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FS6131

varying the voltage on the XTUNE pin. The total change (from one extreme to the other) in effective loading capacitance is 1.5pF

nominal, and the effect is shown in Error! Reference source not found.. The oscillator operates the crystal resonator in the parallel-

resonant mode. Crystal warping, or the "pulling" of the crystal oscillation frequency, is accomplished by altering the effective load

capacitance presented to the crystal by the oscillator circuit. The actual amount that changing the load capacitance alters the oscillator

frequency will be dependent on the characteristics of the crystal as well as the oscillator circuit itself.

determine the total warping capability of a crystal is

(

)

() (

)

C

C

C

C

C

C

C

ppm

f

L

L

L

L

1

0

2

0

6

1

2

1

2

10

)

(

+

×

+

×

×

−

×

=

Δ

Example: A crystal with the following parameters is used with the FS6131. The total coarse tuning range is:

(

)

() (

)

ppm

.

.

.

f

305

10

5

66

22

5

2

10

10

66

22

02

0

6

=

+

×

+

×

×

−

×

=

Δ

4.4.1. VCXO Tuning

The VCXO may be coarse tuned by a programmable adjustment of the crystal load capacitance via the XCT[3:0] control bits. See Table

11 for the control code and the associated loading capacitance.

The actual amount of frequency warping caused by the tuning capacitance will depend on the crystal used. The VCXO tuning

capacitance includes an external 6pF load capacitance (12pF from the XIN pin to ground and 12pF from the XOUT pin to ground). The

fine tuning capability of the VCXO can be enabled by setting the XLVTEN bit to a one, or disabled by setting it to a zero.

Error! Reference source not found. shows the typical effect of the coarse and fine tuning mechanisms. The total coarse tune range is

about 350ppm. The difference in VCXO frequency in parts per million (ppm) is shown as the fine tuning voltage on the XTUNE pin

varies from 0V to 5V. Note that as the crystal load capacitance is increased the VCXO frequency is pulled somewhat less with each

coarse step, and the fine tuning range decreases. The fine tuning range always overlaps a few coarse tuning ranges, eliminating the

possibility of holes in the VCXO response. The different crystal warping characteristics may change the scaling on the Y-axis, but not

the overall characteristic of the curves.

VCXO Range (ppm) vs. XTUNE Voltage (V)

-200

-150

-100

-50

0

50

100

150

200

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

Coarse Tune Setting XCT[3:0]

XTUNE Voltage = 0.0V

XTUNE Voltage = 5.0V

Figure 11: VCXO Course and Fine Tuning