Sunday, January 18, 2015

Low power FM stereo transmitter

BA1404 is quiet famous FM stereo transmitter IC manufactured by ROHM Semiconductor. This 18pin chip contain FM stereo multiplexer, 19 kHz pilot tone generator and RF output stage with oscillator. The FM transmitter in this article is also based on BA1404 and 78L33 voltage regulator.

The FM transmitter in this article is specifically designed to transmit audio signals generated by PC/Laptop sound cards. To archive this goal, this transmitter is design to drive using 5V DC power source.

Except Q1 (38 kHz crystal) and L2 (oscillator inductor), this transmitter uses commonly available electronic components. 38 kHz crystal is vital to generate 19 kHz pilot tone and this crystal is available to purchase in several web stores.

For L2, wind 3 turns of 22 S.W.G enameled copper wire on 5mm bobbin with adjustable ferrite core.  This variable inductor is used to select the output frequency of the transmitter.

To archive good quality output and frequency stability it is necessary to construct this transmitter on PCB.

As per our tests the maximum range of this transmitter is 20m - 25m with obstacles and 50m - 60m in open area. In shorter distances this transmitter performs very well and produces really good audio output. Due to legal reasons we didn't test this transmitter beyond 50m.

PCB design for BA1404 FM stereo transmitter.

As per the time of writing, BA1404 is discontinued product, but there are lots of places to buy this chip in either DIP or SOP form.

The schematic and PCB of this transmitter is available to download at google drive.

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