FM transmitter circuit not working

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polashd

Joined Jan 16, 2014
2
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.
 

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Bordodynov

Joined May 20, 2015
2,377
I don't know what you've been modeling or what oscillator frequency you've got. From my experience, you won't get 100 MHz. The capacity of the resonant circuit is too high. Make C6=1pF -10pF, C7=22pF-220pF
 

AlbertHall

Joined Jun 4, 2014
8,301
It looks to me as if C6 and C7 in series are in parallel with the coil - about 200pF.
I would suggest removing C7 and making C6 much smaller - 18p?
 

Alec_t

Joined Sep 17, 2013
10,350
If you have built the circuit on a breadboard, the board's inherent capacitances and inductances will skew things and may well prevent oscillation (at least at the frequency you expect).
 
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