Motorola 6864115B62-C Detailed Service Manual page 364

2-10
STEERING
LINE
(VCTRL)
PRE_IN is amplified by feedback buffer Q1202, low-pass filtered by L1205, C1222 and C1224, and
fed to U1201 pin 32 (PREIN).
The pre-scaler in the synthesizer (U1201) is basically a dual modulus pre-scaler with selectable
divider ratios. This divider ratio of the pre-scaler is controlled by the loop divider, which in turn
receives its inputs via the serial interface to the microprocessor. 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 reference signal. The reference signal is
generated by dividing down the signal of the reference oscillator, whose frequency is controlled by
Y1201.
The output signal of the phase detector is a pulsed DC signal which is routed to the charge pump.
The charge pump outputs a current at pin 43 of U1201 (I OUT). The loop filter (which consists of
R1205-6, R1208, C1212-14) transforms this current into a voltage that is applied to the varactor
diodes (CR1310 for transmit, CR1302 for receive) and alters the output frequency of the appropriate
VCO. The current can be set to a value fixed in the LVFRAC-N IC or to a value determined by the
currents flowing into BIAS 1 (U1201-40) or BIAS 2 (U1201-39). The currents are set by the value of
R1211 or R1207 respectively. The selection of the three different bias sources is done by software
programming.
To reduce synthesizer lock time when new frequency data has been loaded into the synthesizer the
magnitude of the loop current is increased by enabling the IADAPT (U1201-45) for a certain
software programmable time (Adapt Mode). The adapt mode timer is started by a low to high
transient of the CSX line. When the synthesizer is within the lock range the current is determined
RXVCO
Q1303
~
AGC
TXVCO
Q1306
~
AGC
SFOUT
(U1201 PIN28)
(U1201 PIN2)
Figure 2-4 LowBand VCO/Buffer Block Diagram
1ST RX
2ND RX
BUFFER BUFFER
Q1304 Q1305
2ND TX
1ST TX
BUFFER
BUFFER
Q1307 Q1308
U1377-8
DC SWITCH
THEORY OF OPERATION
RXINJ
(TO 1ST MIXER)
TO Q1202
PRESCALER BUFFER
TXINJ
(TO U1401 PIN16)
RX (TO Q1303-5)
TX (TO Q1306-8)