Wait a minute!
Buying a board scanning system to evaluate different vendors for ferrites
and oscillators? My company doesn't have that kind of money to throw
around. These systems can cost 10's of thousands of dollars. Ferrite
beads cost about a dime for a dump truck load. If it costs $10,000 to
evaluate a second source for ferrites, I 'll stick with the ferrites I have.
I'd like to offer a lower cost alternative.
Koh Nai asked about what specifications were important for qualifying
alternate sources for ferrites and oscillators.
When it comes to ferrites, I look at three things: I look at the PCB
footprint (it won't work if it won't fit). I look at the impedance curve
and the current capacity. If all three of these specifications are equal or
better than what I need, I accept them. I don't even consider re-testing
for emissions if I have checked these three specifications. Alternate
sources for ferrites can be qualified for the cost of reading a spec sheet.
Oscillators are a different story. When one of my digital design colleagues
wants to change oscillators. They consider its PCB footprint, the output
frequency, its ambient stability, its temperature stability and its load
driving capability. If it meets their needs; then I take a circuitboard
with the existing oscillator and run a near field probe over it near the
oscillator until if find a location of maximum near field emissions. (If
you really are strapped for cash, you can make your own near field probe.)
I write down the exact position and orientation of the near field probe and
I either print out or write down the spectrum analyzer readings. I then put
the new oscillator on the exact same board and repeat the experiment. If
the measurements are close (within a dB or two) I don't worry. If the
measurements are more than 4 dB higher, then I look further. Then I
consider: testing the whole unit with the new oscillator with my antenna
set up 1 meter away in-house, or re-testing the unit for emissions at an
OATS, or not using the new oscillator.
ONE WARNING: if the new oscillator is at a different frequency, then the
method above WILL NOT yield any useful results.
One thing that we have done with new designs is to put a 1206 surface mount
PCB footprint in line with the oscillator output. We start our testing with
a 0 ohm resistor. If we run into problems, we can put either a ferrite bead
or higher value resistor in this position to "cool off" the oscillator.
This has worked well with oscillators under 100MHz. I don't know if it will
work for faster oscillator.
I know that there are problems with using near field probes to make such
correlations, however using a board scanning device would cost much more
than a near field probe and still only be measuring near fields. Even so,
if I had the budget, I'd love to try one out.
To me, the real method of doing this starts with the initial testing of your
product. I try to get more than a 5dB margin during the initial testing.
With these margins, I don't need to worry so much about component
differences. I know that this is sometimes not possible. I have sacrificed
margins in order to get a product's testing done and released (I don't get
paid if we don't ship.) The problem is, without margins, I need to worry
more about component differences.
Another point to remember is that EN 55022 and other emissions standards go
by the "80 %" rule. A product "passes" if we are confident that 80% of
the units that we ship meet the emissions requirements. Anybody who wants
to dispute whether your units pass or not is REQUIRED to test up to 7
samples in order to get enough data to use statistical methods to compute
the confidence factor. One failing unit does not equal a guilty verdict.
(Of course, if one unit is failing by 20dB, that's a problem.) The people
at CISPR put this slack into the limits to allow for slight component
differences and measurement uncertainty. They allowed us the slack, but it
is our responsibility to use it with common sense.
These are my two cents worth and definitely don't reflect the opinions of my
employer. (Why would they need me if I thought the same way they do?)
Have a great day!
Chris Maxwell, Design Engineer
GN Nettest Optical Division
6 Rhoads Drive, Building 4
Utica, NY 13502
PH: 315-797-4449
FAX: 315-797-8024
EMAIL: chr...@gnlp.com
> -----Original Message-----
> From: Ralph Cameron [SMTP:ral...@igs.net]
> Sent: Wednesday, September 20, 2000 11:16 PM
> To: Tony J. O'Hara; Koh Nai Ghee
> Cc: EMC-PSTC
> Subject: Re: Component Qualification
>
>
> Tony:
>
> I have sold these systems and also used them and they are good for
> graphing
> a profile of a scanned board and giving the field intensity vs postion
> over
> the board area. You can quickly find trouble spots using the computer
> interface.
>
> Ralph Cameron
>
> ----- Original Message -----
> From: "Tony J. O'Hara" <tonyoh...@compuserve.com>
> To: "Koh Nai Ghee" <koh...@cyberway.com.sg>
> Cc: "EMC-PSTC" <emc-p...@majordomo.ieee.org>
> Sent: Wednesday, September 20, 2000 4:42 PM
> Subject: Re: Component Qualification
>
>
> >
> > You may want to look at using a PCB Electromagnetic Scanning System!
> One
> > of the advertised uses for these test devices is for quickly comparing
> EMC
> > performance when component changes are made etc.! I believe there are 4
> > different manufacturers who make these devices. The one that I'm just
> > starting to learn about is made by EMSCAN in Canada. Their web is
> > www.emscan.com
> > Maybe someone who has & uses one of these scanners can provide an
> > experienced viewpoint?
> > Regards
> > Tony
> > Colorado
> >
> > -------------------------------------------
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> >
> >
>
>
> -------------------------------------------
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