HP TM 11-6625-2779-14&P manual Transmission Loss Measurements, Crosstalk Measurements

Section III

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 3-6shows 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.

Model 3555B

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

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