TDA5212_06 Datasheet, PDF (19/49 Pages) Infineon Technologies AG – ASK/FSK 915MHz Single Conversion Receiver

TDA 5212
Functional Description
quency band of 910 to 932MHz. But please note that using high side injetion of
the L0 yields a sign inversion of the demodulated data signal in case of FSK.
See also Section 4.4.
3.4.4 Crystal Oscillator
The on-chip crystal oscillator circuitry allows for utilisation of quartzes both in
the 7 and 14MHz range as the overall division ratio of the PLL can be switched
between 64 and 128 via the CSEL (Pin 16 ) pin according to the following table.
Table 3-2 CSEL Pin Operating States
CSEL
Open
Shorted to ground
Crystal Frequency
7.xx MHz
14.xx MHz
The calculation of the value of the necessary quartz load capacitance is shown
in Section 4.3, the quartz frequency calculation is expained in Section 4.4.
3.4.5 Limiter
The Limiter is an AC coupled multistage amplifier with a cumulative gain of
approximately 80dB that has a bandpass-characteristic centred around
10.7MHz. It has an input impedance of 330Ω to allow for easy interfacing to a
10.7MHz ceramic IF filter. The limiter circuit acts as a Receive Signal Strength
Indicator (RSSI) generator which produces a DC voltage that is directly propor-
tional to the input signal level as can be seen in Figure 4.2. This signal is used
to demodulate the ASK receive signal in the subsequent baseband circuitry and
to turn down the LNA gain by approximately 18dB in case the input signal
strength is too strong as described in Section 3.4.1 and Section 4.1.
3.4.6 FSK Demodulator
To demodulate frequency shift keyed (FSK) signals a PLL circuit is used that is
contained fully on chip. The Limiter output differential signal is fed to the linear
phase detector as is the output of the 10.7MHz center frequency VCO. The
demodulator gain is typically 200µV/kHz. The passive loop filter output that is
comprised fully on chip is fed to both the VCO and the modulation format
switch.This signal is representing the demodulated signal. This switch is actu-
ally a switchable amplifier with an AC gain of 11 that is controlled by the MSEL
Wireless Components
3 - 11
Specification, December 2006