TDA7110 Datasheet, PDF (25/39 Pages) Infineon Technologies AG – ASK/FSK Transmitter 868/433 MHz

TDA7110
Applications
3.4 Application Hints on the Crystal Oscillator
The crystal oscillator achieves a turn on time less than 1 msec when the
specified crystal is used. To achieve this, a NIC oscillator type is implemented
in the TDA7110. The input impedance of this oscillator is a negative resistance
in series to an inductance. Therefore the load capacitance of the crystal CL
(specified by the crystal supplier) is transformed to the capacitance Cv.
-R
L
f, CL Cv
TDA7110
Cv = ¹
(1)
1 _+ω2L
CL
CL: crystal load capacitance for nominal frequency
ω: angular frequency
L: inductance of the crystal oscillator
Example for the ASK-Mode:
Referring to the application circuit, in ASK-Mode the capacitance C7 is replaced
by a short to ground. Assume a crystal frequency of 13.56 MHz and a crystal
load capacitance of CL = 12 pF. The inductance L at 13.5 MHz is about 4.6 μH.
Therefore C6 is calculated to 8.567 pF, but due to parasitic capacitors of the
board C6 usually has to be smaller (e.g. 6.8 pF in the ASK evalboard)
Cv = ¹ = C6
1 _+ω2L
CL
Wireless Components
3-5
Data Sheet, December 2008