HP TM 11-6625-2779-14&P Range Attenuator, Setting Attenuation, Pads Used, Input Amplifier Gain

Section IV				Model 3555B
		Table 4-1. Range Attenuation and Amplifier Gain
	RANGE	RANGE	ATTENUATOR
	Setting	Attenuation	PADS USED	Input Amplifier Gain
	+30dBm	80dB	1,2,3	3.6dB
	+20dBm	70dB	1,2,4	3.6dB
	+10dBm	60dB	1,2	3.6dB
	0dBm	50dB	2,3	3.6dB
	-10dBm	40dB	2,4	3.6dB
	-20dBm	30dB	2	3.6dB
	-30dBm	20dB	3	3.6dB
	-40dBm	10dB	4	3.6dB
	-50dBm	0dB	0	3.6dB
	-60dBm	0dB	0	13.6dB
	-70dBm	0dB	0	23.6dB
	-80dBm	0dB	0	33.6dB

In positions 4 thru 12 (-SODBM thru +30DBM), A3R11 is bypassed for maximum feedback. The gain of the amplifier in these nine positions is a constant 2.5dB. Potentiometer A3R26 is for calibration of the -80DBM range, 600 ohm function. Resistor A3R27 is used to maintain a charge on A3C22 to prevent transients when changing ranges.

4-23. In order that the meter always indicate in DBM regardless of the impedance selected, additional gain switching must be performed. When the 75 function is chosen, A3K2 energizes and places A3R16 in parallel with A3R14 and A3R15. This reduces the negative feedback (with respect to the 600 function) and increases the amplifier gain by 9dB. When the 135 function is selected, A3R22/R23/R24 are connected in series with A3R16. This combination is then in parallel with A3R14 and A3R15, reducing the feedback and increasing the amplifier gain by 6.4dB with respect to the 600 function. When the 900 function is depressed, A3R17, A3R19 and A3R20 are connected in parallel with A3R13, increasing the negative feedback and

reducing the amplifier gain by 1.7dB. Relays A3K1 thru A3K3 are controlled by the FUNCTION switch when any of the impedance functions except 600 are selected.

4-24. Transistors A3Q1 and A3Q2 form a differential amplifier. The signal is taken from the collector of A3Q1, amplified by A3Q4 and A3Q5 with A3Q5 providing feedback to the base of A3Q2. Transistor A3Q3 provides isolation between A3Q2 and A3Q4 to prevent undesired feedback. This results in a greater bandwidth than could be achieved without its use. The output signal is coupled through A3R17 and A3C10 to the INPUT switch.

4-25. FILTERS. (Schematic No. 3)

4-26. The 3555B contains a 3kHz FLAT weighting filter, a C MSG weighting filter, a PROG weighting filter and a 15kHz FLAT weighting filter. These active filters consist of five amplifiers with controlled feedback for waveshaping. They are used in combinations to form each of the filters (refer to Figure 7-1).Since all of these amplifiers are

Figure 4-4. Simplified Average Detection

4-4