Motorola PRO Series, Detailed Service Manual

Page: 662 / 686

Section 13: 2-8 UHF Band 2 (450–512/520 MHz) 25–40 W LDMOS Frequency Synthesis
6881091C63-F
Figure 13-4 UHF VCO Block Diagram
The external RX buffers (Q5332) are enabled by a high at U5201, pin 3 (AUX4) via transistor switch
Q5333. In TX mode, the modulation signal (VCOMOD) from the LVFRAC-N synthesizer IC (U5201,
pin 41) is applied to the modulation circuits CR5321, R5321, R5322, and C5324. These modulate
the TX VCO frequency via coupling capacitor C5321. Varactor CR5321 is biased for linearity from
the VSF.
4.4 Synthesizer Operation
The synthesizer consists of a low voltage FRAC-N (LVFRACN) IC, reference oscillator (crystal
oscillator with temperature compensation), charge pump circuits, loop filter circuits, and DC supply.
The output signal (PRESC_OUT) of the VCOBIC (U5301, pin12) is fed to U5201, pin 32 (PREIN) via
a low-pass filter (C5229, L5225, and C5226) which attenuates harmonics and provides correct level
to close the synthesizer loop.
The pre-scaler in the synthesizer (U5201) is a dual modulus pre-scaler with selectable divider ratios.
The divider ratio of the pre-scaler is controlled by the loop divider, which in turn receives its inputs
via the SRL. The output of the pre-scaler is applied to the loop divider. The output of the loop divider
is connected to the phase detector, which compares the loop divider’s output signal with the
Presc
RX
TX
Matching
Network
Low-Pass
Filter
Attenuator
Pin8
Pin14
Pin10
(U5201 Pin28)
VCC Buffers
TX RF Injection
U5201 Pin 32
AUX3 (U5201 Pin2)
Prescaler Out
Pin 12 Pin 19
Pin 20
TX/RX/BS
Switching Network
U5301
VCOBIC
Rx
Active Bias
Tx
Active Bias
Pin2
Rx-I adjust
Pin1
Tx-I adjust
Pins 9,11,17
Pin18
Vsens
Circuit
Pin15
Pin16
RX VCO
Circuit
TX VCO
Circuit
RX Tank
TX Tank
Pin7
Vcc-Superfilter
Collector/RF in
Pin4
Pin5
Pin6
RX
TX
(U5201 Pin28)
Rx-SW
Tx-SW
Vcc-Logic
(U5201 Pin28)
Steer Line
Voltage
(VCTRL)
Pin13
Pin3
TRB IN
LO RF INJECTION
Q4301
Q4332