FM Transmitter Circuit (Please Explain working)

In summary: Theoretically you can just increase the antenna length for more range, but it is not a good practice to just rely on antenna length for range. It is better to use a more powerful amplifier.
  • #1
SunnyRulz
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Hi.I have got this circuit for a low cost short range FM transmitter but I am unable to understand the working and functionality of the circuit.Please help me in understanding the working.What I know is that its working as a VCO(voltage controlled oscillator) when input (through electret mic) is applied.
 

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  • #2
The top transistor, inductor and two caps form a Colpitts type oscillator that generate the actual radio waves. The lower transistor is just an audio amplified current source that pulls the bias conditions of the oscillator which will AM modulate it in theory, but FM will still be detected by a radio due to overdriving the oscillator which can generate a lot mess, and one among may FM modulation.
 
  • #3
waht said:
The top transistor, inductor and two caps form a Colpitts type oscillator that generate the actual radio waves. The lower transistor is just an audio amplified current source that pulls the bias conditions of the oscillator which will AM modulate it in theory, but FM will still be detected by a radio due to overdriving the oscillator which can generate a lot mess, and one among may FM modulation.


AM Modulation...??
Please sir elaborate a bit more...cant understand...!
 
  • #4
SunnyRulz said:
AM Modulation...??
Please sir elaborate a bit more...cant understand...!

In AM modulation, the amplitude of the carrier frequency is varied, in FM the frequency is slightly varied to convey any data. In case of audio transmission, the amplitude varies with audio in AM, and frequency varies with audio in FM.

The circuit that you provided modulates input in AM primarily, but because the transistors are overdriven, a lot of ugly harmonics will be generated, and many of them will be FM modulated which can be heard in FM radio.

And a lot of times FM radios are poorly designed, and will actually pick up AM modulation in the FM band due to similar reasons. So this circuit is bound to work.
 
  • #5
Thanks for the reply sir...
also please tell that whether the circuit will produce NBFM(narrow band FM) or WBFM(wide band FM)...??
and how can i match the antenna...??
Like a senior of mine was saying that a simple 6-inch wire is enough to act as an antenna...?? As i have not studied da course on "antennas" yet...!
How to do "impedance matching" for antenna with the circuit...??
If i want to increase the range of transmission...?? can that just be done by merely increasing the antenna length...??
 
  • #6
SunnyRulz said:
Thanks for the reply sir...
also please tell that whether the circuit will produce NBFM(narrow band FM) or WBFM(wide band FM)...??
and how can i match the antenna...??
Like a senior of mine was saying that a simple 6-inch wire is enough to act as an antenna...?? As i have not studied da course on "antennas" yet...!
How to do "impedance matching" for antenna with the circuit...??
If i want to increase the range of transmission...?? can that just be done by merely increasing the antenna length...??


please Help...! :(
 
  • #7
SunnyRulz said:
Thanks for the reply sir...
also please tell that whether the circuit will produce NBFM(narrow band FM) or WBFM(wide band FM)...??

It will produce NBFM because the transistor doesn't have enough capacitance to pull too much frequency. A wideband VCO will have varactor diodes which change a lot of capacitance with input voltage, and the changed capacitance changes the resonant frequency.

and how can i match the antenna...?? Like a senior of mine was saying that a simple 6-inch wire is enough to act as an antenna...?? As i have not studied da course on "antennas" yet...! How to do "impedance matching" for antenna with the circuit...??
If i want to increase the range of transmission...?? can that just be done by merely increasing the antenna length...??

A simple whip antenna up to a meter will do fine. For more range, a more powerful amplifier is needed. Impedance matching is used to improve performance, but this circuit is just a standalone oscillator which probably generates some interference and output is not low pass filtered.
 

Related to FM Transmitter Circuit (Please Explain working)

1. How does an FM transmitter circuit work?

An FM transmitter circuit works by converting an audio signal into a radio frequency (RF) signal. This is done through a process called modulation, where the audio signal is used to vary the frequency of the RF signal. The varying frequency of the RF signal carries the audio information and can be received by a radio receiver.

2. What components are needed for an FM transmitter circuit?

An FM transmitter circuit typically consists of a microphone, an oscillator, a frequency modulator, and an amplifying stage. The microphone is used to convert sound waves into an electrical audio signal. The oscillator generates the RF carrier signal, which is then modulated by the audio signal. The frequency modulator adjusts the frequency of the RF signal, and the amplifying stage boosts the signal to a level strong enough to be transmitted.

3. How far can an FM transmitter circuit transmit?

The range of an FM transmitter circuit depends on various factors, including the power output of the circuit, the quality of the components used, and any obstacles in the transmission path. Generally, a small FM transmitter circuit can transmit up to a few hundred feet, while a larger, more powerful circuit can transmit several miles.

4. Can an FM transmitter circuit be used to transmit music?

Yes, an FM transmitter circuit can be used to transmit music. In fact, this is one of the most common uses for an FM transmitter circuit. By connecting an audio source, such as a smartphone or MP3 player, to the circuit, the music can be transmitted and received on a radio receiver.

5. Are there any legal restrictions for using an FM transmitter circuit?

In most countries, there are regulations for using an FM transmitter circuit, especially if it is being used for commercial purposes. These regulations typically limit the power output of the circuit and require a license for certain frequencies. It is important to research and follow the laws and regulations in your area before using an FM transmitter circuit.

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