FM Bug Transmitter Mint Box

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Frequency Modulation
           Since the FM radio band (88 to 108 MHz) will be used for FM transmitting in this project, we must first understand what the FM means, how it works and how we use it. Frequency Modulation (FM) transmits at a certain frequency, 107.9 MHz for example, and depending upon the audio signal input, the transmitted signal will have a fast frequency (for large positive voltages) or a slower frequency (for large negative voltages). The animation below best illustrates how the signal transmitted by the FM Bug will look.

You can think of the 'Signal' as what comes out of a microphone when you speak into it and the 'FM signal' as what goes out of the Antenna.

Creating FM Signals
           What is so unique about this type of modulation is that, as long as we use a voltage controlled oscillator for creating the carrier frequency (i.e. 107.9 MHz), we just mix the amplified audio signal with that 107.9 MHz carrier and presto the newly created signal is frequency modulated, aka FM. If you look closely at the schematic below you'll see the exact point where this mixing/modulating occurs and the new FM signal is born.