Motorola Radio Mcs2000 Vol 1 68p81083c20 A Manual

							 
Radio Tuning Procedure6-9 
Select 5 Transmit 
Deviation
(GVN6011 only) 
This tuning sequence is ONLY for radios with Conventional 
Select 5 Signaling software being tuned with the RSS 
GVN6011. 
The Select 5 Deviation Softpot is used to tune the FFSK signaling deviation. 
Tuning is performed at one frequency and for 25 kHz channel spacing. The 
radio generates a Select 5 signal for tuning. Values for other frequencies and 
channel spacings are calculated by the radio software.
Alignment of the Transmit Deviation Limit Reference MUST immediately 
PRECEDE the Select 5 Alignment Procedure.
1.From the RADIO TUNE menu, press F4 to select SIGNALING alignment.
2.Press F5 again to select the Select 5 softpot.
3.Press F6 to key the radio on the test frequency. The screen will indicate 
that the radio is transmitting.
4.Measure the Select 5 deviation on your service monitor.
5.Use the UP/DOWN arrow keys to adjust the Select 
   
5 deviation to be 
between 3.30 and 3.70 kHz.
6.Press F6 again to dekey the radio.
7.Press F8 to program the softpot value; press F10 twice to return to the 
SERVICE menu. 
Squelch 
The squelch softpots set the signal to noise ratio at which the squelch opens. 
The 25/30 kHz squelch value needs to be set at 7 frequencies across the 
frequency range. In 800 MHz and 900 MHz radios the 6th and 7th frequency 
are always the same, so only 6 frequencies are being tuned. For some models, 
the 20 kHz and 12.5 kHz values are then set at one frequency and the other 
frequency settings are calculated by the radio software. For other models the 
20 kHz and 12.5 kHz squelch settings are also tuned for all 7 frequencies across 
the band.
1.From the SERVICE menu, press F3 to select RECEIVER alignment.
2.Channel spacing determines the next key press. Press F4, F5, or F6 (as 
applicable) to select the SQUELCH softpot.
3.Select the Þrst test frequency shown, and adjust the UP/DOWN arrow 
key to the minimum squelch value.
4.W
ith Test Box RLN4460A: Set the RF test generator to the test frequency 
plus the following offset: VHF: +200 Hz,   UHF: +200 Hz, 800 MHz: 
+500 Hz.
W
ith Test Box GTF180B: Set the RF test generator to the test frequency 
and modulate the signal generator at 3.0 kHz deviation, 1 kHz tone. 
Adjust the generator for an 8-10 dB SINAD level.
5.Adjust the UP/DOWN arrow key until the squelch just closes.
6.Monitor for squelch chatter; if chatter is present, repeat step 4. 
7.When no chatter is detected, press F8 to program this value. Press 
ÒENTERÓ to select next softpot adjustment.
8.Repeat steps 3 - 6 for all test frequencies shown on the screen.
9.Press F10, F10 to return to the Service menu.
Note
Note 
						

							 
6-10Radio Tuning Procedure 
Rated Audio 
Alignment of the Rated Audio is not required on these radios. 
Front-End Pre-
Selector 
Alignment of the front-end pre-selector is not required on these radios. 
						

							 
Controller Section Theory of Operation 7-1 
Controller Section
Theory of Operation 
7 
Introduction 
The radio is a microcontroller-based dual mode (trunked/conventional) 
transceiver. The microcontroller determines the active state of the radio 
(transmit/receive), monitors radio status, and processes operator commands 
entered from the keypad or the other radio controls.
In the Low and Mid Power versions of the radio all radio circuitry is contained 
on a single rigid printed circuit board (PCB). In the High Power version of the 
radio, the physical arrangement of the circuitry is the same except the power 
ampliÞer (PA) part of the transmitter is contained on a separate PCB.
The radio is composed of the following three major functions:
¥ RF (Receiver and Transmitter)
¥ Controller (Main Controller and DC Power Control and Regulation
¥ Control Head
The RF section is band speciÞc circuitry described in one of the frequency 
range speciÞc Volume 2s depending on the band split (See ÒRelated 
PublicationsÓ on page 1-1).
The Control Head connects to the radio via an 18 pin connector. There are 
three different control head types of varying complexity: Model I, Model II 
and Model III. Control Head information is covered in this volume of the 
manual. 
Theory 
Overview 
The Controller section is physically separated into two functional sections, i.e. 
the Main Controller and the DC Power Control and Regulation.
The controller section of the radio is a microcomputer-based processor that 
controls the radio in accordance with its built in programming as well as 
commands input manually by the radio operator. The radio operator inputs 
manual commands to the controller section using the push-button and other 
controls located on the radio control head. In addition to its controlling 
functions, the controller section provides audio ampliÞcation of the audio 
output signal from the receiver section. It also controls and processes a squelch 
signal output from the receiver back end section to control the receiverÕs audio 
squelch function. 
						

							 
7-2 Controller Section Theory of Operation 
The radio controller consists of 5 main functional sections. These are: 
¥ voltage regulation
¥ power control
¥ interfacing
¥ microprocessor operation
¥ audio and signalling processing
The controller is based on the Motorola 68HC11F1 microprocessor (U0103). 
In addition to the microprocessor, the controller has 3 external memory 
devices, signal MUX IC (U0105), and a Support Logic IC (SLIC IV, SLIC IVa, or 
SLIC V, U0104). The 3 memory devices consist of a 32 Kbyte EEPROM (U0100), 
32 Kbyte SRAM (U0101), and a 256Kbyte or 1MByte FLASH EEPROM (U0102).
The digital section consists of a microprocessor, support memory, support 
logic, signal MUX ICs, the On/Off circuit, and General Purpose Input/Output 
circuitry. 
Voltage 
Regulation 
(Refer to schematic page 10-29 or 10-30 for reference) 
Voltage Regulators 
Voltage regulation for the controller is provided by 4 separate devices; U0500 
(LM2925) +5V, U0501 (LM2941) +9.3 V, U0502 +8V, and UNSW 5V (a 
combination of R0004 and VR0001). Additional regulators are located in the 
RF section and the control head/handset.
Voltage regulation providing 5V for the digital circuitry is done by U500. Input 
and output capacitors (C0500/0501 and C0503/0504) are used to reduce high 
frequency noise and provide proper operation during battery transients. This 
regulator provides a reset output (pin 5) that goes to 0 volts if the regulator 
output goes out of regulation. This is used to reset the controller during power 
on to prevent improper operation. C0502 sets the duration of this reset signal 
to a minimum of about 12 ms.
Regulator U0501 is used to generate the 9.3 volts required by some audio 
circuits, the RF circuitry and power control circuitry. Input and output 
capacitors (C0505/0506 and C0508/0509) are used to reduce high frequency 
noise. R0503/R0504 sets the output voltage of the regulator. If the voltage at 
pin 1 is greater than 1.2 volts the regulator output decreases and if the voltage 
is less than 1.2 volts the regulator output increases. This regulator output is 
electronically enabled by a 0 volt signal on pin 2. Q0500 and associated 
circuitry (R0500/0501/0502 and C0507) are used to disable the regulator when 
the radio is turned off.
Regulated 8V is provided by U0502. It provides a separate supply to the 
microphone circuitry. The microphone cannot be supported directly from A+/
SWB+ because any ßuctuation on A+ would be impressed on the microphone 
line and affect transmitted audio. The microphone circuitry can not be 
connected to the other regulators because, there is a function which allows the 
radio to be switched on by momentarily grounding the microphone line (see 
Electronic On/Off). 
						

							 
Controller Section Theory of Operation 7-3  
UNSW 5V is only used in a few areas which draw low current and requires 5 V 
while the radio is off. 
Electronic On/Off 
Unlike older radios whose on/off feature was controlled only by the 
mechanical on/off switch, this radio has circuitry which allow radio software 
and/or external triggers to turn the radio on or off without direct user action. 
Some examples of this usage are 1) automatic turn on when emergency switch 
is engaged; 2) automatic turn on when ignition is sensed and off when 
ignition is off; 3) radio deafÞliation, with certain features the radio can be 
programmed to stay on for a brief period after the user has Òturned it offÓ. 
During this Òextra onÓ period the radio may transmit a signal to a base station 
letting the station know that the radio has been turned off.
Q0511 is used to provide SW B+ to the various radio circuits. Q0511 acts as an 
electronic on/off switch where pin 4 acts as the control pin. The switch is on 
when pin 4 is low and off when pin 4 is high. When the radio is off Q0510 is 
cutoff and the voltage at Q0511-4 is at A+. This effectively prevents current 
ßow through Q0511 from pins 2/3 to 5/6/7/8. When the radio is turned on the 
voltage at the base of Q0510 is high (at least above 3V) and Q0510 switches on 
(saturation) and pulls down the voltage at Q0511-4 to ~0.07V. With FET 
Q0511 now ÒenabledÓ current ßows through the device. This path has a very 
low impedance (less than 1 ohm) from pins 2/3 to 5/6/7/8. This effectively 
provides the same voltage level at SWB+ as at A+.
The electronic on/off circuitry can be enabled by the microprocessor, the 
emergency switch, the mechanical On/Off switch on the control head, or the 
ignition sense circuitry. If any of the 4 paths leading to the base of Q0510 
provides a high voltage through diodes CR0510 or CR0511, the electronic 
ÒONÓ is engaged. 
Emergency 
(Refer to schematic 
Page 10-26 for 
reference) 
The emergency switch, when not engaged, normally grounds the base of 
Q0414. When the emergency switch is pressed this connection is opened and 
the base of Q0414 is pulled high by R0437. This causes the collector of 
transistor Q0414 to go low (.2V), thereby setting the EMERGENCY IN input of 
U0510 low. U0510 is a dual 555 timer conÞgured as two Òpulse stretchersÓ, one 
for the Emergency input to electronically turn on the radio and the other for 
the mechanical On/Off switch on the control head to cause the radio to 
electronically switch on.
With regard to emergency, when U0510-8 EMERGENCY IN is above 1/3 of the 
supply voltage at U0510-14, that causes the device to internally ground 
U0510-13, which in turn holds capacitor C0513 in a discharged state. The 
device also keeps U0510-9 EMER TRIG at a low voltage. When EMERGENCY 
IN goes below 1/3 of the device supply voltage, U0510 disables the discharge 
mechanism at pin 13 allowing the capacitor C0513 to charge through R0513. 
The charging rate is set by the values of R0513 and C0513. When the voltage 
at pin 12 goes above 2/3 supply voltage, the device sets pin 9 EMER TRIG high 
which in turn switches on Q0510 enabling the radio to turn on to begin 
emergency operation. U0510 will keep pin 12 high for a period set by R0513 
and C0513 which for 200K/1 uF equals 220 ms.
While SW B+ is on for 220 ms, the microprocessor starts execution, reads that 
the emergency input is active, and sets the B+ CNTL output to a logic high. 
This high will keep Q0510 switched on through CR0510 beyond the 220 ms 
period after which U0510-9 will return to a low level. This operation allows a 
momentary press of the emergency switch to power up the radio. When the  
						

							 
7-4 Controller Section Theory of Operation 
microprocessor has Þnished processing the emergency press, it sets the B+ 
CNTL output to a logic 0. This turns off Q0511 and the radio turns off. Notice 
that EMER TRIG also goes to U0104-D5 and U0003-F2. This is where the 
appropriate microprocessor is alerted to the emergency condition. If the radio 
was already on when emergency was triggered then B+ CTRL would already be 
high and EMER TRIG going high due to the emergency condition would have 
no effect on Q0510, i.e. switching on Q0511 when it is already on. 
Mechanical 
On/Off 
(Refer to schematic 
Page 10-26 for 
reference) 
This refers to the typical on/off switch, located on the control head, and which 
turns the radio on and off.
U0502 is fed by A+, and is therefore always on and providing 8 V on MIC 
VOLTAGE. When the Mechanical on/off switch is pressed, the MIC IN line is 
momentarily grounded setting the ON/OFF input of U0510-6 low.
Similar to the emergency operation described previously, when U0510-6 ON/
OFF is above 1/3 of the supply voltage at U0510-14, it will cause the device to 
internally ground U0510-1, which in turn holds capacitor C0511 in a 
discharged state. The device also keeps U0510-5 SW B+ SENSE at a low voltage. 
When ON/OFF goes below 1/3 of the device supply voltage, U0510 disables 
the discharge mechanism at pin 1 allowing the capacitor C0511 to charge 
through R0512. The charging rate is set by the values of R0512 and C0511. 
When the voltage at pin 2 goes above 2/3 supply voltage, the device sets pin 
5 SW B+ SENSE high which in turn switches on Q0510 enabling the radio to 
turn on to begin operation. U0510 will keep pin 5 high for a period set by 
R0512 and C0511 which for 200K/1 uF equals 220 ms.
While SW B+ SENSE is on for 220 ms, the microprocessor starts execution, 
reads that the SW B+ SENSE is active, and sets the B+ CNTL output to a logic 
high to keep Q0510 switched on through CR0510 beyond the 220 ms period. 
After this U0510-5 will return to a low level. This operation allows a 
momentary press of the Mechanical on/off switch to power up the radio. 
Notice that SW B+ SENSE also goes to U0104-B4 or U0003-H2. This is where 
the appropriate microprocessor is alerted to the turn on/off condition.
If the radio is already on when the switch was pressed then B+ CTRL is already 
high and SW B+ SENSE going high due to the switch being pressed will have 
no effect on Q0510. However, since SW B+ SENSE also goes to U0104-B4 or 
U0003-H2, the software can detect the line changing from low to high state 
momentarily, indicating that the radio must now turn off. In this case the 
software asserts B+ CTRL low which switches off Q0510 and Q0511, turning 
off the radio.
Resistor R0518 and capacitor C0518 form a Þlter to roll off any audio on the 
line. This prevents audio signal swing on the MIC IN line from triggering the 
on/off function at U0510-6. 
Ignition 
(Refer to schematic 
Page 10-29 for 
reference) 
Ignition sense is used to prevent the radio from draining the vehicleÕs battery 
because the engine is not running. The radio can be programmed to keep the 
unit entirely off, preventing RX and TX; or programmed to only prevent TX, 
while the vehicleÕs engine is off. See  for functional operation.
When the IGNITION input goes above 6 volts Q0430 turns on. This turns 
Q0517 off and turns Q0519 on, turning on SW B+ by directly forcing Q0511-
4 low. The logic 0 output of Q0430 also turns on Q0431 providing an input to 
the microprocessor. The microprocessor starts execution, reads that the  
						

							 
Controller Section Theory of Operation 7-5  
Ignition input is active and sets the B+ CNTL output to a logic 1 to latch on 
SW B+. If the software determines that the radio should not be operating, it 
will set the CLEAR output to a logic 1 and the B+ CNTL output to a logic 0. 
This sets a latch composed of Q0514 - Q0517, R0519 and R0521. The latch 
output (at the collector of Q0517) will go to a logic 0 (at Q0517) and turn off 
Q0519, which allows R0514 to pull Q0511-4 high, switching the FET off. The 
latch output will remain at a logic 0 state until the IGNITION input has gone 
below 6 volts. The next time the IGNITION input goes above 6 volts the above 
process will be repeated. The microprocessor uses the ignition sense input 
along with codeplug data to determine if the radio is allowed to transmit.  
Table 7-1   Conventional/MDC/SmartNet/SmartZone Ignition Sense Function - 
Operator Action vs Resultant Radio States 
This Action During This State of Radio Causes The Following Radio States to Occur
Present State of Radio Functions Available Through Ignition Sense Programmed in RSS
ActionOn/
OffReceive/
Stand-by/
EmergencyIgnitionBlank
(Default from 
Factory 
*
 
)Soft Power Off
Default from 
FactoryTX Inhibit PTT Inhibit  
Press On/Off Button On Receive/
Stand-byOn Off Off Off Off
Press On/Off Button  
 
 
On Emer On 
Off Off Off Off 
Press On/Off Button Off Off On On On On On
Press On/Off Button On Receive/
Stand-byOff Off Off Off Off
Press On/Off Button On Emer Off Off Off Off Off
Press On/Off Button Off Off Off On On (Off  
à
 
) On - RX or 
StandbyOn -RX or 
Standby
Short Press Emer 
ButtonOn Receive/
Stand-byOn On - Emer TX On - Emer TX On - Emer TX On - Emer TX
Long Press Emer 
ButtonOn Emer On On On On On
Short Press Emer 
Button  
2
 
Off Off On 
Off Off Off Off 
Short Press Emer 
Button  
2
 
On Receive/
Stand-byOff On - Emer TX 
Off 
Emer Displayed 
for 3 Seconds 
then No Ack 
Displayed - No 
TX
(All of the above 
plus sidetones  
3
 
)On - Emer TX
Long Press Emer 
ButtonOn Emer Off On Off On On
Short Press Emer 
Button  
2
 
Off Off Off 
Off Off Off Off 
Turn Ignition On On Receive/
Stand-byOff On On On On - No PTT TX
Turn Ignition On  
2
 
On Emer Off On - Emer TX On On - Emer TX On - Emer TX 
						

							 
7-6 Controller Section Theory of Operation 
*  The ÒDefault from FactoryÓ was changed in Release 4.01.
   Take special note of Emergency operation (shown in oversize  
bold  
 print) under these conditions.
à  For Þrmware versions older than Release 4.0, the feature will operate as shown in parenthesis. To deter-
mine Þrmware version present in radio, proceed as follows:
Within 10 Seconds after powering on the radio, press the test mode entry button Þve times. On Model I 
radio, test mode entry button is third button from left on bottom row of buttons. On radio Models II and 
III, test mode entry button is Þrst button on left side of bottom row of buttons. When the test mode entry 
button is pressed Þve times, the radio will scroll sequentially through six displays. The Þrst display in the 
sequence is SERVICE. The second display is the radio Þrmware version (e.g., R03.11). 
Power Control 
(Refer to schematic 
Page 10-20 for 
reference) 
The power control loop regulates 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. 
Turn Ignition On Off Off Off Off On Off Off
Turn Ignition Off On Receive/
Stand-byOn On Off On - No TX On - No TX
Turn Ignition Off  
2
 
On Emer On On - Emer TX 
Off 
On - Emer TX On -Emer TX
Turn Ignition Off Off Off On Off Off Off Off
Data PTT On Data Mode Off TX Off No TX No TX
Press On/Off Button On Data Mode On Off Off Off Off
Press On/Off Button On Data Mode Off Off Off Off Off
Short Press Emer 
ButtonOn Data Mode On On - Emer TX On - Emer TX On - Emer TX On - Emer TX
Short Press Emer 
Button  
2
 
On Data Mode Off On - Emer TX 
Off Emer 
Displayed 
for 3 
Seconds 
then No 
Ack 
Displayed - 
No TX 
On - Emer TX
Turn Ignition On On Data Mode Off On N/A On On - No PTT TX
Turn Ignition Off On Data Mode On On Off On - No TX On - No TX 
This Action During This State of Radio Causes The Following Radio States to Occur
Present State of Radio Functions Available Through Ignition Sense Programmed in RSS
ActionOn/
OffReceive/
Stand-by/
EmergencyIgnitionBlank
(Default from 
Factory 
*
 
)Soft Power Off
Default from 
FactoryTX Inhibit PTT Inhibit 
						

							 
Controller Section Theory of Operation 7-7  
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.
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 ampliÞed with a gain of set by the PWR RANGE signal (described below) and 
added to 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 
ampliÞer. 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 (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 Þlter R0571 and C0569.
Rise and fall time is controlled by the D/A adjustable control voltage limit 
circuit attached to the reference voltage at U0550 pin 10 via transistor Q0555. 
The reference voltage at U0550 pin 10 is pulled low by Q0555 when the PA 
control voltage approaches the limit set by the D/A output PA CNTL LIM, 
Note 
						

							 
7-8 Controller Section Theory of Operation 
U0551 pin 13. The PA control voltage at U0550 pin 8 connects to opamp 
noninverting input U0202 pin 3 through the voltage divider formed by R0592 
and R0591 and lowpass capacitor C0572. Control voltage limit is set by the D/
A output PA CNTL LIM at U0551 pin 13 which connects to inverting input 
U0202 pin 2 through R0584, Q0556 and R0590. Transistor Q0556 is connected 
to the PA disable line, PA DIS which effectively pulls the control voltage limit 
to zero volts, and activates Q0555 to pull the reference voltage to zero when 
control voltage is greater than zero. 
Protection features 
The transmit power control circuit has three protection mechanisms. They are 
1) thermal cutback, 2) current limit and 3) PA control voltage limit. These 
features operate by adding current to the ALC loop inverting input at U0550 
pin 9 through diodes CR0550 and CR0551 and decreasing the PA control 
voltage. When the voltage exceeds 4.65 volts plus one diode drop at any 
cathode of diodes CR0550 and CR0551, current begins to ßow into the ALC 
loop increasing the voltage at the inverting input U0550 pin 9. As a result the 
PA control voltage at U0550 pin 8 decreases in response to excessive PA 
control voltage, Þnal device temperature, and Þnal device current.
Thermal cutback limits the PA temperature by reducing the PA control voltage 
as temperature increases during extended periods of transmitter operation or 
high ambient temperatures.
PA TEMP connects to an inverting ampliÞer through resistor R0550 to 
inverting input U0550 pin 2. The noninverting input U0550 pin 3 is 
connected to a 4.65 volt reference formed by voltage divider resistors R0576 
and R0582 which connect to ground and the 9.3 volt supply. The output of 
the inverting ampliÞer at U0550 pin 1 is the product of the ampliÞer gain as 
determined by the ratio of R0551 divided by R0550 and the difference between 
the ampliÞer inputs pins 2 and 3. When the PA TEMP input is greater than 
4.65 volts the ampliÞer output is less than 5 volt.
As temperature rises the voltage on PA TEMP falls, the inverting ampliÞer 
output at U0550 pin 1 rises, current begins to ßow through R0552 and CR0550 
into the ALC loop at the inverting input of U0550 pin 9, decreasing the PA 
control voltage PA CNTL and reducing transmitter output.
Current limit is provided to protect the PA Þnal device (Q5540) from 
overcurrent caused by low line voltage and/or mismatched antennas.
Voltage dividers on the current sense lines, CURRENT SENSE+/ CURRENT 
SENSE-, formed by resistors R0557, R0558, R0559, and R0560 protect the 
inputs of U0550 (pins 5 and 6) from excessive voltages. CURRENT SENSE+ 
connects to the noninverting input U0550 pin 5 through resistors R0557 and 
R0558. CURRENT SENSE- connects to the inverting input U0550 pin 6 
through resistors R0559 and R0560. As current through the Þnal device 
increases, voltage drop through R5612 increases and CURRENT SENSE- 
decreases with respect to CURRENT SENSE+, increasing the difference between 
inverting and noninverting inputs, causing the ampliÞer output at U0550 pin 
7 to increase to over 4.65 volts plus one diode drop. As the ampliÞer output 
increases to over 5 volts, the current through resistor R0556 and diode CR0550 
becomes sufÞcient to reduce the PA control voltage reducing the PA device 
current.
The D/A line CUR LIM SET at U0551 pin 4 adjusts the maximum allowed 
current by creating an offset voltage at the noninverting input U0550 pin 5