## Tuesday, May 16, 2023

I made this project three years ago, and it works perfectly. I have found the circuit of this project in a book I bought eighteen years ago when I was in high school.

Most of the available FM transmitter circuits have, in my opinion, a very important drawback: the use of inductors. Some, including myself, find it hard to make the perfect coil and tune it for the intended frequency. I found the solution in the circuit of this project.

The circuit uses the SN7413 NAND chip as an oscillator to generate a carrier frequency of about 100 Mhz which lays inside the FM radio frequency range (88 Mhz - 108 Mhz). This transmitter has a range of about 50 m.

The circuit is extremely easy, and it is based on the SN7413 chip. The range of the transmitter is about 50m.

The transmission frequency of the circuit is around 100Mhz, and can be adjusted by adjusting the 20 PF trimmer capacitor. Using a fixed 22 PF capacitor, you will receive the signal at channel 93.2 Mhz.

The aerial/antenna is a 70 cm long wire. I used a high gain microphone as the one shown in the figures above. If you would like to use a condenser microphone, it is better to use an audio amplifier circuit after the microphone. Example circuits are shown above.

I was planning to use a radio frequency (RF) amplifier to extend the range of this simple circuit, but never proceed.

The theory behind this circuit is very easy. 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 modulated signal is then transmitted across the antenna. The radio receiver receives the modulated signal and modulates it to recover the sound wave.

How the carrier signal is generated?

the SN7413 is a dual four input NAND gate chip. Dual means that it holds two NAND gates inside the chip, and we are going to use only one of these chips. When the inputs of a NAND gate are connected to its output (called feedback), it is transformed into an inverter (or NOT gate). The output of a NOT gate is always the inverse of its input. 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. This means that by feedbacking the NAND gate, it will start generating a square wave. The frequency of this square wave is between 30 and 35 Mhz. Note that the feedback is assured by connecting pins 1, 2, and 4 of the chip to its pin number 6.

The frequency range of FM radio is 88Mhz-108Mhz, so how are you using a 30-35Mhz signal to transmit FM signals?

Note that the signal generated by the NAND oscillator is a square wave. It is known that the square waves embed an infinite number of sine waves. These sine waves are called harmonics, and they have the following properties: the first harmonic is called the fundamental signal, and it has frequency and amplitude equal to the square wave signal. The second harmonics has its amplitude equal to half that of the square wave, and its frequency is double that of the square wave. The third harmonics has its amplitude equal to one third of the square wave, and its frequency three times that of the square wave. This series goes to infinity following the same procedure.

Explanation 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. 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 third modulated harmonics can be detected using FM radios.

For those who can understand Arabic, I added a PDF copy of the reference below in the attachment.

Recently I found a similar circuit using the 7413 chip but with both a coil and a capacitor.< /p>

I made this project three years ago, and it works perfectly. I have found the circuit of this project in a book I bought eighteen years ago when I was in high school.

Most of the available FM transmitter circuits have, in my opinion, a very important drawback: the use of inductors. Some, including myself, find it hard to make the perfect coil and tune it for the intended frequency. I found the solution in the circuit of this project.

The circuit uses the SN7413 NAND chip as an oscillator to generate a carrier frequency of about 100 Mhz which lays inside the FM radio frequency range (88 Mhz - 108 Mhz). This transmitter has a range of about 50 m.

The circuit is extremely easy, and it is based on the SN7413 chip. The range of the transmitter is about 50m.

The transmission frequency of the circuit is around 100Mhz, and can be adjusted by adjusting the 20 PF trimmer capacitor. Using a fixed 22 PF capacitor, you will receive the signal at channel 93.2 Mhz.

The aerial/antenna is a 70 cm long wire. I used a high gain microphone as the one shown in the figures above. If you would like to use a condenser microphone, it is better to use an audio amplifier circuit after the microphone. Example circuits are shown above.

I was planning to use a radio frequency (RF) amplifier to extend the range of this simple circuit, but never proceed.

The theory behind this circuit is very easy. 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 modulated signal is then transmitted across the antenna. The radio receiver receives the modulated signal and modulates it to recover the sound wave.

How the carrier signal is generated?

the SN7413 is a dual four input NAND gate chip. Dual means that it holds two NAND gates inside the chip, and we are going to use only one of these chips. When the inputs of a NAND gate are connected to its output (called feedback), it is transformed into an inverter (or NOT gate). The output of a NOT gate is always the inverse of its input. 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. This means that by feedbacking the NAND gate, it will start generating a square wave. The frequency of this square wave is between 30 and 35 Mhz. Note that the feedback is assured by connecting pins 1, 2, and 4 of the chip to its pin number 6.

The frequency range of FM radio is 88Mhz-108Mhz, so how are you using a 30-35Mhz signal to transmit FM signals?

Note that the signal generated by the NAND oscillator is a square wave. It is known that the square waves embed an infinite number of sine waves. These sine waves are called harmonics, and they have the following properties: the first harmonic is called the fundamental signal, and it has frequency and amplitude equal to the square wave signal. The second harmonics has its amplitude equal to half that of the square wave, and its frequency is double that of the square wave. The third harmonics has its amplitude equal to one third of the square wave, and its frequency three times that of the square wave. This series goes to infinity following the same procedure.

Explanation 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. 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 third modulated harmonics can be detected using FM radios.

For those who can understand Arabic, I added a PDF copy of the reference below in the attachment.

Recently I found a similar circuit using the 7413 chip but with both a coil and a capacitor.< /p>