Motorola Two Way Portable Radio Ht750ls Detailed 6881088c46 E Manual

FMR-2045A-2December 26, 2003
UHF Theory of Operation: Synthesizer11
The Frequency Generation Circuitry is composed of two main ICs:
 Fractional-N synthesizer (U201)
 VCO/Buffer IC (U241)
Designed in conjunction to maximize compatibility, the two ICs provide many of the functions that 
normally would require additional circuitry. The synthesizer block diagram illustrates the interconnect 
and support circuitry used in the region. Refer to the relevant schematics for the reference 
designators.
The...

December 26, 2003FMR-2045A-2
12UHF Theory of Operation: Voltage-Controlled Oscillator (VCO)
The loop filter which consist of C231, C232, C233, R231, R232 and R233 provides the necessary dc 
steering voltage for the VCO and determines the amount of noise and spur passing through.
In achieving fast locking for the synthesizer, an internal adapt charge pump provides higher current 
at pin 45 of U201 to put synthesizer within the lock range. The required frequency is then locked by 
normal mode charge pump...

FMR-2045A-2December 26, 2003
UHF Theory of Operation: Voltage-Controlled Oscillator (VCO) 13
The VCOBIC (U241) in conjunction with the Fractional-N synthesizer (U201) generates RF in both 
the receive and the transmit modes of operation. The TRB line (U241 pin 19) determines which 
oscillator and buffer will be enabled. A sample of the RF signal from the enabled oscillator is routed 
from U241 pin 12, through a low pass filter, to the prescaler input (U201 pin 32). After frequency 
comparison in the...

December 26, 2003FMR-2045A-2
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14UHF Theory of Operation: Voltage-Controlled Oscillator (VCO) 

Chapter 2 UHF Circuit Board Layouts, Schematic 
Diagrams and Parts List
2.1 Notes for All Schematics and Circuit Boards
* Component is frequency sensitive. Refer to the Electrical Parts List for value and usage.
1.Unless otherwise stated, resistances are in Ohms (k = 1000), and capacitances are in picofarads 
(pF) or microfarads (µF).
2.DC voltages are measured from point indicated to chassis ground using a Motorola DC 
multimeter or equivalent. Transmitter measurements should be made with a 1.2 µH choke...

December 26, 2003FMR-2045A-2
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16UHF Circuit Board Layouts, Schematic Diagrams and Parts List: Notes for All Schematics and Circuit Boards 

Circuit Board/Schematic Diagrams and Parts List
17
FMR-2045A-2
December 26, 2003
2.2 Circuit Board/Schematic Diagrams and Parts List
Figure 2-1.  UHF (403-470MHz) Main Board Top Side PCB No. 8486458Z02
C505
R507
C502
VR5011
23 5PB501
43 2
FL304C521
FL302 VR440
C349
R3652
Y301
4 3 2
FL303
C352 R321
R324R345
R355 R360
C362
R342458
U302C324 C325C338
2
3 4 56
FL301
L311
L330
SH323
C320 C322
C381
L309 L310
R308R309 R310SH303
6
4
3 T30164 3
T302C319
C375R348
TP202TP302 1
23 5PB504
C511C512 C513
C514
C520 43...

18Circuit Board/Schematic Diagrams and Parts ListDecember 26, 2003FMR-2045A-2
Figure 2-2.  UHF (403-470MHz) Main Board Bottom Side PCB No. 8486458Z02
C449 C463
C524 C525 E401E402E403 E404E405
E406
26
1
7651
U409
C453
C456R411
R432
L410
L411 C458
C459 C466C526
C527 C528 C529
E400
R409 R457C431C433C436
C437
3
FL401
R414R415R420R426R463
C434C435 CR411
Q416
R419 R460
R461R462
SH402
U410
C328
C337 C359C523
R374
R3751
4 5 8
U303VR442
C277C278
C279C522
L505
U248 VR441
VR506
C358
C383
C390R373 R376
R377 R378...

Circuit Board/Schematic Diagrams and Parts List19FMR-2045A-2December 26, 2003
Figure 2-3.  UHF Controls And Switches Schematic Diagram
B504
if
B503
LI_ION
VR5016.8V0.47uF C502
C505
100pFSWITCH S502
4
1
LOW5
HIGH36
TAB1
7
TAB2
2
R507
0
MECH_SWB+
Vdda VOL
P100M100M101
VR441
6.8V
6.8V VR442PB503
SWITCH
1 2 .01uFC522
VR444
6.8V1 2SWITCHPB501
2PB502
SWITCH
1 6.8V VR440
.01uFC521
2SWITCHPB504
1 .01uFC523
EMER
PTT SB2
SB3SB1
PB505
1 2SWITCH
C520
.01uF6.8V VR443
Vdda
4
3 2
R506
47K 5 1
6IMX1
Q50510K R505
R502...

20Circuit Board/Schematic Diagrams and Parts ListDecember 26, 2003FMR-2045A-2
Figure 2-4.  UHF Receiver Front-End Schematic Diagram
CR303
23
1 100K R302
7.5K R340 CR3022.0K R303 100pF C308
100pFC307 C305
13.9nH L30220pF
9.1pF C304
22nH L303C309
100pF
1 Q301
R328
0 3
2
G
3.9pFC303
3.6pFC301100K R301
C302
10pFCR301
L301
13.9nH
RX_IN
RXFE Bottom Shield
RXFE Top Shield
RX Mixer Diod Shield
SH302
SHIELD
1
1 SHIELDSH304 SH301
SHIELD
1
Mixer Shield
1 SHIELDSH303
G
100pF C321 12K R304C314
0.1uFC315
100pF
180nH...