[SI-LIST] Re: digital to analogue noise coupling

By the way, a useful technique for identifying and isolating guilty =
aggressor circuits in crosstalk is spectral analysis. If your scope has =
FFT capability (most modern scopes do), you can look at the FFT of the =
victim and the FFT of the suspect aggressors. If you see the imprint of =
the suspect aggressor spectrum in the victim signal spectrum, that is =
often a useful clue.=20

This also works in the jitter domain. Most real-time scope jitter =
analysis packages can perform an FFT of the jitter to show the jitter =
spectrum. This is different from the FFT of the "raw" signal. If you =
think about jitter as "FM" (or "PM"), the jitter spectrum resembles the =
spectrum of the "modulating" signal after "demodulation." Again, if =
jitter spectrum components line up with components in the FFT of suspect =
aggressors, this can be a useful clue. =20

Art Porter      =20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Perry Qu
Sent: Monday, June 20, 2005 8:45 AM
To: 'steve weir'; 'si-list'
Subject: [SI-LIST] Re: digital to analogue noise coupling


Steve:

Thanks for your suggestions. Actually all the circuits here are +5V. =
Will do
some more measurement as you suggested to confirm the coupling channel.

Regards

Perry


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Perry Qu

Design & Qualification, Alcatel Canada

600 March Road, Ottawa ON, K2K 2E6

DID: 613-7846720    Fax: 613-5993642

Email: perry.qu@xxxxxxxxxxx

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx]=20
Sent: Monday, June 20, 2005 10:18 AM
To: Perry Qu; 'si-list'
Subject: RE: [SI-LIST] digital to analogue noise coupling

Perry, I assume that none of the voltages here are above 3V.  So you =
have=20
about 2% coupling   If your scope can do a good difference, probe the=20
ground plane on either side of the analog section.    That will tell you =
if=20
the image is just lifting one portion of the analog circuity or another. =
 If
the coupling is capacitive even a few pf looking into 100K will hold-up =
for
244ns.  Just to test, scale the impedances down in your analog circuit =
and
see if that substantially alters the crosstalk.

Steve.
At 09:47 AM 6/20/2005 -0400, Perry Qu wrote:
>Thanks, steve. The crosstalk is in the range of 60 mV. The return path=20
>of the clock is a solid plane and the clock is routed on either side of =

>the plane. There are about total 12 drops on 2 branches of the clock=20
>(series terminated at both branches). The stackup looks like:
>
>Component (short analogue/digital breakout)
>Sig2 (most of analogue routing)
>Sig3 (clock)
>Gnd
>Sig4 (clock)
>...
>
>As I understood, there is some capacitive coupling as clock on sig3=20
>pass underneath component pads on component side. Also, some inductive=20
>coupling is bound to happen as analogue routing on sig2 which is far=20
>away from the plane will see noise induced from the clock loop=20
>(sig2/gnd) -- this is small as most of the analogue traces on sig2 are=20
>routed perpendicular to sig3, and there is only a few routed at an 45=20
>degree angle.  However, even with all these capacitive/inductive=20
>coupling, it still does not explain why I see the copy of the clock=20
>rather than glitch corresponding to rising/falling edges only.
>
>Regards
>
>Perry
>
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
>Perry Qu
>
>Design & Qualification, Alcatel Canada
>
>600 March Road, Ottawa ON, K2K 2E6
>
>DID: 613-7846720    Fax: 613-5993642
>
>Email: perry.qu@xxxxxxxxxxx
>
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
>
>-----Original Message-----
>From: steve weir [mailto:weirsi@xxxxxxxxxx]
>Sent: Monday, June 20, 2005 9:03 AM
>To: Perry.Qu@xxxxxxxxxxx; si-list
>Subject: Re: [SI-LIST] digital to analogue noise coupling
>
>Perry, how much crosstalk are we talking about?
>
>A couple of things to consider:  Planes don't provide effective=20
>magnetic shielding to low frequency interference.  Your amplifier nodes =

>are likely a pretty high impedance making a net TC well above the 244ns =

>half period of your CODEC clock.  What does the return path for that=20
>clock look like?  How much drop in the path does the clock induce?  And =

>is that the image that you are seeing?
>
>Steve.
>
>At 08:45 AM 6/20/2005 -0400, Perry Qu wrote:
> >Good morning,
> >
> >I was asked to help troubleshoot a digital-to-analogue noise coupling =

> >issue on a design. There is a 2MHz clock routed underneath an=20
> >analogue circuitry and we picked up a copy of the 2 MHz clock with=20
> >much smaller amplitude -- small for digital but large enough to=20
> >interfere with analogue
>circuit.
> >
> >The exact same copy of analogue circuitry with no clocks routed=20
> >underneath works fine.
> >
> >Checked the layout of the mal-functioning circuitry, I found that=20
> >there is small capacitive coupling -- only happened  where the clock=20
> >routed underneath some analogue components pads; Inductive coupling=20
> >is also small as most of the analogue circuit are routed either=20
> >perpendicular or at an angle to the digital traces.
> >
> >The most puzzling part is that the crosstalk noise repeat the source
clock.
> >This seems to suggest that this is neither capacitive nor inductive=20
> >coupling as those will induce noise at rising or falling edge only,=20
> >not at the DC portion. It looks like there is some DC link between=20
> >the digital and analogue but could not locate that from schematics or =

> >layout. Also,  the circuit with no clock routed underneath works =
fine.
> >
> >Any thoughts ?
> >
> >Thanks
> >
> >Perry
> >
> =
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >Perry Qu
> >
> >Design & Qualification, Alcatel Canada
> >
> >600 March Road, Ottawa ON, K2K 2E6
> >
> >DID: 613-7846720    Fax: 613-5993642
> >
> >Email: perry.qu@xxxxxxxxxxx
> >
> =
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >
> >
> >
> >
> >------------------------------------------------------------------
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