Just to add a minor note to the thread:
The easiest way I use to determine if a signal is a "pure" clock
harmonic, is to simply punch down the RBW on the receiver/analyzer a
bunch of times. If it is a clock, you can reduce the RBW by a factor of
as much as you have, and the peak amplitude won't change.
Take care,
Brent DeWitt
On 5/6/2022 12:10 AM, Bill Owsley wrote:
If you set the step function to the 63 MHz freq, and then step up, BW
set a little wide to account for slight inaccuracy, do you find more
emissions, energy, harmonics, at the steps ?
If there is only the one peak, and it is a clean signal, suspect an
curious resonance.
Had one once, the trace was resonant and sang rather notably. A little
resistance at each end worked, but oddly, it took several guesses to
find the right value. ps. this was a clock trace that wrapped around
3.5 sides of circuit board near the edge. Don't allow that to
happen. Not fun, interesting, but not fun.
The frequency rings a bell, but can't quite remember why.
Ethernet is 125 MHz, guess what half of that is near ?
If the observed signal is a solid shaped peak, I would first guess a
clock of a steady sort.
If a bit wobbly, modulated, a guess is that it has data on it, or
might be inter-modulation.
If a shape other than sine peak, wobbly or not, it might be frequency
modulated, usually called frequency hopping, and some actually are,
which is a mis-understanding in engineering.
pps. the spread sheet for clock harmonics is a good tool. We had one
long ago, that did all the divisions and multiplications. Then an
easy effort to look it up.
But if the developers don't give you a complete list of clocks, xtals,
or computed and used only in a chip, The sheet will help you tell them
what their chips are doing.
pps, that can get a bit embarrassing, since the designers don't know
what their chips are doing and they are building this to work. A 6
month delay, been there, might happen before the secrets are revealed.
On Thursday, May 5, 2022, 10:08:14 AM EDT, Ken Wyatt
<k...@emc-seminars.com> wrote:
I was just going to suggest what James already did.
You should be able to take a small-diameter H-field probe and zero in
on the source. That may help.
If you email me directly, I’ll provide you a clock harmonic analyzer
spreadsheet developed by my co-author, Patrick André.
Cheers, Ken
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On May 5, 2022, at 3:16 AM, James Pawson (U3C)
<ja...@unit3compliance.co.uk <mailto:ja...@unit3compliance.co.uk>> wrote:
Hi Amund,
Narrowband or broadband noise?
If broadband and you zoom in to the noise on the analyser, is it made
up of separate peaks? What is the spacing between the peaks?
Try using a near field probe to zoom in on the emissions
-https://www.unit3compliance.co.uk/graphical-guide-to-emc-near-field-probing/
<https://www.unit3compliance.co.uk/graphical-guide-to-emc-near-field-probing/>
All the best
James
James Pawson
Managing Director & EMC Problem Solver
**
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*EMC : Environmental & Vibration : Electrical Safety : CE & UKCA :
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www.unit3compliance.co.uk <http://www.unit3compliance.co.uk/> |
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*From:*Amund Westin <am...@westin-emission.no
<mailto:am...@westin-emission.no>>
*Sent:*05 May 2022 09:50
*To:*EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
*Subject:*[PSES] Chasing a specific frequency
During a radiated emission test, we have a problem with one specific
frequency, 63MHz. Not exactly 63.00MHz but like 63.03406….
We have calculated all combinations of intermodulation products from
oscillators on the pcb, but no one match the 63MHz. There are a lots
of DC/DC converters on the board, and I suspect low switching
frequencies (≈1MHz) may be the source and problem. How this low
frequency signal became 63MHz noise, it must be (could be … or?) a
combination of harmonics and layout circuitry that optimize unwanted
antenna performance and then result in a powerful noise component at
63MHz.
At this stage, we have not investigated the trace layouts or the
layers stacking of the pcb. We have done some introduction of clean
power (batteries) and in the same time, bypassed DCDC converters to
observe the difference in noise spectrum.
On the right track?
BR
Amund
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