL (RFI handbook) TPI Presentation

29. Common-mode interference Add a couple of capacitors to reduce high-frequency differential noise: V (Typical values range between 47pF to perhaps 100pF) TPI Presentation

30. Commercial Filters Images courtesy of K-Com filters TPI Presentation

31. Commercial Filters Images courtesy of K-Com filters TPI Presentation

32. Step-by-step process • Make sure the problem is truly due to RFI • Verify that the interference is present only when you’re transmitting. • Run the transmitter output into a well shielded dummy load - if the problem persists, the power wiring may be the culprit. TPI Presentation

33. Step-by-step process • Note that long lengths of telephone wiring can act as tuned antennas... Telephone cable TPI Presentation

34. Step-by-step process • 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. If it seems the installation does not include a valid ground connection, contact the telephone company to have it repaired or installed if needed. TPI Presentation

35. Step-by-step process • 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 don’t, make the appropriate connections so they do) • Ground all unused wires in the cable, just for good measure. TPI Presentation

36. Step-by-step process • Step 2: Install modular filters • Install modular filters on the telephone(s) exhibiting interference. • Check each telephone for interference after installing a filter. TPI Presentation

37. Step-by-step process • Step 3: If RFI persists, break up the telephone cabling by using in-line filters. TPI Presentation

38. Step-by-step process • Make sure you’ve kept a good ground wire connection throughout. TPI Presentation

39. Step-by-step process • 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. *Don’t forget to disconnect fax machines, alarm systems and set-top boxes! TPI Presentation

40. 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 equivalent core height you’ll get more inductance per turn with a smaller diameter toroid. TPI Presentation

41. Building your own filters • 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) TPI Presentation

42. Building your own filters • If you happen to know the core’s inductance factor (AL) you can approximate your filter’s 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 AL of 500. Always count turns on the inside of a toroid. OD:0.825” ID: 0.52” T:0.25” L = N2•AL (nanohenries) L = (20)2•500 = 200,000 nH = 200 µH Fair-Rite material #43 TPI Presentation

43. Building your own filters Add a couple of 47pF to 100 pF 1KV capacitors TPI Presentation

44. Building your own filters Image courtesy KY-Filters TPI Presentation

45. Building your own filters Jack x x Jack Service Entry x x x x Protector block Install filters at the positions marked “X” shown here Jack x x x Jack Jack Jack x Service Entry x x x x x Protector block TPI Presentation

46. Sources • Commercial Filters K-Com P.O. Box 82 Randolph, OH 44265 Tel: (330) 325-2110 Fax: (330) 325-2525 info@k-comfilters.com www.k-comfilters.com (Available at Burghardt Amateur Center in Watertown, SD) TPI Presentation

47. Sources, cont. • Ferrite toroids: Fair-Rite Corporation P.O. Box J 1 Commercial Row Walkill, NY 12589 1-800-836-0427 ferrites@fair-rite.com www.fair-rite.com Amidon Inductive Components 250 Briggs Avenue Costa Mesa, CA 92626 1-800-898-1883 sales@amidon-inductive.com www.amidon-inductive.com TPI Presentation

48. 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 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 TPI Presentation

49. References, cont. • Building your own filters: John Browne, KI6KY http://ky-filters.com/ • FCC Telephone Interference Survey http://www.arrl.org/tis/info/rfitelfcc.html • FCC Telephone Interference Bulletin CIB-10 August 1995 www.fcc.gov/cib/Publications/phone.html TPI Presentation

50. Thanks for your attention ! Questions ?