The level of -40 dbm is 50 db above the reference of -90 dbm (0
dbrn). The test instrument should therefore indicate a noise level
of +50 dbrn.
d. Calculations of Relative Noise Levels.
Example 1: If the normal signal level is -3 dbm at the TLP at
the time the
noise level of +50 dbrn is being received, what is
the relative
level of the noise?
From the previous example, it is evident that +50 dbrn
is equivalent
to -40-dbm noise. Since -40 dbm is 37 db below the
-3-dbm signal
level at TLP, it is -37 dbm0 with the stated TLP.
Example 2: Is there another way to state relative noise levels?
Yes, there is.
Since noise level is -90 dbm at 0 dbrn, relative
value of noise level can also be stated with reference to the TLP.
For example, the difference between 90 and 37 (-37 dbm0 in example 1
above) is -53 dbrn0 where the "0" after dbrn signifies noise level
relative to the stated TLP.
e. Calculations of S/N.
Example 1: What is the S/N for the circuit discussed in b above?
The S/N is the difference between the desired signal level and the
noise level. The relative value of -37 dbm0 therefore expresses the
S/N as long as the signal level remains -3 dbm.
The S/N in this case is 37 db.
Example 2: If the input level drops from a normal of -3 dbm at the TLP to -18
dbm, what effect does this have on the previous S/N of 37 db?
The noise level of -40 dbm is now only 22 db below the signal level,
so the S/N is 22 db. It is important to note, however, that if the
circuit requirement is -37 dbm0, it remains at that value, because
the relative value is taken with respect to the TLP (-3 dbm), not
the actual signal level (-18 dbm) at the time.
To meet circuit
requirements of -37 dbm0, the noise will therefore have to be 37 db
below -18 dbm, or -55 dbm.
2-6. THE MILITARY TRANSMISSION PLAN
The Military Transmission Plan provides an orderly method for allocating
loss to the various type circuits found in any telephone communications
network, and insures that a person anywhere in the network can converse
satisfactorily with any other person in the network. The circuit conditioner
is not responsible for the layout of the network, but he is responsible for
conditioning designated lines within the network. He therefore must be aware
of the important features of the plan.
a. Basic Rules. Conversation between two telephones is satisfactory only
if the speech is loud enough to be heard and understood.
With the military
telephones currently in use, the maximum loss of the overall circuit between
two telephones should not exceed 36 db. The total loss must be divided among
the different types of circuits that connect the two telephones.
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