TDA5250D2_07 Datasheet, PDF (51/94 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

TDA5250 D2
Version 1.7
Application
δf_S ´
δD ₌
δC_L
f_S
δC_L
=
− C₁
2(C₀ + C_L )²
[3 – 15]
Choosing CL as large as possible results in a small pulling sensitivity. On the other hand a small CL
keeps the influence of the serial inductance and the tolerances associated to it small (see formula
[3-17]).
Start-up Time
t_Start
~
L₁
− R − R_ext
where: -R:
R_ext:
[3 – 16]
is the negative impedance of the oscillator
see Figure 3-15
is the sum of all external resistances (e.g. R₁ or any
other resistance that may be present in the circuit,
see Figure 3-14
The proportionality of L1 and C1 of the crystal is defined by formula [3-13]. 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 3-1 Crystal and crystal oscilator dependency
Result
Independent variable Relative Tolerance Maximum Deviation
C1 >
>>
>>
C0 >
<
<
frequency of quartz >
>>>
>
LOSC >
CL >
>>
>
>
<
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 [3-17].
Data Sheet
51
2007-02-26