Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

 

 

The various influences indicated in Fig 8 are described below: 

1.  The line at the top indicates the frequency tolerance of the crystal in this example. 

The distance between two vertical lines represents (at the top) 1 s/day or 11.57 ppm. 
The crystal spread covers about 8.5 lines and thus indicates a spread of about 
100 ppm. 

2.  The production spread of the crystal can be compensated by adjusting the pulling 

capacitor C

T

 (in the graph called C

6

) as long as the value of C

T

 is chosen correctly. 

Here the pulling range is large enough to compensate for a spread of ±175 ppm. 
Typically a variable capacitor has a temperature coefficient of ±300 ppm/°C to 500 
ppm/°C. This capacity change has a very small influence on the accuracy of the 
oscillator and its influence is shown in the third line from the bottom of the graph. The 
solid line shows the impact of C

T

 (here C

6

) if the value is small. If C

T

 is large the 

variation is larger as indicated with the dotted line. 

3.  Zooming in on a range of about 35 ppm… 
4.  The integrated load capacitance has a finite production spread and its potential 

influence on the accuracy of the oscillator depends on the value of the integrated 
capacitor. Whether the internal capacitor is connected to the input (C

IN

 or to the 

output C

OUT

) doesn’t make a difference. The solid line (third solid line from the top) 

covers a range of 1.8 s/day, or around 21 ppm. A greater value of C

IN

 could have a 

larger influence indicated by the dotted line.