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Specifications subject to change without notice. All dimensions in inches. © Copyright 2012 The Connor-Winfield Corporation
Application Note
AN2093
Page
14
Revision
P01
Date
10 May 2013
2111 Comprehensive Drive
Aurora, Illinois 60505
Phone: 630-851-4722
Fax: 630- 851- 5040
www.conwin.com
4.1 Layout Considerations
To achieve the stated frequency stability specifications
the OCXO must be able to reach and sustain thermal
equilibrium over ALL operating conditions. Observation
of the ambient operating temperature range, controlled
air flow, thermal ramp rates and minimizing thermal
energy gains/losses are critical for a successful layout.
“Any attempt to cool, or disperse this heat will cause
oven core temperature to drift and OCXO will not meet
the specified frequency stability. Improper board layout
could also allow heat transfer from nearby components
to overheat the oven core resulting in loss of specified
frequency stability.”
Several thermal design parameters must be carefully
considered.
• Board Layout Considerations
• Controlling Thermal Transfer/Transients
• Controlling Air Flow
4.2 Board Layout Considerations
Careful selection of layer stack-ups, ground fills, and
trace routing is highly recommended for a successful
layout.
“To achieve the stated frequency stability the OCXO
must be able to reach and sustain thermal equilibrium
over ALL operating conditions”
Consider the following layout concerns.
1. Always place the OCXO near the timing circuitry,
and keep all Power / Ground and RF traces as
small as possible.
2. Always adhere to stated loading specifications}
for OCXO RF output. OCXOs are load sensitive
and require an equivalent load capacitor with a
flat capacitance vs. temperature curve to
achieve stated frequency stability specifications.
Consider NPO/COG capacitors when practical
for optimal temperature characteristics. See
FIGURE 13-01 Frequency Response to various
equivalent capacitive loads.
3. Use secondary buffers to fan the OCXO RF
signal to multiple inputs or timing circuits. Avoid
designs that would “switch in” additional
capacitive loads. Avoid designs that would
“switch between” multiple capacitive loads.
OCXOs are load sensitive devices.
See FIGURE XXX
Section 4: Multi-Layer Board Design / Thermal Considerations
4. Always use speed rated level translators or
buffers in applications requiring communication
between digital devices operating from multiple
supply voltages. Never use resistor divider
networks on RF signals.
5. It is recommended to place a 10uF to 47uF bulk
capacitor as close to as possible to the VCC pin
of the device.
6. Place a small ceramic decoupling capacitor
typically NPO/COG/X7R with a 2 to 3 ohm
reactance at the output frequency of the
OCXO to shunt any noise on the supply rail to
ground. Additional decoupling capacitors
can be used to filter out other unwanted supply
noise generated from other devices.
7. Avoid using series current sense resistors in
OCXO monitoring applications. Improper
selection of resistor values could create
significant voltage drops when combined with
the thermal coefficients of the supply, and large
current draw from the OCXO.
8. Careful evaluation of via size should be
considered with all high power devices to
guarantee sufficient current. Maximum
current through a via calculations should be
made using highest expected board temperature
instead of maximum ambient temperature as
heat generating components will typically heat
the board well beyond the ambient temperature
range of the OCXO. Improper via size selection
could cause current starvation issues which
would result in current oscillations as the oven
core is starved of power and unable to reach
thermal equilibrium. Via to pad stringers
or runners should also be evaluated for sufficient
current carrying capacity at maximum expected
board temperatures. Via in pad work best, but
adds additional board cost.