crystal oscillator transmitter

[aimans]

after having a very unsuccessful time with FM transmitters using "wire" inductors and tunning capacitors, i am turning to crystal oscillators,

so can any one tell me whether a crystal oscillator can be used for audio transmission like an FM transmitter? i have 2 crystal oscillators of 27.xxxMHz from an R/C toy, so is frequency the only difference or there are other draw backs?? and why are there not many audio transmissions on low frequency?

too many question ehh, can you help out???
thanks

 

[audioguru2]

Hi Aimans,
You cannot change the frequency of a crystal oscillator very much. Then you cannot have wideband FM.

But you can use a low frequency crystal oscillator that has its frequency changed a tiny amount, then multiply its frequency many times to end up with wideband FM at 100MHz.
You could also use a crystal oscillator in a phase-locked-loop circuit and change its frequency. The PLL has a frequency divider that multiplies its frequency.

ROHM makes a few FM stereo transmitter ICs that use a 7.6MHz crystal in a frequency synthesizer circuit. Most of the little MP3 to car radio transmitters use it.
Here are links to an FM transmitter that uses their simplest IC:
http://electronics-diy.com/schematics/BH1417_fm_transmitter.htm
http://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php

 

[aimans]

you mean we can even increase the frequency of an oscillator, that's nice.

but can't i just use a 27MHz crystal and do not get it in the FM band, because i do not want other people to tune into my frequency,  :-\

 

[aimans]

the circuits you have linked are really good but i do not think i can construct them. most of the vital parts are not available here, like the FET and ZMV833ATA, and most probably not even the BH1417 ic,

so cant i jsut replace the oscillator part of a regular FM transmitter and replace it with the crystal oscillator?

 

[audioguru2]

so cant i just replace the oscillator part of a regular FM transmitter and replace it with the crystal oscillator?

No. An FM transmitter changes its frequency with modulation. A crystal oscillator's frequency can be changed only a tiny amount so the received demodulated signal would be extremely weak.

Take a 1.3889MHz crystal oscillator and modulate its frequency with a max deviation of 1kHz. Then use two frequency triplers and three frequency doublers to end up with 100MHz with a max deviation of 72kHz like an FM radio station.

Or you can use a phase-locked-loop circuit like this:

 

[AN920]

Fundamental mode crystals can typically be pulled by 0.2% and overtone crystals much less than that.

 

[aimans]

but how do i know which particular crystal is oscillating at fundamental mode or not?