Daily Archives: 2-April-2013

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

Posted in Electronics, Telephones | 5 Comments