TDA5212 Datasheet, PDF (29/51 Pages) Infineon Technologies AG – ASK/FSK 915MHz Single Conversion Receiver

TDA 5212
preliminary
Applications
4.6 ASK/FSK Switch Functional Description
The TDA5211 is containing an ASK/FSK switch which can be controlled via Pin
15 (MSEL). This switch is actually consisting of 2 operational amplifiers that are
having a gain of 1 in case of the ASK amplifier and a gain of 11 in case of the
FSK amplifier in order to achieve an appropriate demodulation gain character-
istic. In order to compensate for the DC-offset generated especially in case of
the FSK PLL demodulator there is a feedback connection between the thresh-
old voltage of the bit slicer comparator (Pin 20) to the negative input of the FSK
switch amplifier. This is shown in the figure below:
RSSI (ASK signal)
15 MSEL
ASK/FSK Switch
Data Filter
FSK PLL Demodulator
0.18 mV/kHz
typ. 2 V
1.5 V......2.5 V
-
+
ASK
+ _FSK
-
R₁=100k R₂=100k
v=1
R₃=300k
R₄=30k
ASK mode : v=1
FSK mode : v=11
FFB 22
21 OPP SLP 19
+
Comp
-
DATA Out
25
20 ^SLN
C₁
C₂
R
C
Figure 4-7 ASK/FSK mode datapath
ask_fsk_datapath.WMF
4.6.1 FSK Mode
The FSK datapath has a bandpass characterisitc due to the feedback shown
above (highpass) and the data filter (lowpass). The lower cutoff frequency f2 is
determined by the external RC-combination. The upper cutoff frequency f3 is
determined by the data filter bandwidth.
The demodulation gain of the FSK PLL demodulator is 180µV/kHz. This gain is
increased by the gain v of the FSK switch, which is 11. Therefore the resulting
dynamic gain of this circuit is 2mV/kHz within the bandpass. The gain for the DC
Wireless Components
4-8
Specification, May 2003