# Monthly Archives: May 2014

## 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

Posted in Electronics, Telephones | 9 Comments

## 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

Posted in Electronics, Telephones | 5 Comments

## Measuring Telephone Ring Power

I have been looking at some power data for telephone circuits today and this data provided a useful empirical check on the theoretical calculations that I have done elsewhere. When I showed the data to some other engineers, they had some good questions that I thought would be worth covering here. Continue reading

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## Yet Another Thermistor Discussion

I thought enough people may want to see this slight variation that it would be worth putting into a separate post. It turns out the optimal linearization value for RS is exactly the same as for my original post. This result makes sense if you think about it a bit because a nearly linear output across one resistor in the circuit implies a linear voltage across the other. You can derive this result rigorously using the same approach I used in the Appendix of my original post. Continue reading

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## Phone Line Length Math

We make ONT products that provide telephone service in addition to data and video services. I was asked today what limits the length of a standard POTS phone line from an ONT. Most ONTs today are specified to drive a phone line less than 1000 feet long. As I started to write down my answer, I thought that this was a nice application of simple DC electronics and was worth documenting here. Continue reading

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## Bisecting an Outside Wall Corner Angle

I have just returned from putting up crown molding at brother's house. It is always fun working with my brothers. In many ways, we are little different today than we were 40 years ago. During this task, I encountered a wall corner that was not square. Let's talk about how you can measure and bisect this angle. You need to bisect the angle when you want to cut the molding to fit around the corner (Figure 1). Continue reading

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