Hi Don.
Thanks for the example. Hearing about these kinds of experiences makes
troubleshooting easier for everyone I think. Thanks to Chris Maxwell for
his contribution too. It'd be great to hear more stories from the
experience of some other members of the group.
I think your approach was great for the problem you had, but wanted to say
it doesn't always yield the quickest answer. To add to the examples:
Most of the equipment we test has a large number of (mostly long)
communication cable attachments. We had a recent experience where the
equipment was over the limit due to emissions being conducted onto one such
cable. Although with the cable attached the emissions from the cable were
high, the actual emissions from the source in the near field were low (most
of the noise was directly conducted onto the cable rather than radiated
from the problem circuit board). Near field probing wouldn't have told us
any quicker what the source was than the educated guessing + far-field
measurement approach we took. In our case though, just from the frequency
of the emission we already knew which circuit board was the culprit so it
was just a matter of finding out what the coupling mechanism was to the cable.
Under different circumstances though, I certainly agree that if you can use
near field probes to home in on an unknown source, that would be likely to
yield an answer quicker than trial and error.
I guess I'm trying to say near field probing is useful, but doesn't always
work. Because there can be such differences in the emissions measured in
the near field as opposed to the far field it isn't always the best method.
Regards,
Pete.
-------------------------------------------------------------------------
Please note: The views, opinions and information expressed and/or
contained herein do not necessarily reflect the opinions or views of
Foxboro, the organisation/s through which this communication was transmitted
nor any other third party, unless explicitly stated so.
Peter Poulos (Hardware Design Engineer)
Foxboro Australia
At 12:16 AM 16/09/2000, umbdenst...@sensormatic.com wrote:
Hello Joe,
Consider the following -- in the far field (3 or 10 meters), a plane wave is
monitored. In the near field, using either commercial or lab built
near-field probes, either E field or H field emissions will be monitored
separately. The E, H components will be isolated. The emission may be
identified with a probe, but the effective radiator (culprit antenna) for
that emission might be missed.
The above is an answer to the question. If you would like a real life
experience describing the difference, read the example that follows.
On a recent product we had a band of frequencies of non-compliant emissions
that were somewhat polarity sensitive. We observed a particular signature
of the emission (modulation on a pulse) at 3 meters using a bilog antenna.
Using a direct contact E field probe, the pulse frequency showed up at high
levels around the processor and DSP chip, but not with the signature. We
were able to find a trace of the corresponding polarity that was suspect and
had a similar signature, and at a lower level than we found around the
processor and DSP chip. Looking at the schematic, we identified a
reasonable fix. But that only helped part of the profile. We then sniffed
with a non-contact magnetic loop probe and found another viable culprit.
The fix implemented brought the product into compliance with reasonable
margin.
Neither fix by itself brought the product into compliance. Both were
necessary, required a minimum amount of components and contributed to
rationale "source suppression". We did not introduce "balloon squeezing",
i.e., beat down an emission at one frequency and see it pop up at another
frequency. This kind of isolation is more effective than monitoring the far
field emission, hypothesizing the culprit antenna while analyzing the
schematic. We have done it both ways. The near field approach takes a
little more time to set up but saves time in the long run. Or maybe we were
just lucky!
Best regards,
Don
> ----------
> From:
> marti...@appliedbiosystems.com[SMTP:marti...@appliedbiosystems.com]
> Reply To: marti...@appliedbiosystems.com
> Sent: Thursday, September 14, 2000 1:07 PM
> To: emc-p...@majordomo.ieee.org
> Subject: Near Field Versus Far Field
>
>
>
>
> I am having a difficult time answering the following question for a
> non-technical person. Hopefully, someone can put the answer into a
> language
> that a non-technical person can understand.
>
> We have a 400 MHz clock and are failing radiated emissions at 10 meters by
> 10 dB
> at 400 MHz. We bring the product back to our lab and start making
> modifications
> on the clock circuit and taking measurements with a near field probe.
> With
> these modifications and measuring with a near field probe, we realize a 10
> dB
> reduction in emissions at 400 MHz. Why would we not see the same
> reduction when
> taking the product back to a 10 meter site?
>
> Your help is appreciated.
>
> Regards
>
> Joe Martin
> marti...@appliedbiosystems.com
>
>
>
> -------------------------------------------
> This message is from the IEEE EMC Society Product Safety
> Technical Committee emc-pstc discussion list.
>
> To cancel your subscription, send mail to:
> majord...@ieee.org
> with the single line:
> unsubscribe emc-pstc
>
> For help, send mail to the list administrators:
> Jim Bacher: jim_bac...@mail.monarch.com
> Michael Garretson: pstc_ad...@garretson.org
>
> For policy questions, send mail to:
> Richard Nute: ri...@ieee.org
>
>
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
