Reference Amplifier

TM 11-6625-524-14-2

5-10. Reference Amplifier

followers 3A2Q3 and 3A2Q4. The amplifiers

3A2Q5 and 3A2Q6 amplify this square wave, and

it is coupled to the synchronous demodulator via

The 94-Hz square wave from the modulator driver

is sampled by the amplifier stages via emitter

3A2T1 .

Figure 5-7. Reference amplifier, simplified schematic diagram.

5-11. Dc Feedback and Calibration Controls

of 3A2T2 and filter capacitor 3A1C6. For low

input levels, 50 percent of full scale and lower, the

A portion of the dc output voltage from the

signal passes through 3A3R22, 3A3R21,

synchronous demodulator is fed back to the dc

3A3CR6, 3M1, and 3A3R5. At low levels, the

modulator to provide the excellent gain stability

resistance of 3A3CR6 is fairly high and the

of the voltmeter. This feedback ratio is deter-

c u r r e n t through the circuit is limited. By

mined by values of a resistor and potentiometer

paralleling 3A3R21 and 3A3CR6 with 3A3R17

connected in series and switched through section

and 3A3R25, an exact adjustment of the meter to

H of range selector switch 3S2. One of the three

a point corresponding with the input voltage is

thermistors mounted in the rf probe is switched in

possible. For higher levels, 70 percent through

series with the previous combination by means of

100 percent of full scale, 3A3R1, 3A3R35, and

sections C and E of 3S2. The dc feedback con-

3A3VR2 paralleled with 3M1 and 3A3R5 provide

trols, marked mid on the calibration plate, are

adjustment of the meter to correspond with the

used for accurate calibration of the midscale for

incoming signal. To adjust the midscale point,

each range. The appropriate feedback ration is

the main dc feedback resistors are utilized.

selected when switched to the desired range.

b. Indicating meter 3M1 employes a taut band

5-12. Output, Linearizing Networks, and Meter

movement that is rated at 0-500 microampere (ua)

Circuit

dc. It also utilizes a mirror-backed scale to

The dc output of the diodes in the probe is

minimize parallax errors.

basically nonlinear with respect to the rf input

c. Resistor 3A3R5, in series with 3M1, is

l e v e l . In order to achieve a linear output

connected to the circuit via 3J2. For external

characteristic, correction networks are employed.

monitoring purposes, when a plug is inserted in

Their function is to expand the lower end and

3J2, 3A3R5 is switched out of the circuit and the

compress the upper end of the meter scale. Refer

external monitor must provide the 2K ohms series

to figure 5-8 for better understanding of the

load. This value should be within 1 percent to

circuit. The switch is at the 0.001 volt scale.

maintain the rated calibration accuracy of the

a. The rectified dc, proportional to the dc input

voltmeter.

signal, appears at the center tap of the secondary

5-6