Additional considerations

AN2971

4/12

Doc ID 15627 Rev 1

For the purposes of this application note, the net result of considering the turnover point is

an increase in the amount of uncertainty in the drift at the temperature extremes. With this

added, the straightened uncertainty region shown at the top of Figure 1 becomes that of

Figure 4 below.

Figure 4.

Range of drift after compensation

In Figure 4, the black dashed lines show the entire range of the drift after compensation. As

before, in Figure 1, the gray region represents the parabolas after they have been

straightened along the temperature axis.

After compensation, the drift should be no worse than ±20 ppm in the range –10 to +50°C,

and typically less than ±10 ppm in that range as shown by the gray dashed lines for

Crystal equation

The drift of a tuning fork crystal is described by the equation below.

temperature, in Celsius. k is a constant in the range 0.030 to 0.042, with a typical value of

0.036. When plotted, the equation gives the curves shown in Figure 1 through 4. The typical

case in Figure 1 is for k = 0.036. The other values of k give the minimum and maximum

(dashed line) curves.

-60

-50

-40

-30

-20

-10

0

10

20

30

40

-50 -40 -30 -20 -10

0 10

20 30

40 50 60

70 80

90 100

TEMP (°C)

25°C

to 30°C

AM04630v1

0

0

T

T

k

f

f

−

⋅

−

=

Δ