CF5072BA 参数 Datasheet PDF下载

CF5072BA

FUNCTIONAL DESCRIPTION

Oscillator Equivalent Circuit

The oscillator can be represented by the equivalent circuit shown below. The crystal unit is connected to XIN,

and the other terminal is connected to the L

S

and R

P

network. A varicap is added with cathode connected to

XR, and anode connected to XC.

The control voltage is applied to the VC pin, with high-resistance element connected between VC and XR

built-in.

R

P

L

S

X'tal

XIN

XC

100pF

To differential

converter

C2

R

S

VC

C1

C

VC

XR

R

VC

VC input

VSS1

Note. R

P

is a damping resistor to prevent parasitic oscillation due the combined effects of the external inductor (expander coil) and

varicap capacitance/internal capacitance. It is recommended that R

P

be connected in parallel with L

S

.

Oscillator internal capacitors (design value)

Internal capacitance [pF] (design value)

Version

C1

CF5072BA

11.2

C2

14.4

Selecting external constants

The L

S

and R

P

values should be selected such that both (a) the resonance point in the loop formed by L

S

and

C0, C

L

, C

VC

is higher than the crystal oscillator frequency, and (b) the resonance point does not satisfy the

oscillation condition. (C0 is the crystal shunt capacitance, C

L

is the oscillator equivalent circuit capacitance,

and C

VC

is the varicap capacitance.)

In the oscillator circuit, if the crystal capacitance C0 is 2.85pF, the varicap (C

VC

) is a HVC350B (Renesas),

and the oscillator frequency is 155.52MHz, then values in the order L

S

= 220nH, R

P

= 2.2k

Ω

or L

S

= 180nH,

R

P

= 1.8k

Ω

will satisfy the conditions above. The optimal values for L

S

and R

P

will vary with crystal charac-

teristics, oscillator frequency, and varicap diode, thus the values selected should be thoroughly evaluated.

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