[hsdd] High-Speed Digital Design Newsletter Via Inductance II
- From: "Dr. Howard Johnson" <howiej@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Fri, 12 Sep 2003 17:30:55 -0700
VIA INDUCTANCE II
HIGH-SPEED DIGITAL DESIGN ? online newsletter ?
Vol. 6 Issue 08
Greetings Everyone!
Let me tell you about what's happening with my new
Advanced High-Speed Signal Propagation class.
After piloting the course at Oxford University last
May, I have spent the summer refining the material
in preparation for the U.S. premier in San Jose on
October 27-28.
As you probably know, I love doing live experiments
during my seminars. This feature remains one of the
more popular parts of the classroom experience. I
like to show how things really work.
Unfortunately, a classroom setting limits the length
and complexity of the experiments I can do. To break
free of that limitation I've begun filming new
versions of the experimental demonstrations that I
conduct in my High-Speed Digital Design class, as well
as all-new experiments for the Advanced class. The
first of these videos will be shown during the October
classes, in lieu of live demonstrations.
The filmed versions eliminate wasted movement
between measurement setups, allowing me to pack more
measurements into the same amount of time. The films
proceed much like a TV cooking show?first I explain
what I'm going to do, show the ingredients, and then
we cut directly to the finished pudding. Spectacular
video close-ups give the audience a much better
viewing perspective that is possible in a live
format. A professional movie crew, with full lights
and sound, did the filming and editing.
Initially, the videos will be shown only during my
classes. I look forward to receiving feedback from
those of you who have the opportunity to view them.
The following question is directly related to one
of my video experiments AND to the material covered
in the Advanced class.
______________________________________________________
VIA INDUCTANCE II
The following values for the inductance of an
interplane via were collected using the incremental
inductance technique described in chapter 5 of High-
Speed Signal Propagation: Advanced Black Magic, ISBN
013084408X, and discussed in detail during my video
"Inductance of Via", which will be shown as part of
my new Advanced class. I hope you find the numbers
useful.
The setup is shown in , Fig. 5.33, p. 353, of High-
Speed Signal Propagation (if you don't have the
book, the figure also appears in newsletter Vol. 6
Issue 02, "Via Inductance"). The setup provides for
four layers (signal, plane, plane, signal). The
trace proceeds from layer 1, down through a signal
via to layer 4, and then continues across the board.
The planes are connected by a single interplane via,
located some distance away from the signal via. The
interplane connection is designed to be movable, so
you can see the impact of its placement on the
performance of the via. I can actually put my hand
on the interplane connection and drag it back and
forth while watching the TDR response getting better
and worse--a fantastic way to optimize via geometry.
The purpose of making these measurements was to
corroborate real-world measurements of via
inductance with a simple approximation:
L[VIA] = [u/(2*pi)]*2*h*ln(s/r)
Where LVIA is the inductance, in H, taking into
account only that portion of the via inductance due
to the traversal of the interplane space interior to
the pcb,
ln() is the natural logarithm,
h is the interplane spacing (height),
s is the separation between the signal via and one
interplane connection, and
r is the radius of the via.
In metric mks units (h, s, and r in meters, L in
Henries), the constant (u/(2*pi)) works out to 2E-07
H/m (assuming a non-magnetic dielectric).
In English units, (h, s, and r in inches, L in nH)
the constant (u/(2*pi)) equals 5.08 nH/in.
The dimensions of the real-world via under study
are:
Via diameter 0.010
Via radius 0.005
Interplane separation 0.040
Conductor thickness (all) 0.0062 (~1/2 oz. Cu)
Clearance hole diameter 0.024
Pad dia. on signal layer 0.024
(pads stripped on plane layers)
Trace width 0.0075
Trace height 0.004
Trace length (on either side of via) 0.240
Dimension of planes (plan view) 0.480 x 0.240
The following results were obtained for the
incremental inductance of the via. The data were
taken using an HP 4271B digital four-terminal LCR
meter operating at a frequency of 1 MHz. The results
are listed as a function of the spacing S between
the signal via and the interplane connection
(measured center-to-center). The units for S are
inches, the inductances are listed in nH, and the
capacitances are in pF. All values listed here are
corrected for the model scale and assume the real
via is surrounded by FR-4 having an effective
dielectric constant of 4.1 at 1 GHz.
S L[VIA,PREDICTED] L[VIA,MEASURED] C[VIA,MEASURED]
1.7 0.49 0.46 0.40
2 0.56 0.54 0.38
3 0.73 0.76 0.37
6 1.00 1.07 0.37
9 1.17 1.25 0.37
12 1.29 1.41 0.37
Two known factors account for the deviation between
predicted and measured results.
First, the planes in this experiment (0.480 x 0.240)
were only a few times larger than the interplane
spacing (0.040), a factor which is known to affect
(slightly) the measured results. This effect becomes
more pronounced as the interplane connection is
moved further from the signal via.
Second, the simple approximation takes into account
only that portion of the via inductance due to the
traversal of the interplane space interior to the
pcb, ignoring the inductance of the via stubs
protruding above and below the planes. The via stubs
in this case are much shorter than the interplane
spacing. Additional errors result from mechanical
imprecision of the model, and noise and parasitic
effects in the measurement apparatus.
The definition of L[VIA] is used in determining
the reflection coefficient of a via (High-Speed
Signal Propagation, chapter 5), and also appears as
the term L2 in the overall inductance of a bypass
capacitor (see "Parasitic Inductance of Bypass
Capacitors", EDN July 20, 2000).
Via inductance, via capacitance, trace loss, S-
parameters, and a wealth of other topics related to
high-speed digital operation above 1 GHz are all
included in my all-new class, High-Speed Signal
Propagation. See the synopsis at www.sigcon.com.
______________________________________________________
EXTRA FOR EXPERTS
The stainless steel (type 304, non-magnetic) used in
the model has a resistivity 40 times higher than
copper. This feature enlarges the depth of
penetration of current (skin depth) in the model, so
that the ratio of skin depth to conductor thickness
in my giant 100:1 model, when measured at 1 MHz, is
comparable to a copper via, at a scale 100 times
smaller, operated at a frequency of 250 MHz.
The trace geometry was designed so that, if the
model were stuffed with an FR-4 dielectric, the
model trace characteristic impedance would be 50
ohms. So that I could get my hands on the model to
actually move the interplance via to different
positions between the planes, the dielectric I used
in the model was nothing but thin air (mostly). To
hold the model trace in position, and to separate
the planes, I used small wooden dielectric supports.
These supports are clearly visible in the video. The
use of an air dielectric, while it affects the
capacitance of the via, has no effect on its
inductance. The characteristic impedance of the air-
dielectric trace in the model works out to about 85
ohms.
Best Regards,
Dr. Howard Johnson
______________________________________________________
Join us for the US premier of my Advanced class in
San Jose, CA:
Advanced High-Speed Signal Propagation: Oct. 27-28
...and for the popular
High-Speed Digital Design: Oct. 23-24
We are currently reserving dates for
private classes next spring. Contact Jennifer at
509.997.0505 or info03@xxxxxxxxxx if you'd like to
book a class.
A full schedule of cities and dates for public
appearances is posted at: www.sigcon.com.
If you have an idea that would make a good topic for
a future newsletter, please send it to hsdd@xxxxxxxxxxx
To subscribe to this list send an email to
hsdd-request@xxxxxxxxxxxxx with 'subscribe' in the subject field.
To unsubscribe from this list send an email to
hsdd-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field.
Newsletter Archives: http://www.sigcon.com/
Copyright 2003, Signal Consulting, Inc.
All Rights Reserved.
Other related posts:
- » [hsdd] High-Speed Digital Design Newsletter Via Inductance II
