Synthesizer - LG 512W Service Manual

3. TECHNICAL BRIEF

3.2 Synthesizer

The synthesizer IC, the Si4133G-XM2, comprises three complete PLLs. One, which is active all of
the time and in this application produces a constant 1080MHz. This is the IF oscillator and is divided
down inside the HD 155128 to produce the 270 MHz for the second receive mixer, and with an
optional further division to 135 MHz for the transmitter quadrature modulator. The two remaining
synthesizers provide HD 155128 with the RFLO for GSM900 and DCS1800 bands.
The synthesizers use a number of techniques to improve lock times, making them well suited to
GPRS. The use of two synthesizers switched makes the Si4133G-XM2 dual band compatible whilst
minimizing the size of the IC. A fixed reference frequency of 1 MHz is generated by a reference
divider from the external applied 13 MHz crystal oscillator. The phase frequency detector with
charge pump output provides programmable output current drive capability and pulse width.
The counter and mode settings of the synthesizer are also programmed via 3-wire interface.
SDATA
SCLK
SENB
The PLLs can adjust the IF and RF output frequencies ( 5 % with respect to their VCO center
frequencies. Each center frequency is established by the value of an external inductance connected
to the respective VCO. Because the inductances are in the low nH range, the inductance of the
package needs to be considered in determine the correct external inductance. The total inductance
(Ltot)presented to each VCO is the sum of the external inductance (Lext) and the package
inductance (Lpkg). Each VCO has a nominal capacitance (Cnom) in parallel with the total
inductance, and the center frequency is as follows:
or,
RF1 VCO
RF2 VCO
IF VCO
Table 3-4. Synthesizer 3-wire Bus.
Pin Number
27 Serial data input to the synthesizer IC
26 Serial clock input to the synthesizer IC
25 Input latches the serial data transferred to the synthesizer IC
Fcen =
Table 3-5. External Inductance.
Description
1
Fcen =
2π √ Ltot
Cnom
1
2π √ (Lpkg + Lext)
- 14 -
Cnom
Inductance
0.758 nH
2.37 nH
1.74 nH
(3-1)
(3-2)