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Does anyone know of a source that lists worldwide off-hook telephone line impedances country by country? These would be the impedances seen and matched by an FXO card. Silicon Labs data sheet for the Si3050 data sheet list some indirectly, but that list doesn't agree completely with other lists.
Sadly, there's no such thing (at least not an accurate list), in my experience. I believe the ITU
made an attempt at collating this information at one time but gave up.
There's really no single figure that describes "off-hook impedance" anyway - it typically varies with frequency (did you mean DC impedance or audio impedance?) and with line current and is often expressed as a graph. Also the impedance seen at the other end of the line is strongly influenced by line length so you have to accommodate a big variation anyway.
It's probably a mistake to try to cover all countries, because some are very small markets with unusual requirements. Some countries still only allow one state-owned company to connect equipment to the lines in any case. Also, line impedances tend to vary by equipment manufacturer rather than by country anyway.
If you design to be compatible with European and North American standards (CTR21 and FCC part 68) you'll cover most of the world's lines.
At DC, impedance is just resistace because there is no inductive or capacitive reactance component. The usual DC resistance of Bell 500 and 2500 sets about 200 ohms, but it depends on current, since there are varistors that compensate for high or low current levels. Impedance varies with frequency, so it must be specified at one frequency, usually
1000 Hz. The same Bell sets are around 800 to 900 ohms impedance a