TDA7200 Datasheet, PDF (28/49 Pages) Infineon Technologies AG – ASK/FSK Single Conversion Receiver

TDA7200
Applications
3.8
ASK Mode
In case the receiver is operated in ASK mode the datapath frequency charactersitic is
dominated by the data filter alone, thus it is lowpass shaped. The cutoff frequency is
determined by the external capacitors C₁₂ and C₁₄ and the internal 100k resistors as
described in Section 3.2
0dB
-3dB
-40dB/dec
f
f3dB
Figure 11 Frequency characteristic in case of ASK mode
3.9
Principle of the Precharge Circuit
In case the data slicer threshold shall be generated with an external RC network as
described in Section 3.5 it is necessary to use large values for the capacitor C attached
to the SLN pin (pin 20) in order to achieve long time constants. This results also from the
fact that the choice of the value for R1 connected between the SLP and SLN pins (pins
19 and 20) is limited by the 330kΩ resistor appearing in parallel to R1 as can be seen in
Figure 9. Apart from this a resistor value of 100kΩ leads to a voltage offset of 1mV at
the comparator input. The resulting startup time constant τ₁ can be calculated with:
τ₁ = (R1|| 330kΩ)×C13
In case R1 is chosen to be 100kΩ and C13 is chosen as 47nF this leads to
τ₁ = (100kΩ || 330kΩ)× 47nF = 77kΩ × 47nF = 3.6ms
When the device is turned on this time constant dominates the time necessary for the
device to be able to demodulate data properly. In the powerdown mode the capacitor is
only discharged by leakage currents.
Data Sheet
28
V 1.0, 2007-05-02