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Design >> Analog Design >> Low Jitter Clock Generation with a External Crystal
https://designers-guide.org/forum/YaBB.pl?num=1255978581

Message started by neoflash on Oct 19^th, 2009, 11:56am

Title: Low Jitter Clock Generation with a External Crystal
Post by neoflash on Oct 19^th, 2009, 11:56am

Hi folks:

We are leaning to design a clock generator with a external crystal.
Pierce and Colpitts oscillator seems to dominate the design. Which type better suites our target and why?

Regards,
Neo

Title: Re: Low Jitter Clock Generation with a External Crystal
Post by ywguo on Oct 20^th, 2009, 8:26pm

neoflash wrote on Oct 19^th, 2009, 11:56am:

Hi folks:

We are leaning to design a clock generator with a external crystal.
Pierce and Colpitts oscillator seems to dominate the design. Which type better suites our target and why?

Regards,
Neo

Neo, what is your target? It is vague to say design a clock generator.

What is the clock frequency?
Any requirement on phase noise or jitter?
How many pads are available?

Title: Re: Low Jitter Clock Generation with a External Crystal
Post by raja.cedt on Oct 20^th, 2009, 9:17pm

hi Neo,
if you are able to design colpits oscillator with your specifications then from many points of view it is the best one (like excellent phase noise, frequency stability..etc)

Thanks,
rajasekhar.

Title: Re: Low Jitter Clock Generation with a External Crystal
Post by neoflash on Oct 21^st, 2009, 6:20am

We need a low jitter ~50MHz clock generator. Sub-picosecond is the target when applied with a 10MHz HPF.

Currently, the confusion is that the same device gm can not startup colpitts while having no problem with Pierce.

Two pads is possible.

Title: Re: Low Jitter Clock Generation with a External Crystal
Post by neoflash on Oct 21^st, 2009, 12:24pm

I did some analysis and found that pierce and colpitts are exactly same in small signal.

More confused why Colpitts doesn't start while pierce always start easily.

Title: Re: Low Jitter Clock Generation with a External Crystal
Post by loose-electron on Oct 21^st, 2009, 5:49pm

there is a dialog here on the fourum about "quick starting crystal" oscillators and some simulation tricks - search for it, some useful things there, because High Q crystals often get messed up in simulation.

Title: Re: Low Jitter Clock Generation with a External Crystal
Post by neoflash on Oct 21^st, 2009, 8:33pm

Just realized that body effect in Colpitts has to be removed. Otherwise, the complete small signal negative resistance change.

Stupid mistake and ignorance.