20 mhz oscillator please help

Thread Starter

matbymay

Joined Jul 14, 2009
27
hi everbody

i need sinewave output oscillator which operates 20 mhz or more.
crystal oscillator will be preffered
please help i have many oscillator design but i need very stable.

thank you
 

steveb

Joined Jul 3, 2008
2,431
hi everbody

i need sinewave output oscillator which operates 20 mhz or more.
crystal oscillator will be preffered
please help i have many oscillator design but i need very stable.

thank you
How pure does the sine wave need to be?

How stable does it need to be, and over what temperature range?

Does it need to be tunable, or is it at one frequency always?
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
it is just one frequency not adjustable and i need sinewave very clear.
if there is some IC sinewave generator you can suggest me or have you got good tested one oscillator will be ok

thank you
 

davebee

Joined Oct 22, 2008
539
You could get an inexpensive canned crystal oscillator, like this one -

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=27932&

to generate a stable 20 mHz square wave, then pass the signal through a bandpass filter to clean it up into a sine.

I don't know of any off-the-shelf 20 mHz bandpass filters that you could just buy - you might have to make this yourself from discrete parts. But there are plenty of filter design tutorials on the web that can help with this design.
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
thank you for yor solution

but any other ideas quick and clear.
when i searh to internet i found max038 IC but it is not being manufactured anymore. do you know any equivalent ?

thank you
 

steveb

Joined Jul 3, 2008
2,431
to generate a stable 20 mHz square wave, then pass the signal through a bandpass filter to clean it up into a sine.

I was thinking the same thing when I asked those questions. So far, the simple answers seem to indicate this may work. However, we really need more detailed quantitative specifications for harmonic purity and temperature stability to be sure.

Hopefully, the OP can look at his requirements and generate some real specifications to go by.
 

steveb

Joined Jul 3, 2008
2,431
thank you for yor solution

but any other ideas quick and clear.
when i searh to internet i found max038 IC but it is not being manufactured anymore. do you know any equivalent ?

thank you
You haven't given any real specifications or details for how you will use it, or size or anything. What about cost?

Can you use a benchtop signal generator? This will be designed to be very pure and reasonably stable, depending on how much you are willing to spend.
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
temperture is up to max 65C and it would be pure sine wave as i can as. frequency and amplitude accuarcy should be 1% at whole performance.

i implement some colpitts oscillator but it is not usefull actually i need really good one thats why i m asking IC type generator

thanks for your attention. please keep answer me, i really stuck to implement oscillator
 

bertus

Joined Apr 5, 2008
19,939
Hello,

Can you post a schematic of the oscillator you have now?

The stability of the oscillator strongly depends on the type of capacitors used.

Greetings,
Bertus
 

steveb

Joined Jul 3, 2008
2,431
temperture is up to max 65C and it would be pure sine wave as i can as. frequency and amplitude accuarcy should be 1% at whole performance.

i implement some colpitts oscillator but it is not usefull actually i need really good one thats why i m asking IC type generator

thanks for your attention. please keep answer me, i really stuck to implement oscillator
Looking at the MAX038 specifications and the ones you just gave, the stability and purity specifications do not seem that stringent. An LC based oscillator may meet those specs if you design carefully. Certainly, a fixed frequency crystal oscillator generating a square wave followed by a buffered LC bandpass filter stage will be able to meet those specifications.

The MAX038 says the following:
> 100 PPM/C frequency stability (note that's per Celcius degree!)
2% harmonic distortion

Compare these to crystal specifications.
http://microcontrollershop.com/product_info.php?products_id=2061
 
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Thread Starter

matbymay

Joined Jul 14, 2009
27
i have and implemented this circuit below but it didnt work what you think about this circuit
i used 20 mhz crytal oscillator on this circuit
 

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bertus

Joined Apr 5, 2008
19,939
Hello,

What kind of crystal is used?
Is it an overtone crystal?
Did you get some frequency out of the circuit?

Greetings,
Bertus
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
could you check for me please it is usefull or should i use different type so it should be different could you suggest any ?

thank you
 
matbymay have you found an LED or Phototransistor that runs at 20MHz yet. I would worry about that part of your design more than this?
(from another forum about the same thread)

PS you can just buy a sine out OCXO or oscillator rather than build it yourself.
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
thank you blueroom but i sent list to you did you check it? i think it is alright i checked datasheet and slew times more than 20 mhz. but if suggest any others i can buy them.
on the other hand could you suggest specific OCXO or oscillator because you know my project
 
Although I'm not an expert in PLL design and optics but your design appeared to be based solely on your imagination / wishful thinking vs any real prior designs.
You provided no references to any similar design whatsoever.

Why not post it here for other opinions.

Google OCXO, they're expensive but accurate and both HCMOS & sine outputs available.
http://www.conwin.com/
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
thanks blueroom
but first of all i have to do driver circuit (for that i need osc.) and photodetector circuit with opamp. actually the key role is oscillator which should be stable. other step i m goingto ask here step by step. this is project and every information i ll explain.
 
Again you don't provide any information on what you're basing your design on. I'm trying to spare you the grief of building something based on no solid plan whatsoever. You had a block diagram on the other forums, why not post it here.
 

Thread Starter

matbymay

Joined Jul 14, 2009
27
ok i m sorry i just understood you. i have sent diagram but still same thing i didnt draw good one
 

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