TDA5212_06 Datasheet, PDF (27/49 Pages) Infineon Technologies AG – ASK/FSK 915MHz Single Conversion Receiver

TDA 5212
Applications
4.4 Crystal Frequency Calculation
As mentioned in Section 3.4.3 the local oscillator (UHF PLL) signal has to be
high-side injected for a RF below 921MHz and low-side injected for a RF above
921MHz into the downconverting mixer. Thus the crystal frequency is calcu-
lated by using the following formula:
f _QU
=
f _RF
± 10.7
r
with
ƒ_RF .... receive frequency
ƒ_LO .... local oscillator (PLL) frequency (ƒ_RF ± 10.7)
ƒ_QU .... crystal oscillator frequency
r .... ratio of local oscillator (PLL) frequency and crystal
frequency as shown in the subsequent table.
Table 4-1 PLL Division Ratio Dependence on States of CSEL
CSEL
Ratio r = (f_LO/f_QU)
open
128
GND
64
This yields the following calculation for a RF of 915MHz for instance:
CSEL tied to GND¹:
f _QU
=
915MHz +10.7MHz
64
= 14.4641MHz
Wireless Components
1.In the Infineon Evalboard the L0 is used in low side injection mode and therefore
crystal with 14.1296875MHz is used. But to guarantee the function over the whole
temperature range the L0 has to be used in high side injection mode for a RF of
915MHz (see also VDO frequency range).
4-6
Specification, December 2006