Looking for a FM transmitter that uses crystal oscillator

[seadolphine2000]

HI,...

I've made an FM transmitter at a frequency 87.8MHz using LC oscillator, I'm asking if I can get a crystal oscillator instead of the LC one.

Is it possilble.??????????

 

[BigBoss]

Re: FM Tx OSCILLATOR

HI,...

I've made an FM transmitter at a frequency 87.8MHz using LC oscillator, I'm asking if I can get a crystal oscillator instead of the LC one.

Is it possilble.??????????

Yes possible..
If you design harmonic oscillator that will work at frequency of interest which is exact harmonic of crystal frequency, that oscillator will be functional.And small frequency deviation can be obtained with a varicap diode parallel.

Or better way to use a PLL !! I'd prefer to use it very clean,smooth and stable transmitter. An also, if you design PLL loop filter very slow, FM modulation could be possible..
It's worth to try ...

 

[seadolphine2000]

FM Tx OSCILLATOR

Thanks for your help and consideration, Any ideas how to start.????

 

[IanP]

Re: FM Tx OSCILLATOR

Have a look at this very interesting concept of FM-PLL:
https://hem.passagen.se/communication/fm_pll_vco.html
Regards,
IanP

 

[E-design]

Re: FM Tx OSCILLATOR

Just one comment on the extra diode in the modulation circuit. This will provide fairly flat modulation sensitivity across the tuning band BUT, it will introduce a lot of distortion. I experimented with a design some time ago and used a very similar setup with additional DC feedback from my VCO tune line. I got very flat mod sensitivity (under 10kHz error from 75kHz peak deviation). When I measured the %THD of the demodulated audio, it was well over 2%. Without the extra varactor, it dropped to 0.1% Nothing I tried would bring it down near acceptable levels (≤0.5%) without losing flatness again.

At the end I used a digital pot to compensate the audio mod level against frequency to keep the distortion low using a single varactor diode for frequency control and audio mod.

E