Reference Oscillator; Lv Frac-N Synthesizer Ic; Figure 3-37. Frequency Generation Unit Block Diagram (Uhf Range 1) - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

3-50
3.0V
5.0V
16.8MHz OSC
3.0V
Regulator
3.0V
U5750
CLOCK
SYN_SEL
U5752
From
Controller DATA
MOD_IN
PHASE
Bias 1
DETECTOR BIAS
Q5750 RX and
Bias 2
Q5751 TX
SFIN
9.3V
SFBASE
SFOUT
8.2V
Super Filter
8.2V
Q5752

Figure 3-37. Frequency Generation Unit Block Diagram (UHF Range 1)

3.7.2.1 Reference Oscillator

The radio's frequency stability and accuracy is derived from the 16.8 MHz reference oscillator
(Y5750). The 16.8 MHz reference oscillator circuitry provides a 16.8 MHz reference to the LV Frac-N
(U5752), receiver back-end IC (U5002), and the controller section of the XTL 5000 radio. The
reference oscillator circuitry consists of the reference oscillator Y5750 and the inverter/buffer circuitry
containing the active device U5751. Y5750 is a voltage-controlled, temperature-compensated crystal
oscillator (VCTCXO). Circuitry internal to Y5750 compensates for frequency error over temperature.
Warping of the oscillator on frequency is accomplished via a programmable DAC in the LV Frac-N.
The warp voltage is present at pin 25 (WARP) of U5752 and is applied to pin 1 of Y5750. The
16.8 MHz output frequency of Y5750 is capacitor-coupled to pin 23 of the LV Frac-N (U5752) and
also to the inverter/buffer stage U5751. L5753 and C5768 at the output of U5751 filter the 16.8 MHz
signal, and R5768 along with C5763 set the appropriate amplitude of the signal for the receiver back-
end and controller sections.

3.7.2.2 LV Frac-N Synthesizer IC

The LV Frac-N IC (U5752) functions include frequency synthesis, modulation control, voltage
multiplication and filtering, and auxiliary logic outputs for VCO selection.
U5752 is a mixed-mode IC containing digital and analog circuits. Separate power supply inputs are
used for the various functional blocks on the IC. Inductors L5755 and L5756 provide isolation
between supply pin 20 (AVDD - analog supply input) and pin 36 (DVDD - digital supply input)
connected to F3.0v. This 3.0 V DC supply is provided by U5750, a 3-V regulator IC.
May 25, 2005
3.0V
16.8MHz Reference
to Controller and
Receiver Back End
U5751
VOLTAGE
MULTIPLIER
3.0V
D5750 and
D5751
VMULT3
VMULT4
U5753
A
AUX1
AUX2 B
C
AUX3
AUX4 NC
IOUT
LOOP
FILTER
IADAPT
C5780
Mod Out
C5779
PREIN
Prescaler Input
Lock
Lock Detect
to Controller
Theory of Operation: Frequency Generation Unit (FGU)
8.2V
Q5706
X0
X1 Q5707
X2
X3
X4
8.2V SW
Q5708
8.2V SW
Q5709
8.2V SW
Q5710
8.2V SW
8.2V SW
Keyed
9.1V
Q5825
TX
TX
INJECTION
Q5826
Injection
7.5V
Q5901
RX
RX
Q5903
INJECTION
Injection
Q5905
8.2V
RF
Q5755
Feedback
6881096C74-B