Jfet 87-108MHz FM Oscillator

[geotFM]

Hello.I am trying to build a simple fm oscillator based around j310 but i want some help with the values of the parts in order to tweak it for good harmonic profile.Can you help?
I am looking for best values for C1,C2,C3,L1,R5.Are L1,C3 needed?

I am also attaching the LTSpice file simulation :smile:

 

[Audioguru]

I have never seen your oscillator circuit before. Almost all low power FM transmitters use a modified Colpitts circuit with an ordinary transistor like this:

 

[BigBoss]

I designed this oscillator for a relatively wide-band around 100MHz.If you wish, use it because it has a good performance.

 

[Audioguru]

Your transmitter has the antenna connected to the tuned LC circuit of the oscillator then anything that moves towards or away from the antenna will change the capacitance and change the frequency. My FM transmitter uses an RF amplifier in between the oscillator and the antenna so that the frequency does not change.

 

[E-design]

For good harmonic performance, you want to load the oscillator tank as little as possible.

The circuit below uses another J310 as a buffer amp whose output can easily be fed to more amplifying stages. As can be seen there is very good suppression of higher-order harmonics. While simulations give an idea of what to expect, final tuning and verification of your design should be done with the aid of a spectrum analyzer.

 

[Audioguru]

Then the distortion is about 5% and the waveform might show a little clipping.

 

[E-design]

That is close.

P1 := 1.2E-3 (Power in Watt)
P2 := 0.5E-6
P3 := 0.8E-6
P4 := 0.3E-6

THD% := SQRT((P2 + P3 + P4)/P1)*100

THD% =[3.65]