HP TM 11-6625-2779-14&P - Manual (Page 20)

Section III Model 3555B
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.)
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 3-4 should 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 3-4 cannot 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
3-8