D/A SERVO AMPLIFIER MODULE Al.

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

front panel of the receiver-transmitter. Depending upon

the frequency displayed, each switch applies either -0.7-

servo fault circuit provides a tune-in-progress indi- cation

V dc power supply voltage or an open circuit to the

or an indication of invalid circuit conditions.

cathode of the diodes. When connected to pullup

resistors in the d/a servo amplifier module, each

4-45. LOCAL/REMOTE

FREQUENCY

SELECT

frequency select line applies either a logic 0 or logic 1

LINES. Refer to figure FO-18. Pullup resistors R1

voltage to the receiver-transmitter.

through R13 located in the d/a servo amplifier module

provide termination for the local and remote frequency

4-41. The receive signal flow diagram of figure FO-16

select lines. The resistors convert an open circuit

shows a typical (10-MHz) frequency select line for the

frequency select line into a logic 1 voltage and a

radio set. The choice between local frequency control or

grounded (actually about -0.7 V dc) frequency select line

remote frequency control is provided by the

into a logic 0 voltage. Diodes CR1 through CR13 isolate

REMOTE/LOCAL switch (A10A1S3).

When the

the local (front panel) frequency select switches from the

REMOTE/LOCAL switch is in the REMOTE position, the

remote (C-7999/ GRC-171) frequency select switches.

-0.7-V dc power supply voltage is applied to the

Isolation diodes for the remote frequency select switches

frequency select switches of the C-7999,/GRC-171.

are located in the C-7999/GRC-171.

Refer to

Depending upon the frequency selected, the C-7999/

paragraphs 4-40 and 4-41 for further discussion of

GRC-171 applies either a logic 0 or logic 1 voltage (note

frequency select lines.

pullup resistor in the d/a servo amplifier module) to the

frequency select lines going to the frequency synthesizer

4-46. DIGITAL-TO-ANALOG CONVERTER. Refer to

module and to the d/a servo amplifier module. When the

figure FO-18. The d/a converter consists of four voltage

REMOTE/LOCAL switch is in the LOCAL position, the -

amplifiers (U1A, U2B, U1B, U2A) that amplify the output

0.7-V dc power voltage is applied to the local frequency

voltage from a reference voltage amplifier (U3B). The

select switches on the front panel of the receiver-

gain and, therefore. the output voltage of each voltage

transmitter. Depending upon the frequency selected, the

amplifier is varied by connecting resistors (R27 through

local frequency select applies either the -0.7-V dc power

R32 and R41) to ground through transistor switches (Q1l

supply voltage or an open circuit to the frequency select

through Q13) that are turned on by logic 1 voltage of the

lines to the d/a servo amplifier module. Isolation diodes

applied bcd frequency select information. The output

and pullup resistors in the d/a servo amplifier module

voltage from each voltage amplifier is summed together

convert the -0.7-V dc voltage or open circuit into a logic 0

(R45) and amplified (U4B, U4A) to produce the analog

or logic I voltage. respectively, and apply the result to

voltage to the servo amplifier. The analog voltage varies

the d/a converter and to the frequency select lines to the

from about -8 V dc for a radio control frequency of 225

frequency synthesizer module.

MHz to about +8 V dc for a frequency of 400 MHz. The

following describes individual circuits of the d/a

4-42.

D/A SERVO AMPLIFIER MODULE Al.

converter.

4-43. Refer to the block diagram of figure FO-4 and

4-47. The 0.1-MHz voltage amplifier (U1A). 1.0-MHz

schematic diagram of figure FO-18 while reading the

voltage amplifier (U2B), and the 10.0-MHz voltage

following circuit descriptions.

Unless otherwise

amplifier (UIB) all function identically. Therefore. only the

specified, reference designators apply to components of

0.1-MHz voltage amplifier will be discussed in detail.

the d/a servo amplifier module Al.

Resistors R27 through R30 that are connected to the

inverting input of the 0.1-MHz voltage amplifier vary the

4-44. GENERAL. Refer to figure FO-4. D/a servo

amplifier gain when they are connected to ground

amplifier module Al contains the digital-to-analog (d/a)

through transistors Q1, Q2, Q3, and Q4. The transistors

converter and the servo amplifier that controls the servo

turn on in accordance with 0.1-MHz bcd frequency select

mechanism in rf filter module A7 to tune the rf filter. The

information applied to the d/a servo amplifier module.

d/a converter changes bcd frequency select information

The transistors are connected in the reverse beta mode

from the radio control into a d/a analog control signal that

so that their collector-to-emitter voltage is approximately

represents the frequency as selected on the radio

0 V dc when turned on by a logic 1 applied to their base.

control. The servo amplifier receives the d/a analog

The reference voltage amplifier (U3B) amplifies +12-V

control signal and compares it to a position feedback

and -12-V dc power supply voltages to produce a

signal from the position feedback potentiometer in the rf

reference voltage (approximately +3.56 V de) that is

filter module that represents the tuned frequency of the rf

applied to the non-inverting input of the 0.1-MHz voltage

filter. When the two signals differ (frequencies differ), the

amplifier. As the radio control 0.1-MHz frequency digit

servo amplifier drives the servo mechanism. to position

(bcd frequency select information) varies from 0 (0000)

the rf filter to the desired frequency. The servo amplifier

to 9 (1001), the 0.1-MHz voltage amplifier amplifies the

disable circuit prevents the servo mechanism from being

reference voltage to produce an output voltage that

driven past its end stops. The

varies from about 3.56 to about 7.60 volts in

approximately 0.447-volt steps

4-8