Rf Tx Pa - Motorola W208 Manual

W208 Level 3 Circuit Description
The DCXO system is shown in Figure 10 including the power management. VR2 (from
Triton/Triton-Lite) is used to power this system. At the heart of the DCXO system is the
DCXO core that consists of a Colpitts oscillator with an 8-bit current DAC that can be used
to change the loop gain of the DCXO core. The oscillator frequency can be tuned by
controlling a bank of capacitors organized as an array in a very similar fashion to the
construction of D/A converters. By selecting more capacitance, the oscillator frequency
can be reduced and vice versa. The capacitors are selected by independently controlling
rows and columns of the array through a thermometer encoded row/column selection.
The smallest capacitor is dithered with first order sigma-delta modulation to achieve
fractional resolution. The output of DCXO is available to the external world through the
FREF buffer.
The system level control of DCXO basically can be separated by two categories:
Frequency control and amplitude control. Frequency control is accomplished in three
steps:
Coarse frequency control using segmented feedback capacitor inside the Colpitts
oscillator
Fine Frequency Control using 1024 unit tuning capacitors
Fractional Fine Frequency control using Sigma-Delta Dithering of FFC unit capacitor
LDOX: Because of the low phase noise requirements, DCXO is provided with its own
LDO voltage regulator (LDOX)
Oscillation Amplitude Control is accomplished by varying the current to the Colpitts Gm
transistor. This functionality is implemented using a current DAC (IDAC block)

2.5 RF TX PA

The TX signal outputs at
Plus Pin G17 (high-band). The high-band signal passes through R202, and the low-band
signal passes through R201. The
for the high-band signal and one for the low-band signal). A linear power amplifier in
each path. The SKY77318 U201
(logic0) or DCS/PCS (logic1) as determined from the
module consists of separate GSM850/900 PA and DCS1800/PCS1900 PA blocks,
Figure 80: DCXO Block Diagram
TXLB Locosto Plus
SKY77318 PA IC
also contains band-select switch circuitry to select GSM
Service Engineering & Optimization
- 12 -
Pin F17 (low-band) and
U201, has two independent paths (one
Band Select(BS)
TXHB Locosto
Pin 1 signal. The