AN95089 Datasheet, PDF (10/19 Pages) Ramtron International Corporation – BLE Crystal Oscillator Selection and Tuning Techniques

PSoC® 4/PRoC™ BLE Crystal Oscillator Selection and Tuning Techniques
6 Crystal Analysis for ECO
6.1 Frequency Variation with Temperature
Figure 7 shows the frequency variation that was measured for different crystals with temperature.
Figure 7. ppm Variation with Temperature
ppm vs temperature
10
5
0
KDS
-50
-30
-10
10
30
50
70
90
NDK
-5
Epson
-10
ECS Inc
-15
-20
temperature(◦C)
This graph shows that the ppm variation for the four crystals across the temperature range of the device meets BLE
specification of ± 50 ppm. However, other factors like aging, pullability, and stray capacitances can also change the
crystal ppm. Therefore, the cumulative effect of all these factors on ppm should be within ± 50 ppm.
6.2 Frequency Variation with Load Cap Value
Figure 8 shows the frequency variation for crystals that were measured with the load capacitance value.
Figure 8. ppm Variation with Load Cap Value
ppm vs load cap value
150
100
50
ECS Inc
KDS
0
Epson
0
2
4
6
8
10
12
NDK
-50
-100
load cap value(pf)
This graph shows that the frequency can be changed exponentially from positive offset (positive ppm) to negative
offset (negative ppm) by changing the load capacitance value. This load capacitor is internal to the PSoC 4 / PRoC
BLE device.
However, it is important to note that the ppm variation with load capacitance value depends heavily on the PCB
layout of the device. A PCB layout with a lot of stray capacitances may leave the frequency insensitive to changes in
internal trim capacitances and therefore tuning may become difficult. Some general guidelines to reduce parasitic
capacitance on the board are discussed in Layout Considerations for PCB section.
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Document No. 001-95089 Rev. *A
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