TSH511 datasheet(15/18 Pages) STMICROELECTRONICS

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TSH511

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The

LNA

output

directly

connected

LNA-OUT1 and LNA-OUT2 pins in parallel (pin 8

and 4 respectively). As the LNA minimum imped-

ance load is 1 k

Ω, the load impedance on each pin

LNA-OUT1 or LNA-OUT2 must be 2 k

Ω minimum.

The series 50

Ω resistor and the 100 nF capacitor

decouple the LNA supply voltage (pin 5), improv-

ing noise performance.

AMP and LIM sections: Amplifier and Limitor

The first filter output is connected to the intermedi-

ate amplifier AMP. The gain of AMP is typically 20

dB. The second filter is connected between the

output of AMP and the input of the limitor LIM. The

60 dB limitor LIM provides a constant amplitude

signal to the FM demodulator. It reduces the AM

parasitic effects into the FM demodulator.

Figure 4 : AMP and LIM schematic

FM demodulator section

The FM demodulator is a classical quadrature de-

modulator based on a multiplier. The quadrature is

performed with the tank circuit Llo inductor and

Clo capacitor. The tank circuit is tuned on the re-

ceiving frequency.

The Rlo resistor is connected in parallel with Llo

and Clo to reduce the Q factor of the tank circuit. It

allows to adjustment of the demodulation charac-

teristic. 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 300

mVRMS to prevent clipping by the internal ESD di-

odes of the circuit.

The low output impedance of the demodulator

(MIX-OUT pins) drives the external volume control

and the de-emphasis filter.

Figure 5 : FM demodulator schematic

SQUELCH section

To avoid audio ’FM noise’ at the speakers when

no carrier is received, the TSH511 has a built-in

Squelch circuit. The Squelch detects the wide-

band ’FM noise’ on the demodulated output and

fades-out the audio of both audio buffers.

The audio is filtered from the ’FM noise’ using the

high-pass RC filter. The cut-off frequency is typi-

cally 100kHz. The squelch level depends on the

value of the RMUTE resistor in serie with the input

pin of the noise amplifier MUTE-IN (pin 23). The

detected noise is integrated with the Cpeak ca-

pacitor connected to MUTE-INT pin. The DC volt-

age on this pin follows the amplitude of the noise.

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