Motorola cm200 Commercial Series, Service Manual

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UHF (438-470 MHz) Frequency Synthesis 2-5
This oscillator is temperature compensated to an accuracy of +/-2.5 PPM from -30 to 60 degrees C.
The temperature compensation scheme is implemented by an algorithm that uses five crystal
parameters (four characterize the inverse Bechmann voltage curve and one for frequency accuracy
of the reference oscillator at 25 degrees C). This algorithm is implemented by the LVFRAC-N (U200)
at the power up of the radio.
TCXO Y200, along with its corresponding circuitry R204, R205, R210, and C2053, are not placed as
the temperature compensated crystal proved to be reliable.
4.2 Fractional-N Synthesizer
The LVFRAC-N U200 consists of a pre-scaler, programmable loop divider, control divider logic,
phase detector, charge pump, A/D converter for low frequency digital modulation, balanced
attenuator used to balance the high and low frequency analog modulation, 13 V positive voltage
multiplier, serial interface for control, and a super filter for the regulated 5 volts.
Figure 2-3 UHF Synthesizer Block Diagram
A voltage of 5 V applied to the super filter input (U200, pin 30) supplies an output voltage of 4.5 Vdc
(VSF) at U200, pin 28. This supplies 4.5 V to the VCO Buffer IC U201.
To generate a high voltage to supply the phase detector (charge pump) output stage at pin VCP
(U200, pin 47) while using a low voltage 3.3 Vdc supply, a 13 V positive voltage multiplier is used
(D200, D201, and capacitors C2024, 2025, 2026, 2055, 2027, 2001).
Output lock (U200, pin 4) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. A 16.8 MHz reference frequency is provided at U200, pin
19.
DATA
CLK
CEX
MODIN
VCC, DC5V
XTAL1
XTAL2
WARP
PREIN
VCP
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
DATA (U403 PIN 100)
CLOCK (U403 PIN 1)
CSX (U403 PIN 2)
MOD IN (U501 PIN 40)
+5 V (U503 PIN 1)
7
8
9
10
13, 30
23
24
25
32
47
VMULT2 VMULT1
BIAS1
SFOUT
AUX3
AUX4
IADAPT
IOUT
GND
FREFOUT
LOCK
4
19
6, 22, 33, 44
43
45
3
2
28
14
15
40
FILTERED 5V
STEERING
LOCK (U403 PIN 56)
PRESCALER IN
FREF (U504 PIN 34)
39
BIAS2
41
48
5, 20, 34, 36
+5 V (U503 PIN 1)
AUX1
VDD, DC5V
MODOUT
U200
LOW VOLTAGE
FRACTIONAL-N
SYNTHESIZER
AUX2
1 (NU)
BWSELECT
VCO Bias
TRB
To IF
Section
TX RF INJECTION
(1ST STAGE OF PA)
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
LINE
LOOP
FILTER