HP TM 11-6625-2779-14&P - Manual (Page 22)

Section III Model 3555B
236A FUNCTION switch to 600 HOLD or 900 HOLD,
depending on the impedance required. To receive a
tone, set the 3555B FUNCTION switch to either 600
HOLD or 900 HOLD (whichever is appropriate) and
change the 236A FUNCTION switch to DIAL. To send
again, simply change the 236A to 600 HOLD or 900
HOLD. If holding is not required or dialing is not
required, simply select the impedance and switch back
and forth on the 236A FUNCTION switch.
3-48.
TRANSMISSION LOSS MEASUREMENTS.
3-49. Transmission loss is defined as the ratio of
power from a transmission line by a receiving terminal
to the power available from the sending equipment and
is dependent on three factors; power dissipated by the
dc resistance of the line, power losses because of
impedance mismatch, power transferred to other circuits
by inductive or capacitive coupling. (See Figure 3-6).
3-50.
These factors are difficult to measure
separately. Their sum, however, is relatively easy to
measure with the -hp- 236A/3555B combination.
3-51.
Figure 3-6 shows a typical transmission loss
measurement setup. The oscillator is adjusted for a
reference level and the signal is measured at the other
end of the line with a level meter. Loss measurements
are usually made at various frequencies to determine
the response of the line.
3-52.
Ideally the man at each end of the line will have
both an oscillator and a Transmission Measuring Set
(TMS) so that the loss can be measured in both
directions, If the line that is being tested passes through
central office switching equipment, the oscillator or TMS
at the remote end is placed in the DIAL mode and the
lineman's handset connected to the DIAL posts,
permitting the repairman to bypass the instrument
circuitry and dial his test board at the central office.
Tests are then made in the 600 or 900 ohm HOLD
positions, which provide a dc path to hold the switching
relays.
3-53. CROSSTALK MEASUREMENTS.
3-54. Crosstalk is interference on a transmission line
caused by inductive and capacitive coupling between
pairs of transmission lines in close proximity. Crosstalk
can be classified as near-end and far-end. Far-end
crosstalk is interference at the end of the transmission
line opposite the , signal source while near-end crosstalk
is interference detected at the same end of the line as
the signal source.
Table 3-2. Crosstalk Correction Factor
dB Correction Factor
(Crosstalk + Noise) in dB Crosstalk in dB =
Minus Noise Alone in dB (Crosstalk + Noise)
Minus Correction Factor
1 7
2 4
3 3
4 to5 2
6 to 8 1
9 and above 0
3-55. Since different frequency bands are used for
each direction of transmission on two wire carrier
systems, near-end crosstalk cannot be detected. The
situation is quite different, however, for far-end crosstalk
since it is in the same frequency band as the desired
signal and can be detected.
3-56.
Referring to Figure 3-7, one line is designated
A-B and the other designated C-D with A and C
representing the near-end of one of the pairs, and band
D representing the far-end of the other pair. First
measure the transmission loss between A and B. Then
measure the transmission loss from A to D. The
crosstalk coupling loss in dBx is the difference in the
reading from A to B and the reading from Ato D.
3-57.
IDENTIFYING NOISE CHARACTERISTICS.
Figure 3-6. Typical Test Setup for Measuring Insertion Loss
3-10