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Fm transmitter about


mcsech

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I am a begginer of this electronic world. I set the circuit that i added on bread-board. But I cannot hear anything on my radio. Either the circuit fails or i cannot catch the true frequency. My answer is that,how can I change transmitted frequency? Is it done by changing inductors turns or radii? In figure capacitance values is wrong. The circuit schematic comes from this video.  

post-47105-14279143952085_thumb.jpg

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1) The circuit is supposed to have a 3V battery. Yours does not have a battery.
2) The circuit is supposed to have an electret microphone. Yours has a buzzer instead.
3) The circuit is supposed to have a 330 ohm emitter resistor. Yours is way too high at 10k instead.
4) The emitter resistor is supposed to connect to 0V. Yours is in series with a capacitor instead so the transistor won't do anything.
5) The circuit works at the very high frequency of 100MHz but the stray capacitance and inductance of a breadboard is way too high for that frequency.

The 10pf to 40pf trimmer capacitor tunes different frequencies. When it is set to 40pF then the frequency is near the low end of the FM dial and when it is set to 10pf the frequency is near the high end of the dial.

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The bottom line in the schematic is 0V from the battery. The emitter resistor and the 22nF supply bypass capacitor connect to it.
Make the circuit compactly on a pcb or on stripboard and it will work. It will not work if you make it on a breadboard.

It is a very simple circuit so its performance is horrible.

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The capacitor from the collector to the emitter of the transistor is only 4.7pf. The transistor has an internal capacitance adding to it of about 5pf.
But the capacitance between the tracks and wires on a breadboard have much more capacitance between them and at each and every connection.

Each jumper wire on a breadboard has inductance. It takes a very small amount of inductance at 100MHz to mess up the circuit and every wire has inductance.

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You cannot hear your voice on the radio?
1) Did you use an electret microphone instead of the buzzer on your schematic?
2) Did you connect the electret mic with the correct polarity?
3) Did you correct the wrong wiring on your schematic?
4) Did you tune the trimmer capacitor with a plastic screwdriver since a metal screwdriver will add capacitance to affect the frequency?
5) Did you tune the transmitter while listening to a high quality FM radio since a cheap FM radio will be overloaded by the strong closeby signal from the transmitter?

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What did you use to power the transmitter?

Was it a cheap unregulated wall wart or a battery or regulated wall wart/mains supply?

I've tried powering an FM transmitter from a chap wall wart and it made the radio hum rather than transmit sound from the mic. You need to power it from a steady DC source such as a battery or regulated mains supply.

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  • 4 months later...

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