Projet Analogique

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    In brief

  • Code : N7EE07C


The aim of the project is to transmit an audio signal (speech or music) at a distance via a wireless link. The latter is done by infrared optical beam (λ=900nm) with transmission in the air. In addition to the audio source (for example a mobile phone ...) and the speaker or headset, the device consists of two systems: a transmitter and a receiver.


The transmitter is based on analog frequency modulation (FM) which mainly relies  on a voltage-controlled oscillator (VCO) to encode audio information within the VCO carrier wave by varying its instantaneous frequency. The VCO frequency varies linearly with the amplitude of the input signal. The centre frequency f0 should be relatively large compared to the highest signal audio frequency to be transmitted. The  light containing the information to be transmitted is emitted by a high-efficiency light emitting diode (LED). The lightintensity is modulated via the LED biasing. A preamplifier-filter processes the audio modulating signal to be transmitted before being applied to the VCO input.


The receiver consists of: (1) a low-noise analog front-end to convert and amplify the received light into voltage, (2) a phase locked-loop circuit to demodulate the FM signal, (3) filtering and equalizer stage and (4) audio power amplifier. In the first stage, s a silicon PIN photodiode whose spectral response is adapted to the LED emission spectrum is used to convert light into current proportionally. This photoelectric current is then amplified by a transimpedance circuit which provides an output voltage proportional to the input alternating current. After a high-gain selective amplification based on cascade architecture, a phase locked loop (PLL) is used to recover the baseband signal encoded into the FM signal. After filtering the audio signal, a push-pull audio amplifier in class AB makes it possible to excite a speaker or a headset.

Targeted skills

 -         Design and implement a Voltage Controlled Oscillator (VCO).

-         Know how to characterize a VCO

-         Characterize switching times of MOS and Bipolar transistors

-         Design active filters to meet specifications

-         Design high gain (> 60dB) and high frequency (> 200 kHz) low-noise amplifiers based on transistors.

-         Perform frequency demodulation using PLL

-         Know how install a test bench to completely characterize a prototype.


The National Institute of Electrical engineering, Electronics, Computer science,Fluid mechanics & Telecommunications and Networks

2, rue Charles Camichel - BP 7122
31071 Toulouse Cedex 7, France

+33 (0)5 34 32 20 00


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