Re: New thread on EMI
I don't agree with this in whole, only in part. And we had a disconnect on what I
really was asking when I started this thread.
> Comparing to the amplitude of the EMI emissions generated by multiple clocks
> and data lines, the EMI emission from an IDLE signal is small. If an EMI
> design can satisfy clocks/datas, then IDLE is not a problem.
> You should take a look at the actual EMI test report to appreciate it.
> Ed Chang
The test report, as Dan Dove hinted at in earlier replies, will show the clocks,
the harmonics from the clocks, common mode noise concerns, logic edge rates for
each kind of logic family used, and, of course, the effects from any external
communications channel. (oh yeah - and perhaps your favorite TV/radio stations :)
) And as Dan hinted at, proper design technique will reduce everything very
nicely from the 'contributer list'. However, the channel, in our case ethernet,
has had some nasty components that escape as part of the stream and not radiated
from clocks - careful control of the spectrum analyzer and packet content can help
you determine this.
Look at 100tx. There are five components that escape via the port that are not
clock or clock harmonic related. These are very nasty and climb quite nicely with
port count. So I don't agree that the IDLE is not a problem. We can agree to
My intent for this thread was you indicated to Pat Thaler that there were some
patterns that were worse then idle. My question was do you have data of random
network traffic that suggests a percent of the traffic has a higher contribution
to EMI then the idle and what percent is it / what is the packet content that
could cause this? If the percentage is very low, then perhaps the idle spectral
content is the only area in need of change.
I ask the question because my fear is solving the idle issue may case some other
area within the coding stream to show as the offending contributer when considered
as a percent of data traffic. I want to stay a head of the game. As you have
indicated, EMI is best dealt with at the design, not on the production line :).