Customer- Loop Noise Reduction Handbook

Customer Loop

Noise Reduction

Handbook

A Step-By-Step Procedure for

Solving Noise Problems

INTRODUCTION

The analysis of noise problems on customer loops cannot be made in a haphazard manner. The many factors which contribute to noise can cause considerable confusion in the analysis of noise problems, if they are not properly isolated and remedial action is not taken in the proper sequence. It is essential that a systematic procedure be followed and specific tests made in the proper sequence. It is wise never to take anything for granted when working on noise. Unless such a procedure is followed, much time can be wasted and frequently no solution is found. Noise problems can be very complex in nature, but in most cases the solution to these problems can be very simple and well within the capabilities of installers and repairmen.

This handbook will outline the terminology used, note loop performance standards, identify required test equipment and describe a systematic procedure for quick resolution of loop performance problems. The procedures should be followed closely, as skipping steps can cause poor results and wasted time. Data should be taken in a form similar to the sample data sheets at the end of this booklet.

DEFINITIONS

1. Noise

This is the noise measured across the tip (T) and ring (R) of a circuit, and is the noise the customer hears. Same as Circuit Noise (Ckt Noise).

3.Noise-To-Ground (Ng)

in order to determine power influence. The 40

factor.

4.Power Influence (PI)

correction.

5.Balance

Balance equals Power Influence

PI Nm). The balance of circuits is the major factor contributing to the susceptibility of telephone plant to noise induced from power systems. (See appendix).

Balance

Pwr lnfl or

Balance = Pwr Infl

Alternatively:Balance = (Ng + 40)

to Ng measurement.

6.C-Message Weighting

8

reference noise

9.

10. MGN

and a circuit termination set with its LONGITUDINAL CURRENTSWITCH is ON.

TEST EQUIPMENT

The test equipment described in this section is recommended for use in noise reduction procedures. Other noise measuring sets may be used.

1.Wilcom or

-15 to

15to33

b. Input Impedance: 600 ohms ac and 430 ohms d

c.

c. See Figure 1.

* Not including two range extender push-buttons -10

(or

4.Wilcom T338 Transmission Measuring Test Set.

General purpose noise and transmission test set 50 Hz to 130

Preventive Maintenance.

b. Type of Circuit Measured:

Voice (Subscriber Loops, PBX CO Trunks, Private Lines, Tandem

Tie Trunks).

c. lntertandem Trunks:

Off Premise Stations

Foreign Exchange Lines.

d. Data:

Private Line

Multipoint

Switched

the frequency range 50

a. Used with the it provides a means for measuring

ground return current in power systems without the use of ground rods and probe wire.

b. See Figure 4 (shown with

Figure 3.

F i g u r e 4.

STEP-BY-STEP TECHNIQUE

The most important thing to remember when analyzing a noise problem is

that solutions can be found only by following systematic, step-by-step procedures. Using the circuit balance and shield analysis techniques

described, it is typically possible to solve 80 to 90% of noise problems.

Sufficient information should be obtained to either solve the problem or provide data in a logical sequence to help a specialist who may be called in for a more extensive coordination analysis.

Procedures outlined in this handbook should be followed in the order given,

Measuring Tip and Ring Voltage to Ground

Initially, with a

proceed to Loop Current and Circuit Loss section.

b.If the levels are greater, proceed to the next paragraph.

Using a voltmeter having an internal impedance of greater than 5000

Objective values

listed above

are typical for the

telephone

industry. Refer to

your company

practices for further

information.

Figure 6. Noise Investigation Procedure Flowchart

Make separate readings on both the tip and ring conductors. Reverse voltmeter test leads and repeat reading. If values are different, this indicates dc is affecting the readings. Should this occur, connect a

capacitor in series with one voltmeter test lead and repeat the readings.

For customer loops, voltages up to 50

If the limiting voltage is exceeded, proceed directly to Step

STEP

is the measurement of loop current. The or

available).

https://www.360docs.net/doc/129713100.html,ing the Tl36, set function switch to LOOP

to peg in the reverse direction. The T136 leads will have to be reversed prior to dialing (GRN to ring and RED to tip). The

less than 20

connected as described above for . Loop Current do the following:

a.Set function switch for T136 to DIAL position.

b.Set switch on hand test telephone to OFF-HOOK position. Listen for

dial tone and dial number for milliwatt supply in the CO.

C.Set function switch to CKT LOSS position and depress PUSH TO

for customer loops.

When the requirements for Loop Current and Circuit Loss are met, proceed to Step

Noise Metallic and Noise-to-Ground

Noise metallic (Nm) and Power Influence (PI) can be measured with the

in series

with a

STEP

= Pwr Infl

= PI

= Ng + 40

and

Nm as a balance figure may be misleading.If a small value

resistor is inserted in the ground path of a center tapped balanced inductor,

the current flow through the resistor will give a voltage indication of the

longitudinal current flow through the longitudinal circuit impedance. The latter is assumed to be 750 ohms. If we measure voltage across 7.5 ohms

in the ground path and multiply by 100 (or add 40

Nm

and NI

T279 circuit

termination set.

Table B.Balance 0

1Unacceptable

indicates poor balance, proceed to Step

1 (Origin of Noise) to investigate the excessive PI (Pwr Influence).

If tests show that the balance is below acceptable limits, corrective action must first be taken to raise the balance to an acceptable level, especially if PI (Pwr Influence) is above acceptable limits. In other words, balance must be at an acceptable level to assure ‘acceptable Ckt Noise when PI levels vary from time to time.

Ckt Noise = Pwr Infl

Low Balance Caused by CO or Field?

Theory

The

Nm is heard. If there is an unbalance anywhere in the circuit, all induced currents will not be the same on both wires and Nm will be increased, possibly above objectives. The power influence (PI) and the unbalance do not have to occur at the same place on the cable, and since Nm is a function of Ng and balance, both must be included for a valid measurement.

Technique

The T279 can be used to determine if the unbalance is in the CO or the field. See Figure 7 for the test arrangement that corresponds to the

following procedure.

.MAltihME l STATION TELEPHONE PROTECTOR T

ON HOOK

CABLE PAIR T 279

Using the T279

Connect as follows:

a.Connect the

d.Set the

All CO equipment is isolated from the pair under test. A significant improvement in’balance (Pwr

b.If the FIELD is unbalanced, proceed to Unbalance in Cable or Station

Sectionalization, Step

2. STEP

T 2

7

9

N O

T

E

1

P

R

O

T

E

C

T

O R

P

H O N

E

I

1

N O T E

T

C .O

.

G

R

O U N

D 1. R

E M O V E H E A T C O I L S .2. G R O U N D N O T R E Q U I R E D .3.

I F (2)

(U P )B

A L A N C E . (N O I S E L O N G I T U D .