TSH511CF datasheet(19/25 Pages) STMICROELECTRONICS

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TSH511

General description

19/25

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 Llo inductor and Clo capacitor. The tank circuit

is tuned on the receiving frequency.

The Rlo resistor is connected in parallel with Llo and Clo 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 Rlo gives a lower demodulated amplitude and reduces the

distortion.

The AC amplitude on pin 18 must not exceed 300mVRMS to prevent clipping by the internal

ESD diodes of the circuit.

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