FreeLists / si-list / [SI-LIST] Re: nonlinear in power ground noise of clock driver

[Curt McNamara <CurtM@xxxxxxxxxxx>]

Obviously the dielectric is better, but have you considered the effects of
ESR on the
caps ability to supply switching current? If the tantalum is standard issue,
perhaps it
cannot supply current to the clock driver quickly enough. This becomes
particularly
obvious when you are isolated from the power plane.

Curt McNamara. P.E.
Senior Electrical Engineer
Logic Product Development
411 Washington Ave. N Suite 101
Minneapolis, MN 55401
Tel 612-436-5178
Fax 612-672-0443
curtm@xxxxxxxxxxx
www.logicpd.com

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-----Original Message-----
From: Zhangkun [mailto:zhang_kun@xxxxxxxxxx] 
Sent: Thursday, December 04, 2003 5:50 AM
To: si-list
Subject: [SI-LIST] nonlinear in power ground noise of clock driver


Dear All:
Recently, my colleague found out something interesting about clock driver.

In our product, CDC2516 of TI is used as clock driver. The clock frequency
is 100MHz. The power of CDC2516 is isolated by one bead. There is some local
decoupling caps as following:
    several 0.1uF or 0.01uF ceramic cap
    one 10uF tan cap

We measure the power ground noise of CDC2516. The power ground noise is
about 700mV. The spectrum of the noise is at 100MHz, 200MHz, 300MHz, and so
on.

We use several method to decrease the noise. Theere is no use.

My colleague change the 10uF tancap to one 10uF ceracap(X7R). The noise
decrease to 17mV. I am a little confused. I have simulate the impedance of
decoupling circuit. There is no difference in frequency domain beyond 30MHz
between using 10uF tancap and 10uF ceracap. According to my experience, the
power ground noise should not decrease.

I think there is something nonlinear. I am not familar with the circuit of
clock driver. There is no SPICE model of CDC2516 in TI website. Therefore I
could not simulate the power ground noise.

Could you give me some advice?

Best Regards

Zhangkun
2003.12.4


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