17
iO1000 Detailed Service Manual - THEORY OF OPERATION
Figure 9. Frequency Generator
Low Voltage Fractional-N (LV Frac-N) Synthesizer
This synthesizer provides the temperature compensation and frequency error cor-
rection for the crystal-based reference oscillator, generates the control voltage for
the main VCO, shuts off the buffer circuitry and main VCO in battery-save mode,
and acts as a phase detector for the main VCO. The 16.8 MHz frequency is used
as a reference for all frequencies generated in the portable.
In the phase detector, the LV Frac-N synthesizer compares the divided 16.8 MHz
reference to the divided-down LO. The output of the detector is the control volt-
age for the main VCO. The feedback loop keeps the receive and transmitter fre-
quencies locked and allows frequency transitions in a short period of time (lock
time less than 3 milliseconds).
The LV Frac-N controls the power to the main VCO and buffer circuitry in bat-
tery-save mode. It switches the superfilter supply to the VCO and buffer on and
off using a dual-transistor array, Q302 (see Figure 10 on page 18).
Second VCO
219.3MHz
DSP
58.3MHz
Synthesizer
U850
Offset VCO
301.8MHz
16.8MHz
16.8MHz
Pre-scaler
Main VCO
956.9-975.65MHz
(Discrete)
Q304
LV Fractional - N
Synthesizer
(ASIC)
U301
2.1MHz
:8
ODCT
(ASIC)
U510
150.9 x 2
109.65 x 2
DSP
1.2MHz
Serial
Peripherial
Interface
(SPI) Clock
16.8MHz
Crystal
Ref Osc
16.8MHz
Y300
Buffer
(Discrete)
Q305
To
Tx and Rx
Circuit
iZIF
U601
