[SI-LIST] Re: measurement methods of power supply networks for a wirebond BGA package
- From: "Hill, John" <jhill@xxxxxxxxxx>
- To: <hliang@xxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 5 Jun 2006 18:17:10 -0400
Hello,
There are lots of ways that people measure the impedance of power
planes. There are also a lot of possibilities for errors in the
measurements. The big problem is that you cannot discount the wavelength
of the signal. As our frequencies go up we need to improve the
techniques we use to measure the values.=20
I think it is understood that we need to measure through. That is we
need to use a network analyzer and measure the board power plane
impedance between the two ports of a network analyzer. To do this you
have to calibrate out the cables. Using calibration standards you
numerically remove the electrical length and imperfections of the
cables. This is done by the network answer after measuring calibration
standards. An open-short-load calibration is good to above 5 GHz and is
easy to do.
The next problem to resolve is the impedance of the stuff between the
end of the connector on the cable and the power plane. Some people like
to just use the S21 measurement. But I have some concerns about this
technique.=20
If there is significant shunt capacitance, I like to use the fact that
the through Z in a Pi network is equal to the B value of the ABCD
matrix. See Appendix D (page 332) of High-Speed Digital System Design by
Hall, Hall and McCall.=20
However in measuring power planes with good RF connectors on both top
and bottom of the board, I would expect the series inductance of the
vias would cause more errors than the shunt capacitance. Hence I would
think a better model is a T network where the two series elements are
the top half and bottom half of the vias. The center shunt impedance is
the value we are looking for. It is the impedance of the plane to
ground. This center shunt impedance would be equal to one over the C
value of the ABCD matrix.=20
I would hope that someone on the list has already performed this
calculation.=20
The C value is equal to (((1-S11)(1-S22))-(S12 S21)) / 2 Z0 S21
One other concern that I have about this test setup is the impedance of
the ground connection between the two RF connectors. Remember the ground
path between the two RF connectors needs to be very good. My expectation
is that putting the two connectors near the edge of the board would be a
good thing with a copper strap around the edge of the board. The
connection between the two grounds should be as short as possible with a
wide copper construction.
However, there is an even better way to do this. The technique is called
de-embedding. Microwave people do it all the time. The stuff between the
connector at the end of the cable and the power planes in the board is
measured and removed numerically. To do this you need to make a board
with four RF connectors. Two would be a through measurement of a power
plane and two would be a through measurement without any connection to
the power plane. Then the two port network of the stuff, both shunt and
through, could be de-embed and removed. My problem is that I know that
it can be done, but don't know how to do it. I think you divide the S
matrix by 2 and then invert it, but I am not sure.
I hope there is someone on the list that has made these measurements and
calculation and is willing to share their findings.
Best regards,
John=20
=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Liang, Hongwei
Sent: Tuesday, May 30, 2006 6:35 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] measurement methods of power supply networks for a
wirebond BGA package
I am searching for methods on using network analyzer to measure power
supply networks for a wirebond BGA package from 50MHz to 500MHz. All the
wirebonds need to included as part of power supply network. =20
If any one knows a paper or resource, would you please tell me? I
appreciate for your help.
Thank you,
H.W.
=20
=20
=20
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