Regulator And Power-Control Ic; Figure 3-6. Transmitter Attack Time - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

3-14

3.2.14 Regulator and Power-Control IC

The regulator and power-control IC (RPCIC), U500, contains internal circuitry for the 9.6-V regulator
and the switched +5-V regulator. Refer to Section
theory of operation.
The power-control section of the device is responsible for maintaining a constant RF output power. A
directional coupler and detector network, located within the RF power amplifier circuit, rectifies the
sensed forward power from the last RF gain stage. The detected voltage is routed back to the
command board control circuitry (U500) via P503, pin 8. The voltage is then coupled through a buffer
amplifier and summed, through a resistor network (R509, R508, and R507), with the transmit power
set voltage (U500, pin 6) and the temperature sense voltage. The resulting voltage is applied to the
control amplifier's inverting port (U502, pin 2) for automatic RF gain control.
The U500 current-sense inputs, pin 37 (sense +) and pin 38 (sense -), are sourced from the
current-sensing resistor on the RF power amplifier. The two inputs are applied to a differential
amplifier internal to the RPCIC. The current limit is set by a software-programmable D/A device
(U502) that causes a cut back in RF output power when the set limit is exceeded.
The transmitter attack and off times are software programmable to meet domestic and international
specifications. Transistors Q514 and Q515 are controlled by a serial shift register (U530). The
transistors, when turned on (logic 1 input) cause the output of Q504 (the PA control line) to ramp up
slowly to prevent an abrupt RF PA turn-on. The slower rate is required to meet international spurious
requirements. When the transistors are turned off, the attack times return to a standard domestic
response with fast rise times. Refer to
September 5, 2008
Figure 3-6
Trigger Standard spec.
1.87 mS
T1 T2

Figure 3-6. Transmitter Attack Time

Theory of Operation: Command Board
3.2.4, "Regulators," on page 3-10
for attack time diagrams.
European spec.
MAEPF-25186-O
for detailed
6881076C25-E