TSH511_07 Datasheet, PDF (19/25 Pages) STMicroelectronics – Hi-fi stereo/mono infrared receiver and stereo sub-carrier demodulator

TSH511
General description
The LNA output is directly connected to LNA-OUT1 and LNA-OUT2 pins in parallel (pin 8
and 4 respectively). Because the LNA minimum impedance load is 1kΩ, the load impedance
on each pin LNA-OUT1 or LNA-OUT2 must be 2kΩ minimum.
The series 50Ω resistor and the 100nF capacitor decouple the LNA supply voltage (pin 5),
thus improving noise performance.
8.2
AMP and LIM sections: amplifier and limiter
The first filter output is connected to the intermediate amplifier AMP. The gain of AMP is
typically 20dB. The second filter is connected between the output of AMP and the input of
the limiter LIM. The 60dB limiter LIM provides a constant amplitude signal to the FM
demodulator. It reduces the AM parasitic effects into the FM demodulator.
Figure 27. AMP and LIM schematics
8.3
FM demodulator section
The FM demodulator is a classic quadrature demodulator based on a multiplier. The
quadrature is performed with the tank circuit L_lo inductor and C_lo capacitor. The tank circuit
is tuned on the receiving frequency.
The R_lo resistor is connected in parallel with L_lo and C_lo to reduce the Q factor of the tank
circuit. It allows to adjust the demodulation characteristics. For a given transmitting
deviation, a lower value of R_lo gives a lower demodulated amplitude and reduces the
distortion.
The AC amplitude on pin 18 must not exceed 300mV_RMS to prevent clipping by the internal
ESD diodes of the circuit.
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