[SI-LIST] Re: Wacky power plane impedance measurements
- From: steve weir <weirsp@xxxxxxxxxx>
- To: chris_landrum@xxxxxxxxxx,
- Date: Fri, 09 Apr 2004 05:52:54 -0700
Chris,
As others have noted the dependence on the cable location has got to do
with either flaky connections, or a grounding issue between the driving and
receiving circuits. If you use a 3 meter cable from input to output do you
also see anomalies? Are you operating over a metal plate like a metal
workbench?
Anyway, the impedance that you need across the ASIC power balls all comes
back to the V = I * |Z|, and dV = dI * |Z|. The device package as attached
to the PWB effects a low-pass filter that even zero impedance in the PDS
cannot overcome.
Now, the burning question is, if not for power, what drives people to low
impedance? The answer is signal currents when the high speed signals are
referenced against a power plane instead of ground. Now to maintain a
tolerable noise voltage, you are forced to tightly couple the power plane
to ground at the signal operating frequencies, a much more difficult task
than just supplying the PDS needs. If you do that, then you will need to
calculate how much signal switching current you have. Translate that into
an effective driving impedance then multiply by the acceptable noise
voltage and divide by the supply voltage, and that is the maximum impedance
that you want.
This is the topic that routinely gets Chris Cheng to beat the drum about
the importance of selecting the right plane as the return for signal currents.
Regards,
Steve.
At 05:27 AM 4/9/2004 -0700, Landrum, Chris wrote:
>I am using an SA with a tracking generator. I moved the two connection
>points closer to eachother, within an inch or so and the behavior is
>much better, although not perfect. It seems the higher freq are much
>more sensitive to movement (say above 500MHz). Rather than getting a
>totally different curve when I move the cables, I now just seem to get
>an vertical movement (say 10dB or so) of the curve. The peaks/valleys
>remain the same.
>
>I have yet to try the ferrites....
>
>What might you all expect the freq response to be on a "modern" design
>with a high speed ASIC and a DDR memory. I calculate that above 400MHz,
>I am seeing 0.25 Ohms. I would have expected to see 0.025 Ohms. This
>is no doubt due to the plane capacitor (or lack thereof). Is my
>assumption of needing a plane impedance of somewhere in the 10's of
>milliohms accurate? I know this is a tough question to answer without
>knowing the design, but 250 mOhm seems high.
>
>I am coming into this design with it in it's final stages of completion.
>The plane separation, in my opinion, is less than appropriate for a
>design of this type (14 mils). I am having to reverse engineer a lot.
>
>Thanks for the help.
>
>-----Original Message-----
>From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
>Sent: Thursday, April 08, 2004 8:41 PM
>To: Landrum, Chris; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Wacky power plane impedance measurements
>
>
>Chris,
>
>You mention spectrum analyzer... was the source a built-in tracking
>generator? Or did you mean network analyzer?
>
>Beyond the points that other people already pointed out, you may
>experience sensitivity to cable location if the IF bandwidth is
>unnecessarily wide, and/or if accidentally you mix up the power and
>ground connections inj the two cables. This means if you connect
>accidentally the braid from the source cable to the center of the
>receive cable.
>
>I hope this helps.
>
>Istvan Novak
>SUN Microsystems
>
>
>----- Original Message -----
>From: "Landrum, Chris" <chris_landrum@xxxxxxxxxx>
>To: <si-list@xxxxxxxxxxxxx>
>Sent: Thursday, April 08, 2004 11:02 AM
>Subject: [SI-LIST] Wacky power plane impedance measurements
>
>
> > Hi all,
> > I am running an experiment where I am attempting to measure the power
>plane
> > impedance of a PCB. The PCB is only populated with capacitors. I am
> > using
>a
> > spectrum analyzer to plot the freq response. I have two pieces of 50
> > Ohm shielded cable where I have cut the ends to expose the shield and
> > the
>center
> > conductor. I have then soldered the shield to a large cap at GND and
> > the conductor to the other side of the cap at PWR. I have done the
> > same for
>the
> > second cable at some arbitray distance from cable #1.
> > I have noticed that my measurement is EXTREMELY sensitive to cable
>movement.
> > The results will also change when I simply touch the N-type adapter of
>
> > the SA. I am sure there is a good connection to the PCB, as I have
> > wire tied
>the
> > cable to the PCB so as not to allow movement at the points of contact.
>
> > The cables may be approx 2ft in length (matched), and I have tried
> > shorter lengths with the same result. I have also tried higher quality
>
> > cables.
> >
> > Is this result due to coupling? Has anyone performed such a
> > measurement without the varying result? Perhaps someone who has can
> > describe their set-up.
> >
> > Thanks.
> > Chris
> >
> >
> >
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