Wireless Microphone: Audio in the ISM band 2

Published:2011/8/19 2:01:00 Author:Amy From:SeekIC

Ton Giesberts

The transmitter

Figure 2 shows the functional blocks of the WA-TX-01 transmitter. We will look at each in turn.



Input buffer (BUF)
This circuit is an input buffer for the microphone capsule or other sound signal source. The maximum input level is -15 dBV and the input imped­ance is 7.5 kΩ. If the maximum output level of the signal source is not suffi­cient. a low-noise amplifier must be connected before the buffer. If the sig­nal source level is too high, an attenu­ator should be used.

Compressor
The audio signal from the buffer stage is compressed using a ratio of 2:1. The compressor consists of a reference generator, a full-wave rectifier and a summing amplifier. The reference gen­erator provides a bias voltage and a constant current to the other parts of the circuit.

The full-wave rectifier circuit rectifies the incoming signal with the aid of an external capacitor. The output current of the rectifier controls the gain cell amplifier. The tone constant of the con­trol loop is set. in part, using an exter­nal filter capacitor and an internal 10 kΩresistor. The summing amplifier adds the incoming signal and dies signal from the gain cell amplifier together. The summing amplifier used in the compressor needs different properties from the one used in the expander, and so different components are used in the transmitter and in the receiver.

Pre-emphasis
To reduce noise at the upper end of the audio frequency range, which is a par­ticular problem when using frequency modulation; this circuit boosts higher frequencies using a time constant of 50μs.

AF low-pass filter (AF LPF)
This circuit limits the bandwidth of the audio signal in older to ensure that interference to adjacent channels is kept within the permitted limits. Oscillator and modulator In order to operate directly in the 800 MHz band, a crystal-based SAW (surface acoustic wave) filter resonator with good temperature stability is used as the oscillating element. Fre­quency modulation is achieved using a varicap diode that forms part of the oscillator circuit.

RF power amplifier (PA)
This circuit steps the RF output of the oscillator up to the transmit power of about 5 m W.

RF low-pass filter (RF LPF)
This circuit attenuates the second and higher harmonics of the transmitted signal and provides antenna imped­ance matching.

Voltage regulator (AVR)
This circuit provides a stable 2.7 V sup­ply for the whole circuit, it operates from a battery supply of between 3 V and9V.

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