Wireless Microphone: Audio in the ISM band 3

Published:2011/8/19 2:07:00 Author:Amy From:SeekIC

Ton Giesberts

The receiver

The block diagram of the WA-RX-01 receiver module shown in Figure 3 is practically the mirror image of the transmitter, using similar components.



RF band-pass filter (SAW)
The 800 MHz frequency band used in this wireless audio system is extracted using this filter. A high-selectivity SAW filter is used to ensure that frequencies outside the band are eliminated.

Oscillator (OSC)
A quartz crystal oscillator is used to mix the incoming signal down to an intermediate frequency of 10.7 MHz.

RF amplifier (LNA)
A low-noise amplifier is used to amplify the 800 MHz frequency band by 10 dB.

Mixer (MIX)
This circuit creates an intermediate frequency of 10.7 MH.2, produced by mixing the amplified received signal in the 800 MH2 band with the output of the oscillator.

IF amplifier (IF1 to IF3)
These provide a total gain of 100 dB, the final stage acting as a limiter. Before and after the amplifier chain. 10.7 MHz ceramic filters are fitted to provide selectivity.

FM detector (DET)
This circuit demodulates the frequency-modulated IF signal.

RSSI detector (RSSI)
Signals horn the middle of the IF amplifier chain are rectified producing a DC voltage proportional to the signal strength.

Muting comparator (COM)
The RSSI signal is compared to a preset voltage which can be adjusted using a potentiometer. If the level at the antenna input falls to 17 dB/μV or less, the output signal is turned off.

De-emphasis
This compensates (or the 50 μs pre-emphasis, making the overall fre­quency response of the system flat.

AF amplifier (AF)
This circuit amplifies the demodulated audio signal before it is passed to the expander.

Analogue switch (SW)
If the signal strength falls too far, the audio signal is muted using this switch. An LED indicates when this muting occurs.

Expander
The dynamic range of the audio signal is doubled by this circuit, which oper­ates in much the same way as the compressor.

AF output amplifier (AF)
The output of the expander circuit is amplified again for output.

Voltage regulator (AVR)
This circuit provides the entire circuit with a stable 2.7 V supply from a bat­tery voltage of between 3 V and 12 V.

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