Power Control - Motorola MCS 2000 Service Instructions Manual

Controller Section Theory of Operation: Power Control

Power Control

(Refer to schematic Page 10-24 for reference)
The power control loop regulates RF output power with an automatic level control (ALC) loop and
provides protection features against overcurrent, excessive control voltage, and high operating
temperatures. Power and current limit are adjusted under microprocessor control using a Digital to
Analog (D/A) converter (U0551). Control voltage limit is set by resistor ratio on the transmitter, or D/A
output for those radios that must minimize adjacent channel splatter. The D/A adjustable control
voltage limit increases transmitter rise time and reduces adjacent channel splatter as it is adjusted
closer to the actual operating control voltage.
The microprocessor controls K9.1 enable (K9.1 ENB) to bias the PA and antenna switch, PA disable
(PA DIS) to disable the PA control voltage, and power range (PWR RANGE) to adjust the number of
D/A steps per watt.
NOTE: The Power Range Mode is not used on all models.
The ALC loop regulates power by adjusting the PA control line PA CNTL to keep the forward power
voltage V FORWARD at a constant level. V FORWARD is amplified with a gain set by the PWR
RANGE signal (described below) and the PA PWR SET output U0551 pin 2 through resistors R0577,
R0553 and R0554. The result is connected to opamp inverting input U0550 pin 9 which is compared
with a 4.65 volt reference present at noninverting input U0550 pin 10. The 4.65 volt reference is set
by a divider circuit connected to ground and 9.3 volts by resistors R0567 and R0569.
The power range line PWR RANGE controls the gain of the V FORWARD amplifier. For high power
mode operation (i.e. VHF/UHF 110W models, high power mode is 110W. For VHF/UHF 40W
models, high power mode is 40W) the PWR RANGE line is set to a logic 0 (<1.0V) and the resulting
V FORWARD gain is 1+ (R0564/R0573). For low power mode operation (i.e. VHF/UHF 110W
models, low power mode is 50W. For VHF/UHF 40W models, low power mode is 25W) the PWR
RANGE line is set to a logic 1 (>4.0V) and the resulting V FORWARD gain is 1+(R0564/RP) where
RP equals R0563 in parallel with R0573.
The PA disable line PA DIS prevents transmitter operation by keeping the PA control voltage PA
CNTL near zero volts. On radios that are required to minimize adjacent channel splatter, a control
voltage limit line PA CNTL LIM is pulled to ground by transistor Q0556 when PA disable is high. This
effectively makes the control voltage limit equal to zero and pulls the 4.65 volt reference at
noninverting input U0550 pin 10 to ground through transistor Q0551. The ALC opamp output at
U0550 pin 8 is prevented from rising above zero since the noninverting input is grounded. On radios
that are not required to minimize adjacent channel splatter the transmitter is disabled through
transistor Q0551 which pulls the PA control voltage PA CNTL directly to ground.
During normal transmitter operation the voltages at the opamp inputs U0550 pins 9 and 10 should be
equal to 4.65 volts and the PA control voltage output at pin 8 should be between 4 and 7 volts. If
power falls below the desired setting, V FORWARD decreases, causing the inverting input at U0550
pin 9 to decrease, increasing the opamp output at U0550 pin 8 and increasing the PA control voltage
PA CNTL until V FORWARD increases to the desired level. The D/A output voltage PA PWR SET at
U0551 pin 2 adjusts the power set point. V FORWARD reaches a steady state voltage such that the
current through the R0554 branch equals the current through the R0553-R0577 branch. As PA PWR
SET decreases, transmitted power must increase to make V FORWARD larger and keep the
inverting input U0550 pin 9 at 4.65 volts.
Loop frequency response is controlled by opamp feedback components R0570 and C0568 and the
output lowpass filter R0571 and C0569.
68P81083C20-D
7-7
December 6, 2004