[SI-LIST] Re: EMI coupling of clock on other signals
- From: Cosmin Iorga <ci249534@xxxxxxxxx>
- To: chundi srikanth <chundis@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 22 Jan 2010 09:40:53 -0800 (PST)
Hi Sree,
You may have two issues: 1. The power distribution impedance seen by the SDRAM
drivers is enough high at the operating frequencies to generate significant
noise on both ground and positive supply. 2. Many times ground noise couples
through shared impedance into the rest of the circuits (even if these circuits
are powered from separate positive supplies). I suggest to measure the
frequency characteristics of the power distribution impedance as seen from the
major noise contributor circuits and build a model that covers the shared
ground impedance of transient currents (make sure the modeling tool doesn't
lump the model into the power line keeping the ground net ideal - some tools do
that and they will never show your problem). With a good model you can then
adjust your decoupling capacitors values and types (ESR, ESL, res. freq....);
or if you find major layout issues you may need to respin the board.
Cosmin Iorga,
NoiseCoupling.com
http://www.noisecoupling.com
________________________________
From: chundi srikanth <chundis@xxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Thu, January 21, 2010 10:22:16 AM
Subject: [SI-LIST] EMI coupling of clock on other signals
Hi folks,
Iam working on a board which is of 16-layer and its a mixed signal board
which is having signals running at around 3GHz. Iam testing this board with
initial software. And we have one clock chip whose input is 30.72MHz and its
outputs are at 122.88MHz. And This board got an MCU whose highest running
frequency is 166MHz. And it was interfaced to a 32-bit SDRAM and it was
running on a clock speed of 83.33MHz. Today when i probed the 122.88MHz
clock out in a spectrum analyzer there is a strong spur of frequecy
components which were at 166.7, 215.8, 332.4MHz, 489.7MHz etc..... These are
almost harmonics of SDRAM clock frequency. And everywhere even if i probed
the GND the components can be seen. So what might be the problem and can you
suggest what can be done on board to resolve this.
Thanks in advance for your help.
Regards
Sree
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