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Specifications subject to change without notice. All dimensions in inches. © Copyright 2012 The Connor-Winfield Corporation
Application Note
AN2093
Page
15
Revision
P01
Date
10 May 2013
2111 Comprehensive Drive
Aurora, Illinois 60505
Phone: 630-851-4722
Fax: 630- 851- 5040
www.conwin.com
Multi-Layer Board Design / Thermal Considerations
9.
Low impedance power and ground planes
should be used instead of stringer traces or star
based power distribution methods to reduce
induced voltages caused by high current
devices such as OCXOs.
10. For control voltage equipped models it is
important to connect the ground of the control
voltage source as close as possible to the OCXO
ground to minimize trace impedance and
therefore minimize any generated voltages
which could cause large frequency errors. It is
highly recommended to use the supplied
OCXO VREF source to derive the control voltage
when equipped. For models that do not offer
the OCXO VREF source it is also recommended
that the control voltage source run off the same
rail as the OCXO to eliminate the supply voltage
thermal drift errors that could exist between
multiple rails. This will prevent large unwanted
frequency shift errors as the relative voltage drift
between the supplies will be eliminated.
Scenario: Assume a control voltage equipped
OCXO with a tuning sensitivity of 7ppm / volt is
powered from a 3.3V supply (3V3_OCXOSupply)
with a temperature coefficient of 2.6mV / °C.
Free via size calculations tools are available online.
Saturn PCB Design Inc. offers this free tool available at
the following URL.
Figure: 15-01
The DAC which drives the OCXO CV pin is
powered from a secondary 3.3V rail (3v3_
DacSupply) with a thermal coefficient of
0.2mV / °C (See Figure 15-02 Example
schematic Figure 15-03 Thermal Coefficient vs.
Temperature) In this example the OCXO was
initially tuned to 0 ppm at 0°C with a calibration
error of ± 0.025 ppm. As the ambient
temperature in the system changes to 70
°
C over
the next hour, the voltage error between the two
rails would have drifted from 0 volts between
the two rails, to 0.1834 volts resulting in a
1.2838 ppm frequency error.
DNC
DNC
Figure: 15-03 Thermal Drift between Supply
Rails
Figure: 15-02 Example Circuit
http://saturnpcb.com/pcb_toolkit.htm
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