Asked by aditir | 1st Jul, 2009, 10:07: PM

Expert Answer:


In long-distance lines, losses are so high that satisfactory operation becomes impossible without the use of amplification. Amplifiers for telephone service are known as repeaters, and they must of course function in both directions along the line. Repeater stations are ordinarily installed at 50-mile intervals along the line, except in the "newer wide-band transmission systems, for some of which repeater spacing is as little as 5 miles. Because of the total number of stages of amplification in a long-distance transmission, the performance requirements of telephone repeaters are very severe. Any distortion is cumulative, and any appreciable amount would soon result in unintelligible speech. The demands of high-quality programme transmission for broadcast networks are even more exacting.
One method for obtaining two-way repeater service is shown in the figure. Two amplifiers are employed, one for each direction of transmission. Feedback and oscillation are prevented by the hybrid coils, which are essentially three-winding transformers of balanced construction. A signal traveling from west to east encounters the first hybrid coil, where part of the power enters the west-east amplifier by way of the centre taps on the main windings.
Type 22 telephone repeater
The amplified power is fed into the third winding of the other hybrid coil, and divides there into two equal portions, one passing out on the line to the east, the other into the artificial network. This network is constructed to have impedance properties equal to those of the line over the entire frequency band, so that the combination of line and artificial network is equivalent to a balanced bridge. As a result of this balance, none of the output of the west-east amplifier reaches the input terminals of the east-west amplifier, and feedback around the loop is avoided.
It will be noted that power is diverted and lost at both hybrid coils. Raising the gain of the amplifier by an equivalent amount, which turns out to be 6 db in power, compensates for this loss.

Answered by  | 6th Jul, 2009, 03:56: PM

Queries asked on Sunday & after 7pm from Monday to Saturday will be answered after 12pm the next working day.