Power Control Circuitry; Figure 3-26. Rpcic Block Diagram - Motorola ASTRO Digital Spectra Service Manual

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3-74

3.7.2.2.3 Power Control Circuitry

Command Board Circuitry
Inside U500, the Regulator Power Control IC
inverting inputs, and one non-inverting input (at pin 44) which is the reference input for the entire
power control loop of the power amplifier. The 3.2-V reference voltage at U500-44 is produced by
dividing SW +5-V with the voltage-divider circuit, R514 and R515.
28
29
N.C.
30
N.C.
31
REGULATOR
GROUND
32
13.8V
A+
33
TX P.A. ENABLE
34
35
LOCK
SHOT
36
CURRENT SENSE
CURRENT SENSE +
37
+
FROM R9875
CURRENT SENSE
38
CURRENT
FROM R9875
LIMIT SET
BUFFER
+
39
KEYED 9.4V INPUT
POWER CONTROL
40
The power control loop is controlled by the microprocessor U204 on the VOCON board. Through the
SLIC IC U206, the microprocessor enables the RPCIC by pulling TX PA ENABLE (U500 pin 33) low
while the radio synthesizer is locked (U500 pin 35). U520 writes data to a digital-to-analog converter,
U502, to change and control the power-set voltage from pin 10 of U502 to pin 6 of U500. The voltage
on this line, 1.5 to 5 V, will be inversely proportional to the power out of the PA, with 5 V producing
the lowest power output. This voltage may be set with RSS (Radio Service Software) or CPS
(Customer Programming Software).
July 1, 2002
Q538
RPCIC ENABLE
27
26
25
24
23
+
9.6 VOLT
REGULATOR
PA ENABLE
U500
WIDE-BAND
ONE-
CONTROL AMP
ENABLE
Q
+
AMP
PACKAGE FLAG
GROUND
GROUND
41
42
43
44
1
5V
REGULATOR/POWER CONTROL IC U500

Figure 3-26. RPCIC Block Diagram

Theory of Operation: Power Amplifiers
(Figure
3-26), is an operational amplifier that has four
22
21
20
19
18
PACKAGE
GROUND
+
5V REGULATOR
+
THERMISTER
BUFFER
+
DIRECTIONAL
COUPLER BUFFER
+
POWER SET
2
3
4
5
6
17
9.6V SENSE INPUT
16
5V DRIVE TO Q502
15
5V CURRENT SENSE
14
5V FEEDBACK
13
TEMPERATURE SENSE
INPUT
TEMPERATURE SENSE
R508
OUTPUT TO 500-2
68K
12
11
R516
POWER SET FROM
100K
10
U502 1.5V
5 VOLTS
FORWARD DET.
9
VOLTAGE
8
FORWARD BUFFERED
R509
OUT
68K
R507
POWER SET OUT TO
47K
PIN 2 VIA R507
7
BUFFER
TO PIN 10
U502 DAIC
68P81076C25-C
RESISTIVE
SUMMING
NETWORK
MAEPF-22034-O

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