TDA5250D2_07 Datasheet, PDF (52/94 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

TDA5250 D2
Version 1.7
Application
-R
L_OSC
f, C_L
C_V
TDA 5250
Figure 3-15 Crystal Oscillator
QOSZ_NIC.wmf
C_L
=
1
C_V
1
− ω ² L_OSC
↔ C_V
=
1
C_L
1
+ ω ² L_OSC
[3 – 17]
C_L:
ω:
L_OSC:
crystal load capacitance for nominal frequency
angular frequency
inductivity of the crystal oscillator - typ: 2.7µH with pad of board
2.45µH without pad
With the aid of this formula it becomes obvious that the higher the serial capacitance CV is, the
higher is the influence of LOSC.
The tolerance of the internal oscillator inductivity is much higher, so the inductivity is the dominating
value for the tolerance.
FSK modulation and tuning are achieved by a variation of C_v.
In case of small frequency deviations (up to +/- 1000 ppm), the desired load capacitances for FSK
modulation are frequency depending and can be calculated with the formula below.
CL _±
CL
+−
C0
⋅
N---^∆---⋅^-f--f-
⋅

1

+
2-----⋅_---(_{--C----C-0---1-+-----C----L----})_{-}
= -----------------------------------------------------------------------------------------
1
±
N---^∆---⋅^-f--f-
⋅

1

+
-2----^â‹---⁽--C----C-0---1-+-----C----L----⁾-_{}
[3 – 18]
C_L:
crystal load capacitance for nominal frequency
C₀:
shunt capacitance of the crystal
C₁:
motional capacitance of the crystal
f:
crystal oscillator frequency
N:
division ratio of the PLL
Data Sheet
52
2007-02-26