User #209687 732 posts
posted 2008-Jan-28, 4:32 pm
Let’s talk about phone lines, as they have both an AC (speech) and a DC component.
The DC side is simple DC theory. There is a DC voltage source of 50v (about) negative and earth in the exchange. There is the cable and the phone instrument itself. On hook and with no current flowing in the circuit, you read exchange voltage (say 50v) at the phone. This may as has been said keep some display phones “alive”. They consume uA to keep the phone “charged”, and at a level much less that that required to actually seize the line.
Loop the line, and DC current flows in ONE direction from the –50v at the exchange, through the line interface, along the cable, through the phone and back to the exchange. The value of current is determined by the sum of the resistances presented to the source by the above elements. Typically, you read between about 22mA and 60mA, with between 8 and 12 volts dropped across the phone. The rest of the voltage is dispersed in the cable resistance and the interface device at the exchange. The current is easily read with a mA multimeter. No CROs needed here. And the power (E x I) is insignificant. The 20mA mentioned before is a design minimum to make sure a phone works correctly. Current in excess of that is quite OK, as the phone will manage the variation.
Should say too that a ringing signal varies this a bit. It’s usually about 75 – 90VAC at 25Hz superimposed on the 50v DC. This is so that phones will ring and that a ring trip can be detected. During a ring cycle line voltage can rise to about 130v, which explains why ringer gives you a real finger tickle if you get across it!
Now the AC bit. In regard to the R/C circuit mentioned no attempt is being made to send DC through a capacitor. The circuit is a reactive one designed simply to replicate the behaviour of an average length of unloaded customer line over the voice band (.3 to 3.4 KHz) for design purposes – imitating about 2Km of 0.4m cable I think. It’s also used in line balancing circuits. You apply voice frequency signals to the “artificial line” and observe the behaviour of those signals against the network. On the line, speech signals appear as a ripple on the DC flowing in the line. You can use a CRO to see that, no worries. In the exchange, the AC component is stripped off from the DC and sent to the codecs for digitising.
-13dBm was channel mod input level, +4 was output.
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