TDA7200 - Infineon

TDA7200
Applications
injection of the local oscillator has to be used for receive frequencies between 400 and
420MHz. In this case the local oscillator frequency is calculated by adding the IF
frequency (10.7 MHz) to the RF frequency. Thus the higher frequency of a FSKmodulated
signal is demodulated as a logical zero (low).
Low-side injection has to be used for receive frequencies above 420 MHz. The local
oscillator frequency is calculated by subtracting the IF frequency (10.7 MHz) from the RF
frequency then. In this case no sign-inversion occurs and the higher frequency of a FSKmodulated
signal is demodulated as a logical one (high). The overall division ratio in the
PLL is 32.
Therefore the crystal frequency may be calculated by using the following formula:
f
QU
f
= RF
±10.7
32
with
ƒ RF receive frequency
ƒ LO local oscillator (PLL) frequency (ƒ RF ± 10.7)
ƒ QU quartz crystal oscillator frequency
32 ratio of local oscillator (PLL) frequency and crystal frequency.
This yields the following example:
434 .2MHz
− 10.7MHz
f QU =
= 13. 234375 MHz
32
3.5 Data Slicer Threshold Generation
The threshold of the data slicer can be generated using an external R-C integrator as
shown in Figure 7.
The time constant T A of this circuit including also the internal resistors R F3int and R F4int
(see Figure 9) has to be significantly larger than the longest period of no signal change
T L within the data sequence.
In order to keep distortion low, the minimum value for R is 20kΩ.
Data Sheet 23 V 1.0, 2007-05-02