HP TM 11-6625-2779-14&P manual Measurements in DBC

Model 3555B Section III

Figure 3-7. Test Setup for Measuring Crosstalk Coupling Loss

3-58. Normally, a frequency selective voltmeter is

oscillator frequency dial to an accuracy of approximately

used to identify the characteristics of transmission line

±3%.

In practice, this measurement would probably be

interference in order to trace it down to its origin and

made using a "loop around" technique.

The oscillator

apply the appropriate corrective action. As an expedient

would be connected to a quiet line at the remote location

for troubleshooting, there are several subjective

and this line would be tied to the noisy line back at the

measurements that the 236A/3555B can make to help

central office. This permits one man to operate both the

identify the interference characteristics.

oscillator and the test meter.

3-59. Since power line noise is the most common

3-63. When a current flows through a conductor, it

nuisance, a quick check with the 3555B should be made

sets up two distinct fields around the conductor - - the

first. By noting the difference in noise readings between

electrostatic (capacitive) field and the magnetic

the 3kHz FLAT and C-message weighted modes, an

(inductive) field.

Both

are

capable

of

inducing

indication of line frequency disturbance can be

longitudinal voltages in adjacent conductors, and both

ascertained if the 3kHz flat mode shows a substantially

increase in proportion to the power and frequency of the

higher reading.

current from which

they

result.

They

differ

greatly,

3-60.

As a

further

aid

in

identifying noise,

the

however, in how they affect nearby circuits. The voltage

lineman's handset can be connected to the AC

resulting from magnetic induction varies inversely-with

MONITOR terminals and an aural analysis made.

the impedance of the line.

That is, the higher the line

Although the handset will not respond to 60Hz, line

impedance, the less voltage that can be induced by a

interference is usually very rich in odd harmonics and

magnetic field. Capacitively coupled voltage, on the

180Hz can easily be identified.

This test also helps to

other hand, increases in direct proportion to line

identify "babble" and other audio frequency interference.

impedance-- the higher the impedance, the greater the

3-61. Vagrant noise, such as atmospheric noise, can

capacitive coupling.

By means of a simple test, it is

be analyzed by connecting a strip chart recorder to the

possible to identify the coupling between two lines, as

DC MONITOR

terminals.

Long-term seasonal and

shown in Figure 3-8.Since induced voltages are

temperature effects can also be measured very

inversely proportional to line impedance, the voltage

conveniently with a recorder.

coupled from pair A into pair B (Figure 3-8a) will

3-62. Frequency of strong interfering periodic signals,

increase as the impedance is lowered (i.e., shorted).

such as radio transmitters, can be roughly determined

Conversely, since capacitively coupled voltages are

with the 236A and 3555B.

The 236A is connected to

directly proportional to impedance, the coupled voltage

one end of the line and the 3555B to the remote end, as

in Figure 3-8bwould increase as the impedance is

with

transmission loss

measurements.

The oscillator

increased (i.e., open circuited). Both tests in Figure 3-8

output is increased until the test meter barely indicates a

should be performed to correlate the result.

signal above the noise. The oscillator frequency is then

3-64.

MEASUREMENTS IN DBC.

changed very slowly while the repairman observes the

3-65. The term dBC means dB Collins and is defined

3555B for a beat. By tuning for a beat, the frequency of

as

the interfering

signal

can

be

read

directly off

the

3-11