HP TM 11-6625-2779-14&P - Manual (Page 28)

Section IV Model 3555B
4-13. The output of the Input Amplifier goes to the
INPUT switch where noise filters are set up for selection
by the NOISE WTG switch. In the NOISE position,
either 3kHz FLAT weighting, C Message weighting,
15kHz FLAT weighting or PROGRAM weighting can be
selected by the NOISE WTG switch. In the TMS
position of the INPUT switch the filters are bypassed for
transmission level measurements.
4-14.
The output from the INPUT switch goes to the
meter amplifier. This amplifier provides an ac signal to
the DIAL/AC MON jacks so that a handset can be used
to listen to the signal being measured. This is
particularly useful in determining noise characters.
4-15.
The detector circuit provides an equivalent rms
detected voltage to drive the meter. The meter has
shaped pole pieces to provide a linear meter scale both
for dBm and dBrn.
4-16.
DETAILED CIRCUIT DESCRIPTION.
4-17. The purpose of the function switch is to set up
the input conditions to match the type of measurement
being made. Impedances can be selected to match the
lines to be tested and can be either bridged or
terminated. Separate transformers are selected for
voice frequency and carrier frequency measurements.
A 40dB attenuator is bridged across the input terminals
for longitudinal noise measurements when the Ng
pushbutton is depressed. The HOLD function places a
high inductance holding coil across the input terminals
to simulate an off-hook condition. Each of these
functions is described in detail in the following
paragraphs.
a. HOLD: When the HOLD pushbutton is
depressed a high inductance coil LI is
connected across the Model 3555B
balanced INPUT terminals if the INPUT
switch is in the TERM position. A bridging
HOLD is not possible. The TERM switch
connects the two windings of L1 in series.
b. DIAL BAT: (See Figure 4-2) The DIAL BA1
pushbutton serves two purposes. First it
disconnects the meter from the detector
and connects it to the unregulated power
supply so that the battery voltage can be
monitored. Secondly, the DIAL/AC MON
jacks are disconnected from the amplifier
ac output and connected to the INPUT
jacks. This permits connecting the
lineman's handset to the balanced line for
the purpose of dialing.
c. Ng: (See Figure 4-3) The Ng pushbutton
connects a 40dB attenuator across the
balanced input terminals for longitudinal
measurements. This attenuator consists of
A1R5 thru A1R8 and A1C1. The output is
taken from the junction of AIC1 and A1R8.
This output is referenced to ground and
applied to the voice frequency transformer
A1T2.
d. 900 (Vf/Nm): The 900 function switch S4
selects terminating resistors AIRI and A1R9
for 900 ohm terminations. The INPUT
switch must be in _ the TERM position to
complete the circuit for this termination.
The 900 function switch also places a
ground on the 900 ohm relay A3K1 which
provides gain switching in the Input
Amplifier so that the meter will indicate in
dBm. The 900 ohm signal is applied to the
voice frequency transformer A1T2. HOLD
can be accomplished on this function.
Figure 4-2. Simplified DIAL BAT Function
4-2