The reader "bmaverick" has found our circuit with good explanation in english and s/he, thankfully, provided us with the link (page 75, Warning: an e-book, very big file):Ī video of this circuit along with the output shown on the oscilloscope: The circuit is shown above, and its link is: Recently I found a similar circuit using the 7413 chip but with both a coil and a capacitor. What we are actually doing is changing the frequencies of all the harmonics according to the audio signal (modulating all the frequencies), but only the modulated third harmonics can be detected using FM radios.įor those who can understand Arabic, I added a PDF copy of the reference below in the attachment. This series goes to infinity following the same procedure.Įxplanation above means that our carrier signal (square wave with 30-35 Mhz frequency) has its third harmonics with frequency about 100 Mhz which lays inside the FM radio range. The third harmonics has its amplitude equal to one third of the square wave, and its frequency three times that of the square wave. the second harmonics has its amplitude equal to half that of the square wave, and its frequency double that of the square wave. These sine waves are called harmonics, and have the following properties: the first harmonic is called the fundamental signal, and has frequency and amplitude equal to the square wave signal. It is known that the square waves embed an infinite number of sine waves. Note that the signal generated by the NAND oscillator is a square wave. The frequency range of FM radio is 88 Mhz-108 Mhz, so how are you using a 30-35 Mhz signal to transmit FM signals? Note that the feedback is assured by connecting pins 1, 2, and 4 of the chip to its pin number 6. The frequency of this square wave is between 30 and 35 Mhz. ![]() This means that by feedbacking the NAND gate, it will start generating a square wave. If a digital 1 (5 v TTL) is the input of a NOT gate, its output is digital 0 (0 v TTL) and vice versa. The output of a NOT gate is always the inverse of its input. ![]() When the inputs of a NAND gate are connected to its output (called feedback), it is transformed into an inverter (or NOT gate). Dual means that it holds two NAND gates inside the chip, and we are going to use only one of these chips. The SN7413 is a dual four input NAND gate chip. The radio receiver receives the modulated signal and de-modulates it to recover the sound wave. The modulated signal is then transmitted via the antenna. First we generate a square wave (carrier signal) using the SN7413 NAND gate chip (shown in the first figure above), then we change the frequency of this carrier signal according to the sound wave (this is called modulation). The theory behind this circuit is very easy.
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