mk3727e ETC-unknow, mk3727e Datasheet - Page 3

• Manufacturer Part Number: mk3727e
• Description: Low Cost 27 Mhz 3.3 Volt Vcxo
• Manufacturer: ETC-unknow

MDS 3727E D
External Component Selection
The MK3727E 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), 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
When the PCB trace between the clock output (CLK,
pin 5) and the load is over 1 inch, series termination
should be used. 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 MK3727E 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 (shown below) must be
used, and the layout guidelines discussed in the
following section shown 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 MK3727E incorporates on-chip
variable load capacitors that “pull” (change) the
frequency of the crystal. The crystal specified for use
with the MK3727E is designed to have zero frequency
error when the total of on-chip + stray capacitance is
14pF.
Recommended Crystal Parameters:
Integrated Circuit Systems
Initial Accuracy at 25
Temperature Stability
Aging
Load Capacitance
°
C
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525 Ra ce Stree t, Sa n Jose, CA 951 26
±20 ppm
±30 ppm
±20 ppm
14 pf
3
The third overtone mode of the crystal and all spurs
must be >100 ppm distant from 3x the fundamental
resonance measured with a physical load of 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 MK3727E. 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.
Crystals can be made to resonate either at the
fundamental frequency, or on the third, fifth, or even
higher overtone. VCXO crystals are always
fundamental mode, because overtone modes are much
less pullable and require additional oscillator circuitry
for proper operation.
The third overtone mode is not necessarily at exactly
three times the fundamental frequency. The
mechanical properties of the quartz element dictate the
position of the overtones relative to the fundamental,
and in a VCXO circuit, the third overtone is not typically
exactly three times the fundamental, or the oscillator
circuit may excite both the fundamental and overtone
modes simultaneously. This will cause a nonlinearity in
the transfer curve such as the one in Figure 3. This
potential problem is why VCXO crystals are required to
be tested for absence of any activity inside a +/-100
ppm window at three times the fundamental frequency.
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.
The procedure for determining the value of these
capacitors can be found in application note MAN05.
Shunt Capacitance, C0
C0/C1 Ratio
Equivalent Series Resistance
L
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27 MH
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3.3 V
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Revision 042105
OLT
MK3727E
35 Ω Max
7 pF Max
250 Max
VCXO