Section III
3-35. As an aid in identifying the source of noise, the DIAL/AC MON jacks can be used with a monitoring receiver to listen to the noise which will have approximately the same quality as that heard by a subscriber. Particular types of noise like power line induction, switching noise, atmospheric static, crosstalk or random noise may be identified by this listening test. To aid in bringing up the level of the lower frequency power line noise, the 3kHz flat weighting is used. A substantial increase in meter indication with the 3kHz flat weighting indicates the presence of low frequency noise and it will also sound louder in the monitoring headphone.
3-36. In some cases recording of the noise during a busy period is necessary. The recorder connections and operation is discussed in Paragraph 3-27.The calibration should be done using the dBm scale rather than the dBm scale and it should be noted that the RESPONSE switch also damps the recorder.
3-37. Noise-to-ground measurements are made by a special input circuit arrangement which is used when either the Ng or Ng HOLD pushbutton is depressed. Dial and talk may be accomplished on the metallic circuit and the metallic connection held by using the Ng HOLD pushbutton. It is necessary to establish a good earth or system ground and connect it to the black binding post marked G. The noise-to-ground measurement is 40dB less sensitive than the noise metallic measurement because of the voltage divider in the input circuit. This requires adding 40dB to the meter indication to arrive at the correct noise-to-ground level. The level is the algebraic sum of the blue RANGE switch setting and the blue meter scale indication plus 40dB. For example, RANGE is set to 20dBm and the meter indicates +3dBm. The noise-to-ground level is 20 + (+3) +40= 63dBm. Some telephone company operating procedures disregard the 40dB correction factor in which case the noise-to- ground level would be 20 + 3 = 23dBm.
3-38. The Nm and Ng indications can be used to compute the balance of a facility since balance is defined as the degree of rejection of longitudinal signals. The degree of balance in dB where the major part of noise- metallic is due to noise-to-ground, is given by the equation, Balance in dB = Nm - Ng. For example, if the noise-metallic level of a circuit is +26dBm and the noise- to-ground of the same circuit is +9OdBmC, the balance in dB is (+26) - (+90) = 64dB. In the case mentioned above where the 40dB correction factor is neglected, the balance in dB = (Nm) (Ng + 40).
3-39. Other general purpose uses of the 3555B are volume and crosstalk measurements. The ballistic characteristics of the set make it approximately correct for VU measurements. The RANGE switch should be adjusted until the meter pointer fluctuations are on-scale and should be observed for the maximum of the frequently occurring peaks, disregarding the occasional high peaks. The meter indication in dBm is equal to VU (volume units.)
Model 3555B 3-40. Crosstalk measurements involve low level measurements and part of the meter indication may be 3- 8 Model 3555B caused by noise in addition to crosstalk. The general technique is to measure with crosstalk and
noise present and then measure noise alone. A correction factor must then be applied and can be found in Table 3-2.
3-41. RECORDER COMPATIBILITY.
3-42. If an external recorder is to be used to monitor the dc output of the 3555B, the Recorder Compatibility graph, Figure 3-4should be consulted to determine if your particular recorder can be used. Recorders with input characteristics that fall below the compatibility area can be used provided a suitable resistor is used between the 3555B dc output and the recorder input.
3-43. To choose the value of this resistance, simply follow the line designating the full scale current of your recorder, horizontally until it intersects the top line in the Recorder Compatibility graph. From this intersection follow the vertical line to find the total impedance RT required for full scale deflection (see Figure 3-3).The input impedance of the recorder should be subtracted from this value RT to determine the value of R1. For example, assume that your particular recorder has an input impedance of 2000 ohms with a full scale sensitivity of 20uA. Follow the 20uA line to the right until it intersects the top line at 48 kilohms. The value of R1 will then be 48 kilohms -2 kilohms input impedance = 46 kilohms.
3-44. Recorders with input characteristics that fall above the compatibility area in Figure 3-4cannot be used to monitor the 3555B dc output since full scale deflection of the recorder cannot be accomplished by the 3555B.
Figure 3-3. Impedance Matching 3555B to Recorder
3-45. APPLICATIONS.
3-46. Sometimes it is necessary to transmit or send a tone on a line and then measure the received signal coming back on the same line. Rather than change connections back and forth between the 3555B and 236A Oscillator when changing from SEND to RECEIVE and thus take a chance on dropping the line, it is much more convenient to make one set of connections and then select SEND or RECEIVE by means or a switch. Refer to Figure 3-5.
3-47. By utilizing the test set-up shown in Figure 3-5,send and receive can be accomplished with a minimum number of operations. To dial, set both function switches to DIAL and dial the desired line on the butt-in. To send, change the
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