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D/A SERVO AMPLIFIER MODULE Al.
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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SERVO AMPLIFIER DISABLE


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


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