Motorola W208 Manual page 11

W208 Level 3 Circuit Description
main TX oscillator in Locosto. DCO stands for "digitally controlled oscillator", which uses
some digital switched capacitances to do frequency tuning, but it should be noticed that
the oscillator core is still analog. And Locosto DRP uses ADPLL (all digital phase lock loop)
architecture to design a digital synthesizer, and its reference frequency is provided by
26MHz DXCO. The ADPLL output signal will be pre-amplified by a digital controlled
pre-PA and then fed into PA module. The TX signals output at TXLB Locosto Plus Pin
F17 (low-band) and TXHB Locosto Plus Pin G17 (high-band).
Figure 9: Locosto Transmit Block Diagram
2.4.2 Digitally- Controlled Crystal Oscillator (DCXO)
The DCXO system comprises an external crystal Y101, DCXO core based on Colpitts
oscillator, a switching capacitor array, amplitude control loop and a current DAC. It also
includes a startup system to control the startup sequence of the bandgap reference and
the LDO voltage regulator for the DCXO that is based on a 32 KHz clock. DCXO (Digitally
Controlled Crystal Oscillator) is a digitally tunable crystal oscillator centered at 26MHz for
GSM applications with the step size of ~0.01ppm of the 26MHz. Both the amplitude and
the frequency of oscillation are digitally controllable. Figure 10 shows the top level
schematic of DCXO. Major components of DCXO includes a Colpitts oscillator core with
negative resistance, 14-bit AFC fine frequency control capacitor DAC, plus an 10-bits
coarse frequency control capacitor DAC; an 8-bit programmable current source (IDAC), a
peak detector circuit, an ADC, a digital amplitude control loop, and an output buffer.
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