Prescaler Feedback; Figure 3-41. Frequency Generation Unit Block Diagram (700-800 Mhz) - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

3-58

3.7.3.13 Prescaler Feedback

RF feedback for the synthesizer loop is provided by prescaler amplifier Q5755. Feedback from both
the transmitter and receiver injection strings are coupled to this amplifier through capacitors C5863
and C5957. The output of Q5755 is coupled to U5752 at pin 32 (PREIN), which is the prescaler input
for the synthesizer.
3.7.4 700–800 MHz Band
The FGU (Figure
1st local oscillator, and a modulated transmitter RF carrier that is further amplified by the power
amplifier section of the radio.
The FGU consists of the following:
• Reference oscillator (Y6750)
• Low-voltage Fractional-N (LV Frac-N) synthesizer (U6751)
• Two receiver voltage-controlled oscillators (VCOs) contained in U6755
• Two transmitter VCOs contained in U6754
• Two receiver LO amplifiers (Q6762 and Q6763)
• Three transmitter injection amplifiers (Q6764, Q6765 and Q6766)
5.0V
16.8MHz OSC
3.0V
Regulator
U6750
CLOCK
SYN_SEL
From
Controller
DATA
MOD_IN
PHASE
DETECTOR BIAS
Q6757 RX and
Q6758 TX
9.3V
8.2V
Super Filter
Q6759
Figure 3-41. Frequency Generation Unit Block Diagram (700–800 MHz)
May 25, 2005
3-41) provides the XTL 5000 radio with a 16.8 MHz reference frequency, receiver
3.0V
3.0V
U6752
VOLTAGE
MULTIPLIER
3.0V
D6751 and
3.0V
VMULT3
VMULT4
AUX1
AUX2
U6751
IOUT
IADAPT
Mod Out
C6783
Bias 1
Bias 2 AUX3
AUX4
SFIN
SFBASE
PREIN
Lock
SFOUT
8.2V
Theory of Operation: Frequency Generation Unit (FGU)
16.8MHz Reference
to Controller and
Receiver Back End
8.2V
D6752
8.2V SW
8.2V SW
U6754
Q6753
Q6755
LOOP
FILTER
U6755
8.2V SW
Q6756
8.2V SW
Q6760
Prescaler Input
Lock Detect
to Controller
Keyed
9.1V
TX
TX
INJECTION
Injection
8.2V 9.3V
RX
RX
INJECTION
Injection
8.2V
RF
Q6761
Feedback
MAEPF-27804-O
6881096C74-B