MK3727H
LOW DRIVE 3.3 VOLT VCXO VCXO AND MULTIPLIER
IDT™
LOW DRIVE 3.3 VOLT VCXO 3
MK3727H REV F 122109
External Component Selection
The MK3727H requires a minimum number of external
components for proper operation.
Decoupling Capacitor
A decoupling capacitor of 0.01µF must be connected
between VDD (pin 2) and GND (pin 4 & 7), as close to these
pins as possible. For optimum device performance, the
decoupling capacitor should be mounted on the component
side of the PCB. Avoid the use of vias in the decoupling
circuit.
Series Termination Resistor
Use series termination when the PCB trace between the
clock outputs and the loads are over 1 inch. To series
terminate a 50
Ω trace (a commonly used trace impedance),
place a 33
Ω resistor in series with the clock line, as close to
the clock output pin as possible. The nominal impedance of
the clock output is 20
Ω.
Quartz Crystal
The MK3727H VCXO function consists of the external
crystal and the integrated VCXO oscillator circuit. To assure
the best system performance (frequency pull range) and
reliability, a crystal device with the recommended
parameters (as described in application note MAN05) must
be used, and the layout guidelines discussed in the following
section must be followed.
The frequency of oscillation of a quartz crystal is determined
by its “cut” and by the load capacitors connected to it. The
MK3727H incorporates on-chip variable load capacitors that
“pull” (change) the frequency of the crystal. The crystal
specified for use with the MK3727H is designed to have zero
frequency error when the total of on-chip + stray
capacitance is
14 pF.
The external crystal must be connected as close to the chip
as possible and should be on the same side of the PCB as
the MK3727H. There should be no vias between the crystal
pins and the X1 and X2 device pins. There should be no
signal traces underneath or close to the crystal.
Crystal Tuning Load Capacitors
The crystal traces should include pads for small fixed
capacitors, one between X1 and ground, and another
between X2 and ground. Stuffing of these capacitors on the
PCB is optional. The need for these capacitors is
determined at system prototype evaluation, and is
influenced by the particular crystal used (manufacture and
frequency) and by PCB layout. The typical required
capacitor value is 1 to 4 pF.
To determine the need for and value of the crystal
adjustment capacitors, you will need a PC board of your final
layout, a frequency counter capable of about 1 ppm
resolution and accuracy, two power supplies, and some
samples of the crystals which you plan to use in production,
along with measured initial accuracy for each crystal at the
specified crystal load capacitance, CL.
To determine the value of the crystal capacitors:
1. Connect VDD of the MK3727H to 3.3 V. Connect pin 3 of
the MK3727H to the second power supply. Adjust the
voltage on pin 3 to 0V. Measure and record the frequency of
the CLK output.
2. Adjust the voltage on pin 3 to 3.3 V. Measure and record
the frequency of the same output.
To calculate the centering error:
Where:
f
target
= nominal crystal frequency
error
xtal
=actual initial accuracy (in ppm) of the crystal being
measured
If the centering error is less than ±25 ppm, no adjustment is
needed. If the centering error is more than 25 ppm negative,
the PC board has excessive stray capacitance and a new
PCB layout should be considered to reduce stray
capacitance. (Alternately, the crystal may be re-specified to
a higher load capacitance. Contact IDT for details.) If the
centering error is more than 25 ppm positive, add identical
Error 10
6
x
f
3.0V
f
tetarg
–()f
0V
f
tetarg
–()+
f
tetarg
------------------------------------------------------------------------------
error
xtal
–=
