SERVO AMPLIFIER

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

per 0.1-MHz digit. In a like manner, the output voltages

from the 1.0-MHz voltage amplifier (U2B) and the 10.0-

is zero. Resistor R70 combines the position feedback

MHz voltage amplifier (U1B) vary from about 3.56 to

signal with the d/a analog signal in such a manner as to

about 7.60 volts as the corresponding radio control

compensate for nonlinear tracking characteristics of the

frequency is changed from 0 to 9.

rf filter.

4-48. The 200/300-MHz voltage amplifier (U2A)

4-52. The d/a servo potentiometer comparator (U7)

amplifies the output voltage from the reference voltage

develops the servo error signal by comparing the

amplifier. Resistor R41 varies the amplifier gain when it

amplified position feedback signal with the d/a converter

is connected to ground through transistor Q13. When

analog signal. Diodes CR19 and CR20 limit the input to

U7 to approximately 0.7 Vdc. Capacitor C12 in parallel

the 100-MHz frequency select line is changed from logic

0 (200 MHz) to logic 1 (300 MHz). transistor Q13 turns

with resistor R77 provides lag for servo amplifier stability.

from off to on to ground resistor R41. This causes the

output voltage from the 200/300-MHz voltage amplifier to

4-53. The servo error amplifier (US) amplifies the

vary from about 3.56 to about 8.05 V dc. Adjustable

servo error signal from the d/a servo potentiometer

resistor R42 varies the output voltage to compensate for

comparator (U7) to develop either a positive or negative

the dc offset of U2A.

output voltage to the servo motor drive circuits (Q15

through Q23). When the output voltage is positive, the

4-49. The summing network (R45) and the voltage

down frequency servo drive circuit (Q15, Q22, Q18)

swing amplifier (U4B) together form a summing amplifier

applies voltage to the servo motor to position the rf filter

that sums and amplifies the output voltages from the four

down in frequency. When the output voltage from U8 is

voltage amplifiers (U1A, U2B, U1B, and U2A) and the

negative, the up frequency servo drive circuit (Q17, Q16,

reference voltage amplifier (U3B). When summed

Q23, Q20) applies voltage to the servo motor to position

together, the five input voltages produce an output

the rf filter up in frequency. Servo motor voltage applied

voltage from the voltage swing amplifier that varies from

through resistors R84 and R86 provides negative

about +2.07 volts for a frequency of 225.0 MHz to about -

feedback to U8 to control the gain of the overall servo

5.75 volts for a frequency of 399.9 MHz in approximately

amplifier. For large servo error signals, the servo

4.47-millivolt steps per 100 kHz. Diode CR15 provides

amplifier applies about 13 V dc to the servo motor. As

temperature compensation for the d/a converter.

the servo error signal approaches the null, the servo

amplifier reduces the voltage applied to the servo motor

4-50. The dc centering amplifier (U4A) sums and

to stop the servo motor and prevent overshoot when the

amplifies the voltage swing amplifier (U4B) output

null is reached.

voltage, the reference voltage amplifier output voltage,

and the +12and -12-V dc power supply voltages. The

4-54. Rf filter tuning occurs as follows. The rf filter

+12and -12-V dc power supply voltages are applied to

position with respect to frequency is calibrated so that

the dc centering amplifier to compensate for changes in

the output voltage from the position feedback

the reference voltages. When summed together. the

potentiometer (A7R20) when amplified by a factor of 2

four input voltages produce an output voltage from the dc

(U6 output) tracks (equals) the output voltage from the

centering amplifier that varies from about -8 V dc for a

d/a converter (U4A output) for each 100-kHz frequency

frequency of 225.0 MHz to about +7.995 V dc for a

step between 225.0 and 399.9 MHz. Therefore, to tune

frequency of 399.9 MHz in approximately 9-millivolt steps

the rf filter, it is only necessary to drive the servo motor

per 100 kHz. To obtain this voltage range versus

which drives the position feedback potentiometer in the

frequency, variable resistor R47 allows adjustment for a

direction that equalizes the two voltages at the input to

total output voltage swing of about 15.995 V dc when the

d/a servo potentiometer comparator U7. When the

frequency is changed from 225.0 to 399.9 MHz, and

voltages are equal, the servo error signal nulls to stop

variable resistor R50 allows adjustment to center the

the servo motor. When this occurs. the rf filter is tuned

output voltage at about -1.15 V dc for a frequency of

to the same frequency as the radio control frequency.

299.9 MHz. The analog output voltage from the dc

centering amplifier is applied to the servo amplifier.

4-55. When the radio control is set to a higher

Each 9millivolt step between -8 and +8 V dc represents a

frequency the d/a analog voltage at the non-inverting

specific radio control frequency.

input of U7 becomes positive with respect to the position

feedback voltage at the inverting input of U7. This

4-51. SERVO AMPLIFIER. Refer to figure FO-18. The

results in a positive servo error signal to the inverting

servo potentiometer amplifier (U6) amplifies the position

input of the servo error amplifier, U8. The resulting

feedback signal by a voltage gain of about 2. Variable

negative output voltage from U8 reverse biases

resistor R73 allows adjustment of the gain of U6.

transistors Q15 of the down frequency servo drive circuit

Potentiometer R74 allows adjustment for zero output

(Q15, Q22, Q18) and forward biases transistor Q17 of

voltage from U6 when the input voltage

the up frequency servo drive circuit (Q17, Q16, Q23,

Q20). When Q17 is forward biased, transistors

4-9