Synthesizer Operation - Motorola 6864115B62-C Detailed Service Manual

2-8
Rx-SW
Tx-SW
(U4201 Pin 28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
Q4301
TX VCO
TX Tank
Circuit
a low enables RX tank and RX output (pin 8). A sample of the signal from the enabled output is
routed from U4301 pin 12 (PRESC_OUT), via a low pass filter, to pin 32 of U4201 (PREIN).
A steering line voltage (VCTRL) between 3.0V and 10.0V at varactor diode CR4311 will tune the full
TX frequency range (TXINJ) from 403 MHz to 470 MHz, and at varactor diodes CR4301, CR4302
and CR4303 will tune the full RX frequency range (RXINJ) from 358 MHz to 425 MHz. The tank
circuits uses the Hartley configuration for wider bandwidth. For the RX tank circuit, an external
transistor Q4301 is used in conjunction with the internal transistor for better side-band noise.
The external RX buffers (Q4332) are enabled by a high at U4201 pin 3 (AUX4) via transistor switch
Q4333. In TX mode the modulation signal (VCOMOD) from the LVFRAC-N synthesizer IC (U4201
pin41) is applied modulation circuitry CR4321, R4321, R4322 and C4324, which modulates the TX
VCO frequency via coupling capacitor C4321. Varactor CR4321 is biased for linearity from VSF.
4.4

Synthesizer Operation

The complete synthesizer subsystem comprises mainly of low voltage FRAC-N (LVFRACN) IC,
Reference Oscillator (crystal oscillator with temperature compensation), charge pump circuitry, loop
Pin 20
Pin7
Pin13
Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Active Bias
Pin5
Pin6
TX
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
(U4201 Pin 28)
Figure 2-4 UHF VCO Block Diagram
AUX3 (U4201 Pin 2)
Prescaler Out
TRB IN
Pin 19
TX/RX/BS
U4301
VCOBIC
Rx
Tx
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
THEORY OF OPERATION
U4201 Pin 32
Pin 12
LO RF INJECTION
Presc
Matching
Network
Q4332
RX
Pin8
(U4201 Pin28)
Pin14
VCC Buffers
TX
TX RF Injection
Pin10
Attenuator
Low Pass
Filter