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neoflash
Joined: 02 Jul 2005
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 19 Oct 2009 21:40 Low Jitter Clock Generation with a External Crystal |
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Hi folks:
We are leaning to design a clock generator with a external crystal. What's the term people use for such kind of circuits?
Just want to better shoot at the searching.
Regards,
Neo
Last edited by neoflash on 02 Nov 2009 23:40; edited 1 time in total |
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19 Oct 2009 21:40 Ads |
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JoannesPaulus
Joined: 19 Mar 2008
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19 Oct 2009 22:23 Low Jitter Clock Generation with a External Crystal |
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| crystal oscillator
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neoflash
Joined: 02 Jul 2005
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20 Oct 2009 16:23 Low Jitter Clock Generation with a External Crystal |
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Which type of crystal oscillator is best fit for CMOS technology?
Pierce, Colpitts or some other form?
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JoannesPaulus
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20 Oct 2009 16:51 Low Jitter Clock Generation with a External Crystal |
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| I mostly designed pierce oscillators. The disadvantage is that they require 2 package pins but they are fairly easy to design. The colpitts oscillator could be designed for a one package pin but - I believe - it has worse jitter performance (if this is an issue for you).
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neoflash
Joined: 02 Jul 2005
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20 Oct 2009 18:08 Re: Low Jitter Clock Generation with a External Crystal |
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two types of pierce shown there,
one doesn't have a resistor, the other has R1. What's their difference?
Thanks.
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JoannesPaulus
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20 Oct 2009 18:17 Low Jitter Clock Generation with a External Crystal |
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| The Rbias is used to set the input impedance (it is usually a big resistor, like 500kohm). The amplifier based design will, most likely do that with some kind of feedback.
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neoflash
Joined: 02 Jul 2005
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20 Oct 2009 18:49 Low Jitter Clock Generation with a External Crystal |
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Sorry for the confusion.
I mean R1 in figure 1. Some Application note stated that R1 will introduce some phase shift.
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JoannesPaulus
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20 Oct 2009 21:29 Low Jitter Clock Generation with a External Crystal |
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| it is used to adjust the phase shift in the loop and to set the crystal drive level.
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neoflash
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20 Oct 2009 21:45 Low Jitter Clock Generation with a External Crystal |
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| why phase shift need to be adjusted? What if we don't?
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JoannesPaulus
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20 Oct 2009 23:07 Low Jitter Clock Generation with a External Crystal |
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| Well, R1 is often a 0ohm resistor but if the oscillator does not work (because the parasitics on the board are different from what you expected), you might want to be able to tweak here and there and this is a very simple adjustment.
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neoflash
Joined: 02 Jul 2005
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21 Oct 2009 3:04 Low Jitter Clock Generation with a External Crystal |
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New question about XO.
http://www.designers-guide.org/Forum/YaBB.pl?num=1256090508/0
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hock
Joined: 28 May 2001
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21 Oct 2009 13:11 Re: Low Jitter Clock Generation with a External Crystal |
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R1 is used to insure the startup of oscillator by providing the phase shift.
Rbisa is used to reduce the gain of internal amplifier.
hock
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JoannesPaulus
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21 Oct 2009 16:32 Low Jitter Clock Generation with a External Crystal |
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| you should try to kick start the oscillator (i.e. use a current pulse, say, 10nA for 10ns between ground and the gate) or try to ramp up the power supply.
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neoflash
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21 Oct 2009 20:23 Low Jitter Clock Generation with a External Crystal |
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Tried to set initial condition. But the oscillation will die out.
Added after 3 hours 8 minutes:
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.
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JoannesPaulus
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21 Oct 2009 21:23 Low Jitter Clock Generation with a External Crystal |
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yes, they are even though they have a very different behavior.
Try to have a look at this paper (a must read for xtal oscillator design):
"High-performance crystal oscillator circuits: theory and application" E.A. Vittoz, M.G.R.Degrauwe, S. Bitz
IEEE Journal of Solid-State Circuits, Volume 23, Issue 3, June 1988 Page(s):774 - 783
It has some very helpful basic theory.
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