[SI-LIST] Re: Traces don't cause EMI - really?

  • From: MikonCons@xxxxxxx
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 16 Oct 2003 14:33:13 EDT

In a message dated 10/16/2003 10:14:54 AM Pacific Standard Time, 
jeff.loyer@xxxxxxxxx writes:
Just to be clear - are you saying that if I connected 2 properly designed 
chips (driver and receiver) together with many properly designed single-ended 
transmission lines, they would likely fail FCC standards?
Yes, but only with qualifications. Picture a single-ended, 50-Ohm microstrip, 
not multiple closely spaced traces. If excited with a 1 ns risetime signal at 
33 MHz clock frequency from a 50-Ohm generator, and terminated with a 50-Ohm 
chip resistor, expect failure. For example, the EMCAD1 software application 
(circa 1992) is based on radiated emissions equations and predicts ~39 dBuV at 
33 MHz at 30 meters antenna distance. The first few harmonics are also 
predicted to be out of limits. Note that this software uses worst-case 
predictions. 
The same software also indicates an increase in the single-trace emission level 
by SQRT(N), where N = the number of traces; however, the assumption is that 
all traces are carrying the same excitation (which is unrealistic). 
Now, if you place grounded planar areas around the trace, or have a densely 
routed board, the additional copper provides a substantial reduction in the 
measured emissions from that single trace because many field lines that would 
otherwise leave the board terminate on the added copper (even though they may 
be 
additional 50-Ohm traces). And, in a normal board design, many of the signals 
on these surface traces will generate fields that cancel each other. 
Therefore, the bare (i.e., unshielded, and unenclosed) PCB MAY or MAY NOT fail 
an FCC 
or CISPR radiated emissions test. 

The IBM paper I mentioned recorded up to 20 dB variation in radiated 
emissions from a single microstrip on a bare board as the trace was moved 
in-board 
from the PCB edge. This measured reduction also illustrates the field-capturing 
effect of surrounding copper, reference planes, or other conductors.

The PCB I created contained significant open surface space around the traces 
(including the guarded trace structures) which tended to increase the radiated 
emissions relative to a real board design, but this was on purpose to 
illustrate the effect.

As an added note, I found that the radiated emissions from a single trace 
increased more than linearly with the impedance of the line; i.e., a 100-Ohm 
trace radiated >6 dB more than a 50-Ohm trace. This result makes sense as a 
better 
match to the 120*pi = 377 Ohms of free space is achieved with the 100-Ohm 
line.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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