General description

TSH511

18/25

The demodulated signal can be amplified by the audio buffer section after de-emphasis.

Each audio buffer can drive a 16

Ω headphone with 20mW power.

The two standby pins SBY1 & SBY2 allow the second receiver RX2 to be put into standby

for mono operation. In mono mode, it is possible to use both audio buffers or only one

depending on the combination on SBY1 & SBY2.

To avoid noise at the audio output, a squelch section mutes the audio buffers when no

carrier is received. The squelch section uses the demodulated signal of the first receiver

(RX1). This signal is high-pass filtered, rectified and compared to a threshold to produce the

Mute signal (pin 25). When no carrier is received on RX1, the wideband ’FM noise’ on the

demodulator increases and the Mute signal mutes both audio buffers. When the carrier is

present, the wideband noise on the demodulator output decreases, enabling the audio

buffer.

8.1

LNA section: low noise amplifier

The low noise amplifier (LNA) has a typical gain of 22dB to amplify the incoming RF signal

from the photo diode. The LNA is common to both receiver sections RX1 and RX2.

Figure 26.

LNA schematics

Table 5.

Infrared audio frequencies

IR frequency in MHz

Applications

1.6

AM mono

1.7

FM mono

2.3

FM right channel

2.8

FM left channel or mono