DA6283.002

11 November, 2010

5/13

EXTERNAL COMPONENT SELECTION

Quartz Crystal and VCXO Module Information

To ensure the best system performance, the crystal

parameters should be considered carefully. Pulling

is an important parameter which can be calculated

according to an equation 1. Layout guidelines in the

following section should be followed. The frequency

of the crystal is tuned by load capacitors. There are

integrated variable load capacitors on the MAS6283

and they are controlled by an external voltage at the

VC pin. It is recommended to connect a 1 nF

crystal should be located as close to the chip as

possible. In case of a PCB mounted module, it is

usually advisable to mount a crystal on the same

side

with

the

VCXO

IC

to

minimize

stray

capacitance. Often vias between the crystal pins

and the XIN and XOUT pins of the VCXO IC

increase stray capacitance. There should be no

noisy signal traces underneath or close to the

crystal.

Equation 1

Crystal Pulling Sensitivity

pF

ppm

C

C

C

S

L

6

2

0

1

10

)

(

2

+

−

=

[values are given in the units described below]

Where,

Example 1

If we choose a crystal with the following values

the equation 1 yields

(

)

pF

ppm

S

5

.

33

10

10

0

.

8

10

0

.

2

2

10

7

.

6

6

2

12

12

15

−

=

×

+

×

×

−

=

−

−

−

frequency. The following guidelines show how to define the crystal’s nominal frequency.

Equation 2

XTAL

L

IC

L

L

C

C

C

_

_

−

=

∆

Calculate frequency difference ∆f [ppm] as in an equation 3. Pulling S comes from the equation 1.

Equation 3

S

C

f

∆

=

∆