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Broadcasting FM Transmitter
 


 

Broadcasting FM Transmitter

 


With this three stage transistor transmitter we can create and broadcast our mini radio station. We using S9018 transistor because it can handle very high frequencies, up to and including the VHF band. The first transistor on the left is a microphone audio amplifier for modulation. The gain can be adjusted with the potentiometer. The second transistor is the oscillator with a range of 80 to 103 MHz. The frequency can be changed with the upper coil 4T5 by pulling it slightly apart. The signal from the oscillator is very small, so that still needs to be amplified. The right most transistor is therefore an RF amplifier. This amplifies the signal from the oscillator to feed to the antenna. This transistor also immediately provides more stability, because the oscillator is not directly connected to the antenna.


Broadcasting FM Transmitter

Function of the RF oscillator
A transistor is not an oscillator by itself. Only when you feed back its output to the entrance will you get it to strengthen itself. Compare that with a microphone that you hold in front of a speaker; the screaming sound is also oscillation. You can influence the tone of the screaming sound with your hands. And that's how it works with an RF oscillator. By placing a coil (the 4T5 mentioned here) and a capacitor in parallel with the input of the transistor, the transistor only becomes sensitive to a specific frequency. This frequency is called resonance frequency. You can therefore vary the frequency if you change the value of the coil and / or the capacitor. And that's exactly what you do when you pull 4T5 apart. There is a small paracitic capacitance in the transistor, which is voltage dependent. By offering music (with a varying voltage), the capacity therefore varies slightly at the base of T2. This influences the resonance circuit and the transmitter will vary slightly in frequency with the rhythm of the music. This is Frequency Modulation (FM). With 'real' FM stations this is done in a nicer way with for example varicaps.

RF Filter
Thus a coil and a capacitor in parallel form a frequency-specific whole. This is called a resonance circuit. But you can also use this principle as a filter by connecting them to something else. So that, for example, everything above 120 MHz is filtered out. The coils at the far right of the output have such a filter function. But also to let the amplifier work optimally at around 100 MHz.

Stability
Stability is always an issue with an FM bug, but due to the design with a buffer amplifier at the output and an audio amplifier at the input, the oscillator is somewhat isolated. But it is better not to sit at the antenna, because then the frequency will be strong.

Sound quality
I only tested it with the microphone. The sound quality was not disappointing at all, despite the method of modulation. There is, for example, no varicap in it or whatever. The audio is full, but could get a little more high. For speech it is sufficient. The amount of noise on the audio is not too bad, for an FM bug it is neat.

Power Supply
As a power supply it is best to use a 9V battery instead of an adapter or power supply. Due to the moderate disconnection you will otherwise have a very quick hum (even though the power supply is clean, something with a loop I think to the mains). The current consumption is approx. 34mA at 9V.








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