a. Whenever an arbitrary scale is indicated, you are advised to search for
the reference level of that scale.
b. SSTS 53105 has used db's to indicate changes in power level as well as
power level itself. The key to understanding the material in SSTS 53105 is to
determine whether level or change of level is specified.
c. Because the arbitrary scale in SSTS 53105 does not agree with prior
teaching in this subcourse, calculations of levels will not be required in the
exercises.
2.
SEQUENCE OF STUDY
a. Study the Attached Memorandum and answer exercises 1 through 10.
b. Study Appendix C and answer exercises 11 through 20.
ATTACHED MEMORANDUM
3-1. TELEPHONE NETWORKS
Telephone networks include all the telephones, switchboards, and trunks
connecting the switchboards.
Technical controllers and circuit conditioners
must be able to think in terms of complete networks and all of the equipment
within them.
The total characteristic of a telephone network reflects the
combination of characteristics of all individual items within the network.
a. Short-Distance Networks.
Short-distance networks include telephones
and switchboards together with wire pairs connecting the switchboards.
One
wire pair connects each telephone to the switchboard, and another connects the
two switchboards together. As long as the overall circuit loss does not exceed
the maximum permissible value of 36 db, the two-wire (pair) circuit will be
satisfactory.
b. Long-Distance Networks.
Long-distance networks include the usual
telephones and switchboards found in short-distance networks.
However, the
over-all losses in long-distance networks may be so great that repeaters will
have to be inserted in the wire or cable pair to raise the level. Further, the
system may include telephone carrier systems and radio sets. Since repeaters
and telephone carrier or radio systems use amplifiers for each direction of
transmission, it is necessary to provide a pair of wires for each direction of
transmission, thus making a four-wire (two-pair) circuit.
A hybrid coil
circuit will be necessary at each point of conversion from two-wire to four-
wire operation. Each hybrid coil introduces losses which will be overcome by
amplifiers in repeaters, or amplifiers in either carrier terminals or radio
sets.
3-2. HYBRID COIL CIRCUIT
A hybrid coil serves as an "electronic traffic cop" that directs traffic
through repeaters, as shown in figure 3-1. If the hybrid coil were not used,
the output of one repeating amplifier would feed the input of the second,
fording a closed amplifying loop which would "sing" or "howl." When the
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