FM transmitter circuit not working

I made the FM transmitter circuit (schematic attached) to attach with the audio out of music player or TV card. The receiver would be FM radio of smart phone. My intended carrier frequency is around 100MHz. I don’t find many diagram for stereo input (most are using mic).

I don’t receive anything. It seems the circuit is not oscillating (LtSpice shows it’s working). In this configuration current draw is little more than 1ma. I don’t see any difference whether the audio input jack is connected or not. Current increases if value of R4 is reduced. Most of the current is drawn by the oscillator part (ie. Q1). Q2 draws very minimal current.

I don’t have 10p caps, used 2 15p in series (which makes it 7.5p) as C5 across Q1.
I also tried without the second part (Q2, etc.) disconnecting C6 and connecting the antenna at the open end of C6, result is almost same.

1n for C6 may be too much! Is it ok if I use 15p or 18p?
Pls suggest me.

How have you built it? - construction is absolutely critical, and it WON'T work on a breadboard.

Post a picture!.

Build it on pcb

polashd said:

Build it on pcb

Click to expand...

Then post a picture - as I said, layout is critical.

Guess what? If the 100MHz oscillator was designed properly to be powered from only 3V then C7 completely feeds the entire oscillation signal to ground. Remove C7.
Yes, the value of C6 is too large, my similar circuit uses 30pf but 150pF works better.

Your R3 should also be removed since it reduces the base voltage of Q1 so low that some transistors will not work, especially if the battery voltage is a little low.
Your value of R5 is very low, my similar circuit uses 220 ohms.

Instead of mixing left and right signals, your very low values for C1 and C2 are not passing audio signals but are passing only ultra-sonic signals to the very high value of C3 that shorts them to the positive supply. Use two 4.7k resistors with the left channel feeding one and the right channel feeding the other, then their other ends are joined together to mix the two channels and feed a 1uF coupling capacitor to the base of Q1.

Change the value of your C3 to 470pF that my similar circuit uses and connect it to ground instead of to the positive supply. Your Q2 is biased poorly like I was taught to never do because each transistor has a different hFE. Use 39k between its base and emitter instead.

All FM radio stations use pre-emphasis (treble frequencies boost) and all FM radios use matching de-emphasis to cut the high frequencies down to normal and reduce hiss. Your circuit is missing pre-emphasis so it will sound like your stereo with its treble tone control turned all the way down.

I agree that the very high radio frequency circuit will not work if built on a breadboard. Mine is built compactly on a stripboard.

Here is your partially fixed circuit and my circuit that works well and sounds perfect, except its radio signal is strong enough to cover a real radio station which is illegal:

Thanks a lot audioguru for your detailed instruction. I will try your advice. But I'm worried about Q1 biasing, as r3 is removed and r4 only 15k it may draw too much current ( I guess!). How do I know if the current/ power is ok legally & for a range of a house of a couple of rooms.

For nigel
Layout attached. Values of some parts will vary as did some experiments (like r3 r4)

A 2N2222 transistor is so old that its datasheet does not have the entire range of hFE and Vbe so I am looking at the datasheet of a more modern 2N3904 that I used.
If the battery is brand new at 3.0V in your circuit and the 2N3904 oscillator transistor has a 15k base resistor and a 220 ohm emitter resistor then the current in the transistor is:
1) hFE=80, Vbe= 0.7V. Ve= 1.1V then the emitter current is 1.1V/220= 5mA and the Vb is 1.1V + 0.7V= 1.8V. The base current is (3V - 1.8V)/15k= 80uA.
2) hFE= 210, Vbe= 0.7V. Ve= 1.75V then the emitter current is 1.75V/220= 8mA and the Vb is 1.75V + 0.7V=2.45 V. The base current is (3V - 2.45V)/15k= 37uA.
So the current with some transistors is 5mA and is 8mA with other transistors and is less as the battery voltage runs down.

If you use a vacant radio frequency then you are not causing interference to legal FM radio stations then your circuit is not illegal. But your circuit is so simple that its radio frequency changes as the battery voltage runs down, which is why my circuit has a voltage regulator.

What will you do about the bad sounds caused by missing pre-emphasis??