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I agree with you.
Keep in mind that open ended data cables are a very common as well as a valid test setup for radiated emissions.
That is why I tested using this setup, I wanted the worst case to help shed some light on an upper bound for DTE
power supply common mode noise. This applies equally to the PSE and the PD, since either end can be a source of noise.
More testing is needed.
For instance, I would like to see the test results from injecting a sine wave signal, in the "pair to pair" mode,
into the "spare" (as well as data pairs) so that the same mode is excited as will be with a power supply.
The collected data would consist of voltage levels of the injected signal that cause any increase in 10/100/1000 data errors,
at injection frequencies of 100 kHz to 30 MHz. This frequency range is not strictly covered by radiated emissions, yet
is important for power supply output spectrums.
From: Jack Andresen [SMTP:jandresen@xxxxxxxxxx]
Sent: Tuesday, September 19, 2000 12:36 PM
To: Brooks, Rick [SC5:321:EXCH]; stds-802-3-pwrviamdi@xxxxxxxx
Subject: Re: RJ-45 common mode radiated emissions testing
Thanks for the reply and thanks for the testing. I certainly am not
requiring you to undertake additional work.
Its just that reflections for open (unterminated) wires is at least
double giving 6 dB more radiation. And resonances will make it worse.
The TIA found serious resonances due to short patch cords without proper
terminations. I have found resonances with common mode chokes (big ones,
in the order of half a mH) as low as 50 KHz.
Rick Brooks wrote:
> I was trying for the worst case for common mode radiation.
> There were not any differential mode signals generated, except by mode
> The results are ball park in any case.
> What you ask for is possible, but not what I was trying to do.
> I wanted to add one data point for what common mode noise could be
> tolerated from the PSE power supply,
> given Roger's conclusion that radiated emission requirements are the
> more restrictive limit for power supply noise.
> A conclusion that I agree, by the way, with for frequencies of 30 MHz
> and higher.
> I did this EMC testing as a freebie, I don't have a plan to redo this
> EMC testing.
> If I do any testing in the future, I will keep your request in mind.
> - Rick
> -----Original Message-----
> From: Jack Andresen [SMTP:jandresen@xxxxxxxxxx]
> Sent: Friday, September 15, 2000 1:55 PM
> To: Brooks, Rick [SC5:321:EXCH]
> Cc: stds-802-3-pwrviamdi@xxxxxxxx
> Subject: Re: RJ-45 common mode radiated emissions testing
> The data helps. However, is it possible to redo with proper
> on the end of the cable. You need both differential and common
> terminations. Unterminated end reflections will increase mode
> Jack Andresen
> Rick Brooks wrote:
> > I have posted the test setup and results of a radiated
> emissions test
> > that I did recently in a 10 meter anechoic chamber.
> > The test was to try to determine what common mode signal
> voltage would
> > produce a radiated field that would be at the
> > compliance limit, class B in this case.
> > I hope that this data will help us to specify a noise
> requirement for
> > the DTE power from the PSE, as well as the PD.
> > It is important to understand that the radiated emissions
> > only cover from 30MHz and higher.
> > The spectral region from 500 kHz to 30 MHz, is obviously of
> > to 10/100/1000, but I don't cover it here.
> > The pdf file is at:
> > comments, questions?
> > thanks,
> > - Rick