2-4. CIRCUIT ADJUSTMENTS
The only adjustment in the circuit illustrated in figure 2-1 is the
attenuator, which controls the overall gain of the amplifier arrangement in bay
1.
The gain of the amplifier remains fixed at 35 db, and the input signal
level is adjusted by the attenuator.
a. Equal-Level Patching. The equal-level patching panel in bay 2 permits
circuits to be patched easily and quickly.
As long as equal levels are
maintained, the technician need not make any level measurements before he makes
the patches. Whether the levels remain equal depends on three factors: level
of the arriving input signal, gain of the amplifier, and adjustment of the
attenuator. One of the duties of the technical controller is to make frequent
checks of levels of incoming signals at J1. On the other hand, J3 can be used
for the same purpose, because any variation of input signal level at bay 1
reflects in a similar change of level in J3.
b. Attenuator Adjustment.
Example 1:
Assume that the input signal level drops from -12 dbm to -20 dbm.
How must the attenuator be adjusted?
The level in bay 2 drops 8 db from -2 to -10 dbm, because the input
signal level dropped the same amount.
The attenuator loss must
therefore be reduced 8 db so the amplifier can raise the signal
level back up to the required -2 dbm in bay 2.
Example 2: Assume that the input signal level rises from -12 dbm to -6 dbm.
Row must the attenuator be adjusted?
The level in bay 2 rises 6 db from -2 to +4 dbm. The attenuator
loss must therefore be increased 6 db so the previous level of -2
dbm will be produced by the amplifier.
2-5. SIGNAL-TO-NOISE RATIO
The signal-to-noise ratio (S/N) in a telephone communication circuit must
always be maintained at a high value. This is especially important where data
communication is intended.
The higher the S/N, the more satisfactory the
circuit will be for communication. When S/N approaches 1/1 (0 db), noise level
is as high as the signal level, making the circuit worthless for communication.
An absolute minimum S/N that can be tolerated is 4/1 (6 db).
a. Noise Level Statement. Noise level can be stated in either absolute or
relative terms.
Each method has its advantages under different test
conditions. Noise can be stated with respect to the expected level at the TLP,
or it can be seated with respect to an established noise level reference.
b. Absolute Level. The absolute, or actual, level of noise existing in a
circuit is measured by a test set that is calibrated in terms of a power
reference. Most test equipment of this type uses -90 dbm as a zero reference
noise (rn), making all quoted figures a positive value (+). Further, each test
set must state the type of weighting, which affects the relative value of
interfering noise. A comparison of noise reference levels used for some of the
more widely used test sets is shown in the chart below.
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