HP TM 11-6625-2779-14&P manual Section Theory of Operation Introduction, Block Diagram Description

Model 3555B Section IV

SECTION IV

THEORY OF OPERATION

4-1. INTRODUCTION.

4-2. The Model 3555B Transmission and Noise Measuring Set is a special measuring set designed for uses in testing telecommunications equipment. Inputs between -90dBm and +30dBm full scale can be selected in twelve ranges for level measurements and correspond to the black markings on the meter scale and the RANGE switch. Noise measurements between 0dBrn and +120dBrn full scale can be made, selectable in twelve ranges and corresponds to the blue markings

on the meter scale and RANGE switch. When measuring rapidly fluctuating noises, a damping circuit can be inserted by the RESPONSE switch.

4-3. Impedances of 75, 135 and 600 ohms, terminated or bridging can be selected for carrier level measurements. The 135 and 600 ohm functions can be either balanced or unbalanced while the 75 ohm function is unbalanced only. For voice frequencies, impedances of 600 and 900 ohms are provided. These impedances are selectable by the pushbutton FUNCTION switch and can be terminated or bridging, balanced or unbalanced.

4-4. A noise-to-ground (Ng) function is included to permit measurement of longitudinal noise. When the Ng pushbutton is depressed, a 40dB attenuator is placed across the INPUT terminals.

4-5. The HOLD function places a high inductance holding coil across the INPUT terminals to simulate an off-hook condition while measurements are being made. The HOLD function is not operative on any of the carrier functions.

4-6. A variety of INPUT and DIAL jacks are provided which accept Western Electric type 241 and 289 dual plugs, 309, 310, 347, and 358 single plugs, dual banana plugs, clip leads and bare wires.

4-7. BLOCK DIAGRAM DESCRIPTION.

4-8. Figure 4-1illustrates a simplified block diagram of the Model 3555B Transmission and Noise Measuring

Set. Refer to this figure for the following block diagram description.

4-9. The input signal is first applied to the FUNCTION switch where the input circuitry is set up to accommodate the type of measurement being made. For voice frequencies, impedances of 900 ohms or 600 ohms can be selected, bridged or terminated. Voice frequencies are then applied to a transformer with a frequency range of 20Hz to 20kHz. The HOLD function places a high inductance bridge across the INPUT terminals to simulate an off-hook condition. For carrier frequencies impedances of 600 ohms, and 135 ohms can be selected, terminated or bridged, balanced or unbalanced. Carrier frequencies at these impedances are applied to a transformer having a frequency range from 5kHz to 600kHz. For 75 ohm carrier frequencies an unbalanced input is provided. This input can be either terminated or bridged. HOLD is not possible on any of the carrier functions.

4-10. For longitudinal measurements, an Ng function is provided which places a 40dB attenuator across the INPUT terminals. The HOLD function bridges the input with a holding coil while measurements are being made. The output of the 40dB attenuator is always applied to the voice frequency transformer.

4-11. The DIAL/BAT function serves two functions. First it connects the DIAL/AC MON jacks to the INPUT jacks so that a handset can be used for dialing. Secondly, the meter is connected to the unregulated power supply so that the battery condition can be monitored.

4-12.After the signal is conditioned by the input circuitry it is coupled to the RANGE attenuator where the signal level is adjusted to provide the proper input for the Input Amplifier. The RANGE attenuator provides from 0dB to 80dB of attenuation. It also provides gain switching for the Input Amplifier.

Figure 4-1. Simplified Block Diagram

4-1