Figure 3-25. Rpcic Block Diagram - Motorola ASTRO Digital Spectra Service Manual

3-70
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).
28
29
N.C.
30
N.C.
31
REGULATOR
GROUND
32 13.8V
A+
33
TX P.A. ENABLE
34
35
LOCK
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
Control Voltage Limiter
R5807 and R5808 form a voltage divider that connects to control voltage drive. The output of this
voltage divider is connected to the control-voltage-limit input (pin 4) of the RPCIC. If the voltage at
this input reaches 3.2 V, then the control voltage will be clamped to a maximum value. For the
high-power UHF PA, this maximum value is 10 V. This voltage control limit is set by the values of
R5807 and R5808.
Current Limiter
U204, the processor on the VOCON board, sends data to U502, the digital-to-analog converter, to
properly set the voltage on U502, pin 15, which is the TX CURRENT LIMIT control line to the RPCIC
(U500, pin 40). Sixteen different voltages, ranging from 1.5 to 4.5 V, can be programmed from U502.
July 1, 2002
Q538
RPCIC ENABLE
27 26 25 24 23
+
9.6 VOLT
REGULATOR
PA ENABLE
U500
WIDE-BAND
CONTROL AMP
ONE-
ENABLE
Q
SHOT
+
AMP
+
PACKAGE FLAG
GROUND
GROUND
41 42 43 44 1
5V
REGULATOR/POWER CONTROL IC U500

Figure 3-25. RPCIC Block Diagram

Theory of Operation: Power Amplifiers
22 21 20 18
19
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
TEMPERATURE SENSE
13
INPUT
TEMPERATURE SENSE
R508
OUTPUT TO 500-2
68K
12
11
R516
POWER SET FROM
100K
U502 1.5V 5 VOLTS
10
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