3. TECHNICAL BRIEF
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Only for training and service purposes
3.4 Crystal(26 MHz, X201)
The X-GOLDTM215 RF-Subsystem contains a fully
3. TECHNICAL BRIEF
y
y
integrated 26 MHz digitally controlled crystal
oscillator, designed for 8 pF crystals. The only
external part of the oscillator is the crystal itself.
Overall pulling range of the DCXO is
approximately
·
55 ppm, controllable by a 13-bit
tuning word
Figure. 3.4.1 Crystal Oscillator External Connection
tuning word.
This frequency serves as comparison frequency
within the RF-PLL and as clock frequency for the
digital circuitry.
The 26 MHz reference clock can also be applied
to external components like Bluetooth or GPS, via
the two buffered output signals FSYS1 and FSYS2
Figure. 3.4.2 Digital PREDISTORTION with LUXO
The DCXO tuning characteristic should be a first order linear function of the programming word AFC. The
variable capacitance array is a first order linear function of the digital word DIG, which leads to a nonlinear curve
ppm vs. DIG (and also a nonlinear ppm vs. AFC for DIG=AFC). In order to linearize the ppm vs. AFC curve the
implementation of a predistortion is necessary.
To get the wanted linear ppm vs. AFC tuning curve some digital predistortion of the AFC word is required. This
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To get the wanted linear ppm vs. AFC tuning curve some digital predistortion of the AFC word is required. This
predistortion is performed by the linearization unit for crystal oscillator (LUXO). The LUXO calculates the
corresponding DIG value according to the given AFC value.
FSYS1
FSYS
6 MHz XO
XOX
TDC
VBA
T
3.05V
-5.5V
LUXO
DIG = Y
DIGFILT
Cap
Array
AFC = X
DCXO_LUXO_PREDISTORTION