Motorola Two Way Portable Radio Ht750ls Detailed 6881088c46 E Manual

FMR-2045A-2December 26, 2003
VHF Theory of Operation: Frequency Generation Circuitry 71
Radio signal strength indicator, RSSI, a voltage signal, is used to drive Q3301 to saturation i.e. 
turned on. RSSI is produced by U3220 and is proportional to the gain of the RF amplifier and the 
input power to the radio. 
Resistors R3304 and R3305 are voltage dividers designed to turn on Q3301 at certain RSSI levels. 
In order to turn on Q3301 the voltage across R3305 must be greater or equal to the voltage across...

December 26, 2003FMR-2045A-2
72VHF Theory of Operation: Synthesizer
7.4 Synthesizer
(Refer to the VHF Synthesizer Schematic Diagram on page 82.)
The Fractional-N Synthesizer uses a 16.8MHz crystal (Y3761) to provide a reference for the system. 
The LVFractN IC (U3701) further divides this to 2.1MHz, 2.225 MHz, and 2.4 MHz as reference 
frequencies. Together with C3761, C3762, C3763, R3761 and D3761, they build up the reference 
oscillator which is capable of 2.5ppm stability over temperatures of -30 to...

FMR-2045A-2December 26, 2003
VHF Theory of Operation: Voltage-Controlled Oscillator (VCO) 73
7.5 Voltage-Controlled Oscillator (VCO)
(Refer to the VHF Voltage-Controlled Oscillator Schematic Diagram on page 83.)
Figure 7-5.  VCO Block Diagram
The VCOBIC (U3801) in conjunction with the Fractional-N synthesizer (U3701) generates RF in both 
the receive and the transmit modes of operation. The TRB line (U3801 pin 19) determines which 
oscillator and buffer will be enabled. A sample of the RF signal from the...

December 26, 2003FMR-2045A-2
74VHF Theory of Operation: Voltage-Controlled Oscillator (VCO)
The RF section of the VCOBIC(U3801) is operated at 4.54 V (VSF), while the control section of the 
VCOBIC and Fractional-N synthesizer (U3701) is operated at 3.3V. The operation logic is shown in 
Table 7-1.
In receive mode, U3801 pin 19 is low or grounded. This activates the receive VCO by enabling the 
receive oscillator and the receive buffer of U3801. The RF signal at U3801 pin 8 is run through a 
matching...

Chapter 8 VHF Circuit Board Layouts, Schematic 
Diagrams and Parts List
8.1 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 multime-
ter or equivalent. Transmitter measurements should be made with a 1.2 µH...

December 26, 2003FMR-2045A-2
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76VHF Circuit Board Layouts, Schematic Diagrams and Parts List: Notes For All Schematics and Circuit Boards 

Circuit Board/Schematic Diagrams and Parts List
77
FMR-2045A-2
December 26, 2003
8.2 Circuit Board/Schematic Diagrams and Parts List
Figure 8-1.  VHF (136-174MHz) Main Board Top Side PCB No. 8486473Z03
C502C505
C512 C513
Q502Q505
R505R506
VR501
123
5PB501
4 3 2
Y3204C521
C3225C3278
R3275
58
U3221 VR440
2
Y3202 Y3205
C3208
R3205
R3208
R3211
4C3206
L3200
L3203
123 5PB504VR444
43
2
Y3203
C3201 C3202 SH3203
2
3 4 56
Y3200123
5PB502
C522
B503B504
TP3701C523
123
5PB503
6
431
Y3761
C432 TP3702
C3706 C3710...

78Circuit Board/Schematic Diagrams and Parts ListDecember 26, 2003FMR-2045A-2
Figure 8-2.  VHF (136-174MHz) Main Board Bottom Side PCB No. 8486473Z03
C449
C458
C459C463
C466C4020C4021
E400 E401 E402E403 E404 E405
E406
1
7651 26
U409
C453
C456 L410
L411R409R411
R432 R457C431 C433
C436C437
3
FL401 R414
R415
R420
R426R463
C434C435
CR411 Q416R419 R460
R461R462
SH402
U410C328
C359C390
C3763D3761R373
R374R375
R377 R3781
4 5 8
U303 VR442
C3212
R3214 R3215 R3761
U3201
C337C358
C383
C3213C3215
CR501L505 R370
R371...

Circuit Board/Schematic Diagrams and Parts List79FMR-2045A-2December 26, 2003
Figure 8-3.  VHF Controls And Switches Schematic Diagram
VOL MECH_SWB+
Vdda
6.8VVR501
HIGH
TAB1
6 TAB2
7
2 mgc_comps
4
1LOW5
3S502
C502
0.47uF
C505100pF
UNSWB+
1000pF1000pFC535
100pFC503
CR501VR439
10V2 124V F501
BATT_CODE
SB1 EMER
LI_ION
VR506
6.8VL505
390nHB501
DATA NEG POS
B503
B504.01uFC520
.01uFC521
1 2SWITCHPB505
VR440
NU6.8V
PB501
1 2
SWITCH
PB504
1 2
SWITCH
NU 6.8V VR444
PTT
PB502
1
2
SWITCH
VR441
NU6.8VC522
.01uF
SB2...

80Circuit Board/Schematic Diagrams and Parts ListDecember 26, 2003FMR-2045A-2
Figure 8-4.  VHF Receiver Front-End Schematic Diagram
G UNSWB_PLUS_1_RX
C3336
10uF UNSWB+UNSWB_PLUS_2_RX
G
L3301
12nH RX_INC3302
150pF RX_IN_1_FE
 
5.6pF C3301C3303
15pF
D3301CR3303 L3303
12nHC3304
150pF
33KR3307 NU C3305
NU C3326
470pFC3338
470pF
R3304
100KR3308
1.5K
R3311
NU 7.5K R3309
5.1 R3312 0.1uF C3306
Q330239 R3313 L3304
470nH
R3315
330 C3308
330pF
100pF
  C3307
2.7K R3310
16pF C3312
150pF C3311
R3314
27
D3302
NU...