Category Archives: Telephones

New Zealand Complex Phone Line Impedance

I received an email today asking me about the phone line impedance differences between New Zealand (Figure 1) and Australia. This is an easy question to answer, and I wrote up a quick Mathcad worksheet to perform the calculation. Continue reading

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Why Positive Ground in Telcom Power Systems?

Today, we use wires with excellent insulation and these moisture-induced corrosion arguments carry less weight. Back in the old days, however, telecom wiring had cloth-based insulation that would get wet – along with the copper wire. When you mix water and electric fields, you get electrochemical action (i.e. electroplating). While the early systems were configured with negative grounds (i.e. ground as the lowest potential in the system), it was soon discovered that these systems suffered from corrosion. Positive ground (i.e. ground is the highest potential in the system) voltages were found to be effective in combating this corrosion. Continue reading

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Estimating the RMS Ring Voltage at a Telephone

Introduction I just saw a customer report of phone ringing issues with an old 2500 series phone (Figure 1). While these are good phones, they can be difficult for Fiber-to-the-Home (FTTH) deployments because they have 1 Ringer Equivalence Number (REN) … Continue reading

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Maximum Phone Line Length Math

I can see the days of the classic copper phone line coming to an end over the next few decades. Like analog video, it will eventually be replaced by digital services. For a fiber optic deployment (i.e. no copper), the old phones lines can re-purposed to carry the power needed by the fiber optic interface if AC power is not available. To carry power, the resistance of the line -- which is determined by the length and cross-sectional area of the line -- becomes very important. When carrying voice signals, phone line resistance was limited to ensure that the central office could detect the phone going off-hook. This post will use the maximum allowed line resistance to determine the maximum possible line length. Continue reading

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Telephone Ring Trip Math

As you can tell by my recent posts, I am doing quite a bit of traditional landline phone math lately. The problems are not very complex, but their resolution is important to delivering quality voice service. Today, I have been working on the circuitry that determines when a phone has gone off-hook. Continue reading

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Phone Line Impedance Levels: 600 Ohms and 900 Ohms

Engineering is a pretty conservative profession -- I have been accused of "abhorring change". Once something gets standardized it stays in place even when it does not make sense. This morning provided me a good example of this. Phone lines in the United States are usually characterized as having a characteristic impedance of 600 Ω or 900 Ω. These impedance levels go back to the early days of telephony (Figure 1). However, all the phone cables we work with are Category 3 and therefore have identical characteristic impedance (~725 Ω). So why the different impedance levels? I had a discussion with one of our telephony engineers about it this morning and all we could do is speculate. I thought I would document this speculation here. Continue reading

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