TDA5220 Datasheet, PDF (29/44 Pages) Infineon Technologies AG – ASK/FSK Single Conversion Receiver

confidential
TDA 5220
preliminary
Applications
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
content of FSK signal remains at 180µV/kHz. The cutoff frequencies of the
bandpass have to be chosen such that the spectrum of the data signal is influ-
enced in an acceptable amount.
In case that the user data is containing long sequences of logical zeroes the
effect of the drift-off of the bit slicer threshold voltage can be lowered if the offset
voltage inherent at the negative input of the slicer comparator (Pin20) is used.
The comparator has no hysteresis built in.
This offset voltage is generated by the bias current of the negative input of the
comparator (i.e. 20nA) running over the external resistor R. This voltage raises
the voltage appearing at pin 20 (e.g. 1mV with R = 100kΩ). In order to obtain
benefit of this asymmetrical offset for the demodulation of long zeros the lower
of the two FSK frequencies should be chosen in the transmitter as the zero-
symbol frequency.
In the following figure the shape of the above mentioned bandpass is shown.
gain (pin19)
v
v-3dB
3dB
0dB
DC
20dB/dec
f1
f2
-40dB/dec
f
f3
0.18mV/kHz
2mV/kHz
Figure 4-8 Frequency characterstic in case of FSK mode
frequenzgang.WMF
The cutoff frequencies are calculated with the following formulas:
f₁ =
2π
1
R ⋅330kΩ _⋅C
R + 330kΩ
Wireless Components
4-9
Target Specification, October 2001