TDA5250D2 Datasheet, PDF (54/97 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

confidential
TDA 5250 D2
preliminary
Applications
δD
δC_L
=
δf_S ´
δC
f
L
S
=
− C₁
2(C₀ + C_L )²
[4 – 15]
Choosing C_L as large as possible results in a small pulling sensitivity. On the
other hand a small C_L keeps the influence of the serial inductance and the
tolerances associated to it small (see formula [4-17]).
Start-up Time
t_Start
~
L₁
− R − R_ext
[4 – 16]
where: -R:
R_ext:
is the negative impedance of the oscillator
(see fig. [4-15])
is the sum of all external resistances (e.g. R₁ or any
other resistance that may be present in the circuit,
see figure [4-14]
The proportionality of L₁ and C₁ of the crystal is defined by formula [4-13]. For
a crystal with a small C₁ the start -up time will also be slower. Typically the lower
the value of the crystal frequency, the lower the C₁.
A short conclusion regarding crystal and crystal oscillator dependencies is
shown in the following table:
Table 4-1 Crystal and crystal oscilator dependency
Result
Independent variable
Relative
Tolerance
Maximum
Deviation
C₁ >
>>
>>
C₀ >
<
<
frequency of quartz >
>>>
>
L_OSC >
>>
>
C_L >
>
<
t_Start-up
<
-
<<
-
-
The crystal oscillator in the TDA5250 is a NIC (negative impedance converter)
oscillator type. The input impedance of this oscillator is a negative impedance
in series to an inductance. Therefore the load capacitance of the crystal C_L
(specified by the crystal supplier) is transformed to the capacitance C_v as
shown in formula [4-17].
Wireless Components
4 - 14
Specification, July 2002