MTC-20144TQ-I AMI Semiconductor, Inc., MTC-20144TQ-I Datasheet - Page 16

• Manufacturer Part Number: MTC-20144TQ-I
• Description: Integrated ADSL CMOS Analog Front-End Circuit
• Manufacturer: AMI Semiconductor, Inc.

VCXO
A voltage controlled crystal oscillator
driver is integrated in MTC-20144. Its
nominal frequency is 35.328MHz. The
quartz crystal is connected between the
pins XTAL1 and XTAL2.
The principle of the VCXO control is
shown on the figure below. The
information coming from the digital
processor via the CTRLIN path is used to
drive a 8 bits I-DAC which generate a
control current.
This current is externally converted and
filtered to generates the required control
voltage (range: -15 to 0.5V) for the
varicap. The VCXO characteristics are
given in Table 4.
N.B: Frequency tuning range is
proportional to the crystal dynamic
capacitance Cm.
Parameters
Absolute frequency accuracy
Frequency tunning range
VCXO output current
Table 4: VCXO Characteristics
Fig. 10: Principle of VCXO Control
The tuning is monotonic with 8-bit
resolution with the worst-case tuning step
of < 2ppm/LSB (8-bit). The time
constant of the tuning can be selected
through an external capacitor Ct.
CTRLIN=010
bit3
-15ppm
Min
16
35.328Mhz
Nominal
±50ppm
100 µA
MTC-20144
+ 15ppm
Max
Rref=16,5 kΩ
AVDD=3.3V
Note