Figure 3-21. Regulator/Power Control Ic Block Diagram - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Theory of Operation: Power Amplifiers
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).
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
FROM R9875
39
KEYED 9.4V INPUT
40
Control Voltage Limiter
R3813 and R3814 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
25/10-Watt VHF PA, this maximum value is 9.2 V. This voltage-control limit is set by the values of
R3813 and R3814.
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.
6881076C25-E
Q538
RPCIC ENABLE
28 27 26 25 24
+
9.6 VOLT
REGULATOR
PA ENABLE
U500
WIDE-BAND
CONTROL AMP
ONE-
ENABLE
Q
SHOT
+
AMP
+
CURRENT
LIMIT SET
BUFFER
+
POWER CONTROL PACKAGE FLAG
GROUND
GROUND
41 42 43 44
REGULATOR/POWER CONTROL IC U500

Figure 3-21. Regulator/Power Control IC Block Diagram

23 22 21 20
19
PACKAGE
GROUND
+
5V REGULATOR
+
THERMISTER
BUFFER
DIRECTIONAL
COUPLER BUFFER
1 2 3 4 5
5V
18
17
9.6V SENSE INPUT
16
5V DRIVE TO Q502
15
5V CURRENT SENSE
14
5V FEEDBACK
13 TEMPERATURE SENSE
INPUT
TEMPERATURE SENSE
OUTPUT TO 500-2
12
11
+
POWER SET FROM
U502 1.5V 5 VOLTS
10
FORWARD DET.
9
VOLTAGE
+
8
FORWARD BUFFERED
OUT
POWER SET OUT TO
PIN 2 VIA R507
7
POWER SET
BUFFER
6
TO PIN 10
U502 DAIC
September 5, 2008
3-61
RESISTIVE
SUMMING
NETWORK
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68K
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100K
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