As shown In the block diagram above, the audio output (pin 10) of the RX FM AUDIO module is connected to a de-emphasis network, comprising of a 47nF capacitor connected to ground. This de-emphasis network together with the pre-emphasis network at the TX-FM AUDIO module end improves the linearity of the Low Frequency (audio) response and increases the related signal-to-noise ratio. An audio power amplifier in series with the de-emphasis network, is used to drive a loudspeaker or acoustic box. There is a 1.2Vdc component superimposed on the true demodulated signal at the receiver audio output (pin 10) therefore it is necessary to use a power amplifier with an AC-coupled input. The audio amplifier supply, indicated here to be 24V, is taken through a relay driven by the squelch circuit of the RX FM AUDIO module: the maximum current which can be supplied from pin 18 (squelch output) is 100mA. If the RF signal power is lower than the pre-set squelch threshold, the squelch circuit causes the relay to open thus removing power from amplifier and the loudspeaker is silenced……

The schematic diagram above provides an application example using the RX-FM AUDIO receiver module together with the companion TX-FM AUDIO and interfacing components to make up a half duplex two-way radio or ‘walkie talkie’….

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