HP TM 11-6625-2779-14&P manual Range Attenuator A2, Input Amplifier A3. Schematic No

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 4-1for 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.

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