Any idea to use AC to DC adaptors in basic FM Tx??

[Willen]

It makes noise lot. But professional FM are transmitting their signal clearer using Ac to dc source, how it happens? Can anybody show me the idea or any stablizer circuits to get clear transmission using AC to DC transformer? Or anything like that?

 

[MikeMl]

What are the details about the DC adapter? Some are AC, some rectified AC(labelled DC, but have no filtering), some are DC with capacitive filtering, but not regulated, some are DC regulated.

You need to test your adapter into a resistive load which draws a similar current as your transmitter. Use a 'scope or DMM to measure AC ripple on top of the DC level. If you have ripple, then you will need an IC regulator (like LM317) and possibly some big electrolytic capacitors (hundreds to thousands of uF) between the DC adapter and your transmitter.

 

[Willen]

I have a simple Linear Transformers bought for general purpose 220V AC to 3, 4.5, 7, 9, 12V DC output. There is a rectifier bridge diode and 400uF capacitor for filtering. Even if i use this DC in my simple fm transmitter, it transmits noise. Other people generally used to say 'do not use ac to dc adaptors to power it, just use battery for no noise transmission'. But I am searching the idea. If i use LM317, can i get success?

 

[MikeMl]

I'm guessing that since your transmitter is just a simple VHF oscillator (instead of being a proper crystal-controlled transmitter), it is extremely sensitive to supply voltage drift, fluctuations, ripple. Even tiny changes in the supply voltage cause the oscillator to shift frequency. Any thing you do to make the supply "pure DC", with no drift or ripple should minimize the incidental FMing.

 

[Willen]

Ya, it is almost oscillator with few capacitor as a filter. And searching a circuits to minimize this type of noise produced by such adaptptors. This is the transmitter to which I made.
Any circuits to make "pure DC"? because I am not more experienced.