[SI-LIST] Re: Wacky power plane impedance measurements
- From: "Landrum, Chris" <chris_landrum@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 9 Apr 2004 05:27:13 -0700
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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