HP TM 11-6625-2779-14&P - Manual (Page 27)

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 4-1 illustrates 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