CF5072BA datasheet(2/8 Pages) NPC

2 / 8 page

CF5072BA

SEIKO NPC CORPORATION —2

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 LS and RP 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.

Oscillator internal capacitors (design value)

Selecting external constants

The LS and RP values should be selected such that both (a) the resonance point in the loop formed by LS and

C0, CL, CVC is higher than the crystal oscillator frequency, and (b) the resonance point does not satisfy the

oscillation condition. (C0 is the crystal shunt capacitance, CL is the oscillator equivalent circuit capacitance,

and CVC is the varicap capacitance.)

In the oscillator circuit, if the crystal capacitance C0 is 2.85pF, the varicap (CVC) is a HVC350B (Renesas),

and the oscillator frequency is 155.52MHz, then values in the order LS = 220nH, RP = 2.2kΩ or LS = 180nH,

RP = 1.8kΩ will satisfy the conditions above. The optimal values for LS and RP will vary with crystal charac-

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

Note. RP 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 RP be connected in parallel with LS.

Version

Internal capacitance [pF] (design value)

CF5072BA

11.2

14.4

XIN

VSS1

RVC

100pF

To differential

converter

VC input

X'tal

CVC

CF5072BA 데이터시트(PDF) 2 Page - Nippon Precision Circuits Inc