I don't know much about crystal oscillators, so I've got a question:

Can crystal oscillators change frequency? like from 88-108 MHZ? Or even a bigger range hopefully?

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Can any one give some link about crystal oscillators?

 

No! - the whole point of a crystal is that it's a stable accurate frequency.

To provide a wide range, with crystal accuracy, you would normally use a Phase Locked Loop system, with a crystal as reference.

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Amateur radio operators use crystals and a variable capacitor to provide some variation however the variation is quite small - a few kHz at most. They use the term VXO.

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stevez

 

I believe the term is VCXO: Voltage-Controlled Crystal (Xtal) Oscillator.

 

well then, is there any way, other than w/ an LC circuit to get wide frequency range? I heard of something w/ an LC circuit that used a crystal as a reference or something like that. Any ideas???

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As I posted above, it's called a Phase Locked Loop.

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Oh sry ops: i missed that. My transmitter is extremely small, a little bigger than a pink eraser. I want my transmitter small; is a phase locked loop system small enough for a small transmitter? also, what kind of variation in frequency can you get from it? I read a definition on google, and it mentioned that it is used to step up frequency or something like that.

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Here is an example.I found this about years ago somewhere on the net.

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pllfmtx.jpg (105.6 KB, 498 views)

 

That's quite nice! - but rather too simple, it just provides a single frequency based on the crystal.

A full PLL tuning system uses programmable dividers to adjust the frequency, and gives a large tuning range in channel steps as small as you want to make them.

But I suspect a PLL system is probably too complicated for what he wants!.

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The simpler form of 'adjustable' crystal oscillator is probably VXO - just small variable capacitor that's adjusted by hand to get a little bit of range. I agree that replacing the variable capacitor with a varactor (or other) diode and a pot is still pretty simple.

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stevez

 

Yes, VXO (Variable Xtal Oscillator) is the correct name, I've never seen VCXO used - and it obviously wouldn't apply to your original mention of a variable capacitor across a crystal!.

BTW, this technique is called 'pulling' a crystal, and as you say it only works over a very small range - and it also reduces the crystals accuracy somewhat.

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My apologies, Steve. My thoughts jumped to modulating the crystal oscillator frequency by using a varicap (varactor) in place of the manually adjustable cap, and controlling it with audio. I assure you VCXOs are common, and are commercially available from many sources. As I think you pointed out, getting the deviation required for an FM transmitter is probably impossible.
Here is Harry Lythall's article that accompanied the schematic posted by Sebi. It has a good discussion of the problem, and how he got around it with a PLL.

 

Ron - don't worry, wasn't bothered by the comment, just helps others to get different points of view so I tossed another bit in. "Rubbering" is another word I've seen used by journal article authors to describe the process.

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stevez

 

you can possibly run off of harmonics but i wouldnt suggest it, i personally would go with a voltage controlled oscillator. a simple one to work with would be a 74624. i tend to use them alot myself. if you do go into pll circuits like what they mentioned try a NE565 and i think a NE564 are both pll circuits and are pretty simple to use.

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