Fractional-N Synthesizer - Motorola Commercial CM Series Service Information

UHF (403-440MHz) Frequency Synthesis
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, 13V positive voltage
multiplier, serial interface for control, and a super filter for the regulated 5 volts.
DATA (U403 PIN 100)
CLOCK (U403 PIN 1)
CSX (U403 PIN 2)
MOD IN (U501 PIN 40)
+5V (U503 PIN 1)
5, 20, 34, 36
+5V (U503 PIN 1)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
A voltage of 5V applied to the super filter input (U200, pin 30) supplies an output voltage of 4.5Vdc
(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 3.3Vdc supply, a 13V 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.
7
DATA
8
CLK
9
CEX
10
MODIN
13, 30
VCC, DC5V
VDD, DC5V
23
XTAL1
U200
LOW VOLTAGE
24
XTAL2
FRACTIONAL-N
25
SYNTHESIZER
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
PRESCALER IN
Figure 2-3 UHF Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6, 22, 33, 44
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
1 (NU)
AUX2
2
AUX3
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
15
48
BWSELECT
LOCK (U403 PIN 56)
FREF (U504 PIN 34)
STEERING
LINE
LOOP
FILTER
VCO Bias
TRB
VOLTAGE
CONTROLLED
FILTERED 4.5V
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA)
To IF
Section
2-5
LO RF INJECTION