Synthesizer; Figure 4-4. Synthesizer Block Diagram - Motorola HT Series Service Manual

UHF Band 1 Theory of Operation: Synthesizer
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 synthesizer is powered by regulated 5 V and 3.3 V which come from U247 and U248
respectively. The synthesizer in turn generates a super-filtered 4.5 V which powers U241.
In addition to the VCO, the synthesizer must interface with the logic and ASFIC circuitry.
Programming for the synthesizer is accomplished through the data, clock and chip select lines from
the microprocessor. A 3.3 V dc signal from the synthesizer lock detect line indicates to the
microprocessor that the synthesizer is locked.
Transmit modulation from the ASFIC is supplied to pin 10 of U201. Internally the audio is digitized by
the Fractional-N and applied to the loop divider to provide the low-port modulation. The audio runs
through an internal attenuator for modulation balancing purposes before going out to the VCO.
4.4

Synthesizer

(Refer to UHF Synthesizer Schematic Diagram on page 9-86.)
DATA (U409 PIN 100)
CLOCK (U409 PIN 1)
CSX (U409 PIN 2)
MOD IN (U404 PIN 40)
+5V (U247 PIN 4)
5, 20, 34, 36
(U248 PIN 5)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
The Fractional-N Synthesizer uses a 16.8MHz crystal (FL201) to provide a reference for the system.
The LVFractN IC (U201) further divides this to 2.1 MHz, 2.225 MHz, and 2.4 MHz as reference
frequencies. Together with C206, C207, C208, R204 and CR203, they build up the reference
oscillator which is capable of 2.5ppm stability over temperatures of -30 to 85°C. It also provides 16.8
MHz at pin 19 of U201 to be used by the ASFIC.
7
DATA
8
CLK
9
CEX
U251
LOW VOLTAGE
10
MODIN
FRACTIONAL-N
13, 30
SYNTHESIZER
V , DC5V
CC
V , 3.3V
DD
23
XTAL1
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
15
PRESCALER IN

Figure 4-4. Synthesizer Block Diagram

4
LOCK
19
FREFOUT
6, 22, 23, 24
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
DUAL
TRANSIS
2
AUX3
TORS
FILTERED 5V
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
48
DUAL
R5
TRANSIS-
5V
TORS
LOCK (U409 PIN 56)
FREF (U201 PIN 21 & U404 PIN 34)
STEERING
LINE
2-POLE
LOOP
FILTER
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA
4-5