HP TM 11-6625-2779-14&P - Manual (Page 29)

Model 3555B Section IV
Figure 4-3. Simplified NG Function
e. 600 (Vf/Nm): The 600 function switch S5
selects terminating resistors A1R2 and
A1R10 for a 600 ohm termination. The
INPUT switch completes the circuit for this
termination. The 600 (Vf/Nm) signal is
applied to T2. No gain switching is
performed in this function since the set is
normalized at 600 ohms HOLD can be
accomplished on this function.
f. 600 (Carrier): This function is identical to
the 600 (Vf/Nm) function except that the
signal is applied to A1T1 and HOLD cannot
be accomplished on this function.
g. 135 (Carrier): The 135 function is identical
to the 600 (carrier) function except that the
gain switching in the Input Amplifier is
accomplished by one section of the 135
function switch S7.
h. 75 UNBAL: The 75 UNBAL function
bypasses the balanced input circuitry and
transformer AlT1 and A1T2. Gain
switching is performed by one section of
this function switch. When the 75 UNBAL
function is selected the output of the
balanced circuitry is disconnected. A 75
ohm termination is provided thru the
INPUT switch.
4-18. RANGE ATTENUATOR A2.
4-19. The RANGE attenuator adjusts the input signal
to a suitable level for the Input Amplifier. This
attenuator is composed of four L pads, selectable in
combinations to provide from 0dB to 80dB of
attenuation. Two 30dB pads are selected by A2S1A and
A2S1B, a 20dB pad is selected by A2S1C and a 10dB
pad is selected by A2S1D. Another section of the
RANGE attenuator switch provides gain switching for
the Input Amplifier in the -80dBm, -70dBm and -60dBm
positions. Refer to Table 4-1 for more detailed
information on range attenuation and amplifier gain.
4-20.
INPUT AMPLIFIER A3. (Schematic No. 2)
4-21. The purpose of the Input Amplifier is to provide
the necessary gain at each setting of the RANGE switch
and to provide the necessary gain at all impedances.
This amplifier is normalized at 600 ohms and the
following discussion is for the 600 ohm function.
4-22.
Diodes A3CR1 thru A3CR4 serve as protection
for the input amplifier. Signals greater than 7 volts
peak-to-peak will be conducted to ground through these
diodes. The gain of this amplifier is determined by the
negative feedback from the emitter of A3Q5 to the base
of A3Q2. This feedback is first determined by the ratio
of A3R13 to the sum of A3R14 and A3R15. In position
1 of the RANGE switch (-80DBM) this feedback is
further divided by the ratio of A3R11 to the sum of
A3R25 and A3R26. In position 2 (-70DBM) of the
RANGE switch the feedback is determined by the ratio
of A3R11 to the sum of A2R13, A3R25 and A3R26. In
position 3 (-60DBM) of the switch the feedback is
determined by the ratio of A3Rll to the sum of A2R13,
A2R14, A3R25 and A3R26.
4-3