How to eliminate noise in FM transmitter?

I had made a FM transmitter using IC NE555. When i power the circuit with 9V battery,the transmitted audio is clearly received in my radio receiver.The problem is the battery will soon drain out and also as the battery starts to drain the transmitted frequency changes without my knowledge.So as the battery voltage decreases the frequency is also changing.To eliminate this problem i used transformer powered source with 9V Regulator 7809.When i power the circuit with transformer the received audio signal on my radio receiver has so much of noise added.The audio quality which i transmitted is completely lost.Anyone Help me to resolve this problem.

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iceng3 months ago

A 555 is not a steady oscillator because of probably ordinary capacitors which are thermally sensitive. I doubt that you tried to balance the resistor thermal drift against the NPO capacitor opposing thermal drift to provide a stable FM frequency over room temperature to maintain a stable RC. This is just one of a dozen items that need to be considered when FM transmitting is beyond a simple demonstration circuit..

-max-3 months ago

How did you manage to use a 555 timer to transmit FM? You need something oscillating at several hundred MHz. Are you relying on harmonic content from the output? The whole idea behind the 555 timer is that the output frequency is unaffected by the input voltage. That was the whole reason it ever become popular. One of the first early designs to enable that.


if you want better performance, try using an actual transformer or at least a better power supply instead of the switching converter that I think you are mistaking for a transformer. This will reduce high frequency noise. Make sure to add bypass capacitors to components, as close to the input leads as possible. Make sure to use appropriate filter capacitors on the output of the regulator as well.

iceng3 months ago

The five passive components in pic_1 all suppress line noise.

Pic_2 is what resides in the general corcom.

Be sure to click the pic to see the entire image !

balunXFMR circuit.GIFbauln circuit.PNG
Rajkumar2506 (author)  iceng3 months ago

What for the BAULN transformer is used.It is used to match impedance in olden CRT Televisions according to my knowledge.How it will eliminate noise?

About the size of two TO-220 power transistor or the size of the knuckle of your thumb.

One of the noises that can upset your 555 FM is the common mode noise that travels over both wires at the same time and over whelms the pi-filter but gets totally stopped be the magnetic Bauln action.

balun common mode circuit.gif
Downunder35m3 months ago

Simply add a good filter after the rectifier.
A LC combination should work fine and if you add another filter at the output of the voltage regulator the noise should be gone.
If in doubt use the round ferrite inductor from an old mains filter (found in almost any mains powered electronics junk) and an electrolytic capacitor as big as possible.
You can of course use multiple capacitors.
Something in the range of 10.000 - 25.000µF should be enough.

Rajkumar2506 (author)  Downunder35m3 months ago

Can i use RC filter combination because finding a correct inductor in shop is slightly difficult and also the capacitor 10.000-25.000µf is a big range,the cost is also very high