HP TM 11-6625-2779-14&P manual Range, Noise Measurement

Model 3555B

when no input signal is present and a mechanical zero adjust is not required. The actual input level to the set is the algebraic sum of the black dBm meter scale and black RANGE setting. For example, RANGE is set to 40dBm and the meter indicates -6.3dBm. The input level is then (-

40)+ (-6.3) = -46.3dBm. If the RANGE switch is at +20dBm and the meter indication is 4.7dBm, the level is (+20) + (4.7) = +15.3dBm.

3-24. All panel markings corresponding to the proper dBm markings on the RANGE switch and meter face are in black, as is the TMS position of the INPUT switch. The blue markings correspond to the settings for noise measurements as discussed in paragraph 3-28.The response of the meter rectifier circuit is RMS which allows the set to measure the true power of any arbitrary input waveform provided the crest factor does not exceed 4:1. Crest factor is defined as the ratio of the peak value of the waveform to the RMS value of that waveform. In most telephonic measurements, consideration of this crest factor is not necessary.

3-25. The balanced input to the set is achieved through the use of two repeat coils, one for voice frequencies from 20Hz to 20kHz and the other for carrier frequencies from 10kHz to 600kHz. The maximum high frequency range is achieved through the use of the 75 ohm functions and the 75 ohm jack. This input bypasses both input repeat coils, thus allowing measurements from 30Hz to 3MHz. This high frequency range is limited to 600kHz on the +20 and +30dBm ranges. The maximum longitudinal input voltage is 150 volts peak between tip and ring and 200 volts rms at 60Hz between either tip or ring and ground.

3-26. The switch marked RESPONSE determines the speed of the meter response and is usually left in the NORM position for transmission measurements.

3-27. The jack marked DC MON accepts a Western Electric 310 or 347 plug with connections to the tip and sleeve. The dc voltage supplied by this jack can be used to operate a dc potentiometric recorder requiring 1V or a dc galvanometric recorder requiring 500uA. The dc output is proportional to input level on any one range and not meter deflection since the meter is logarithmically scaled. Knowing the current required to drive the recorder full scale and the input impedance of the recorder, enter these numbers into the recorder compatability chart Figure 3-4to determine if the recorder is suitable for use with this set. If these numbers do not fall within the compatability area, refer to Paragraph 3-41.Connect an input voltage to the set and adjust the RANGE switch until a near full scale indication is observed on the meter. Connect the recorder plug with the tip negative to the DC MON jack and adjust the input level until the meter indicates 0dBm. Mark this point, which should be near full scale, on the recorder paper. Decrease the input level until the meter indicates - 1dBm. Mark this point on the recorder paper. Continue until the recorder has been calibrated for each major dBm division on the meter. The actual input level to the set as

Section III

indicated on the recorder will be the algebraic sum of the

RANGE.

3-28. NOISE MEASUREMENT.

3-29. One of the primary functions of this set is to measure message circuit noise, both metallic and noise-to- ground. The weighting filters built into this set are switch selected and their characteristics conform to the standards set up by the Bell System and Edison Electric Institute.

3-30. In general, noise-metallic measurements are made by connecting the circuit under test to the INPUT jacks with a suitable patch cord, selecting the proper bridging or terminate condition and impedance, selecting the proper weighting filter and operating the RANGE switch to provide an on-scale meter indication. Noise measurements involve many of the same operations as the level measurements discussed in Paragraph 3-14and only the differences will be discussed.

3-31. Four filters are supplied for noise measurements; C-MESSAGE and 3kHz FLAT for message circuit noise measurement, a PROG and 15kHz FLAT for broadcast studio-transmitter links and telephone company program

circuits. These filters are necessary to allow the measuring set to approximate the response of the human ear and give an indication representative of a person's subjectiveness to noise. The frequency response of these filters is shown in Figures 4-5and 4-6.

3-32. Once a circuit has been connected, the RANGE switch is adjusted until the noise fluctuations appear on- scale on the meter with normal response, and a two-to- three minute observation of the pointer fluctuations is made to establish the point at which the pointer appears most of the time, disregarding the occasional high peaks. For rapidly fluctuating noise such as atmospheric static or switching noise, operate the RESPONSE switch to DAMP. In this position of the switch, the level of the most frequently occurring peaks should be read. Noise is specified in dBm (decibels above reference noise) and the type of filter used is noted, for example, dBmC meaning C-message weighting is used.

3-33. The noise-metallic level is the algebraic sum of the indication on the blue dBm meter scale and the blue dBm RANGE switch setting. For example, RANGE is set to 20dBm and the meter indicates +7dBm. The noise- metallic level is (20) + (+7) = +27dBm. The RANGE switch marking indicates the level at the 0dBm mark on the left end of the meter scale.

3-34. Occasionally other message circuit weightings such as the older Bell System F1A weighting or the International Telecommunication Union's CCITT or psophometric weighting may be required. To convert from C-message to F1A, subtract 6dBm from the C-message indication. The units for F1A weighting are dBa, meaning decibels adjusted. To convert from C-message to CCITT or psophometric weighting, subtract 1dBm from the C- message level as read on the black dBm meter scale and RANGE switch setting. This will give the noise level in dBm which is acceptable for psophometric measurements.

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