Voltage Controlled Oscillator (Vco) And Buffers - Motorola GM328 Service Manual

2-8
of 13 volts DC.
(VMULTI1 and VMULTI2) which are 180 degree out of phase.
Output LOCK (U1201-4) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. A buffered output of the 16.8 MHz reference frequency is
provided at pin 19.
The operating frequency of the synthesizer is loaded serially from the microprocessor via the data
line (DATA, U1201-7), clock line (CLK, U1201-8) and chip select line (CSX, U1201-9).
The reference oscillator circuit within U1201 uses an external 16.8 MHz crystal (Y1201). Varactor
CR1201 allows software-controlled frequency adjustments (warp) and temperature compensation of
the oscillator frequency. Warp adjustment is performed using serial data from the microprocessor.
This controls the setting of an A/D converter, with its output (WARP, pin 25) applied to CR1201.
4.2

Voltage Controlled Oscillator (VCO) and Buffers

Separate voltage controlled oscillator (VCO) and buffer circuits, shown in Figure 2-4, are used for
receiver injection and transmitter carrier frequency generation. Since the receiver uses high-side
injection, the receiver VCO frequency range is 10.7 MHz above the transmit VCO range. The VCO/
buffers are bandsplit into three ranges depending on radio model, covering radio operating
frequencies of 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to 50.0 MHz. The corresponding three
frequency ranges from the receive VCO are 40.4 to 46.7 MHz, 46.7 to 52.7 MHz, and 52.7 to 60.7
MHz.
STEERING
LINE
(VCTRL)
This multiplier is driven by two 1.05 MHz clock signals from
RXVCO
Q1303
~
AGC
TXVCO
Q1306
~
AGC
SFOUT
(U1201 PIN28)
(U1201 PIN2)
Figure 2-4 Low Band VCO Block Diagram
1ST RX
2ND RX
BUFFER BUFFER
Q1304 Q1305
2ND TX
1ST TX
BUFFER
BUFFER
Q1307 Q1308
U1377-8
DC SWITCH
Frequency Synthesis
U1201 pins 15 and 14
RXINJ
(TO 1ST MIXER)
TO Q1202
PRESCALER BUFFER
TXINJ
(TO U1401 PIN16)
RX (TO Q1303-5)
TX (TO Q1306-8)