Re: [SI-LIST] : attenuation calculations

Howard Johnson (howiej@sigcon.com)
Fri, 22 May 1998 15:53:53 -0700

Hi Craig,

Yes, your numbers sound to me like they are in the right
ballpark.

The skin effect (AC resistance) of a trace
at a freuqency of 1 GHz (i.e., rising edge rate of about 1/2 ns)
is greater than the dielectric losses (by quite a bit).

At a freuqency of 1 GHz (i.e., rising edge rate of about 1/2 ns)
I calculate the skin-effect resistance of a 0.006"-wide PCB
trace, at a height of 0.006" above ground, to be 1 ohm per inch.

If you'd like more details, there is a write-up about these calculations
(and lots of other stuff) on my web site at:
signalintegrity.com/newsletter.com --- see "Skin Effect Calculations" at the
bottom of the page.

Fibre-channel designers typically run traces of this type for
distances of 12" to 24" on big FC-AL backplanes. These lines
run at a speed of 1.06 Gb/s, using differential signaling. The
signal at the other end displays noticeable edge-degradation and
a fair amount of inter-symbol interference, but nothing that a
well-offset differential receiver can't cope with. No equalization
is used.

Best regards,
Dr. Howard Johnson

At 06:11 PM 4/23/98 +0100, you wrote:
>Hello,
>
>I am currently investigating attenuation of pcb traces, at speeds of
>~1-3GHz, with the use
>of GETEK and FR4. The traces are 1/2oz copper, 6mil width, 50 Ohm
>impedance. Unfortunately
>the only calculations I can find are low-loss approximations. With
>these formulas
>I am finding the attenuation due to dielectric loss being 0.1db/in and
>attenuation due to metal being 0.15db/in, for FR4. These attenuations
>were calculated
>with the FR4 tangent loss of .018 and AC Resistance approximated at 1
>Ohm per in.
>I was wondering if anyone out there has accurate formulas for
>attenuation and do my
>calculations seem reasonable.
>
>Thanks
>
>Craig Callen
>ccallen@ibnets.com
>
>
>
>
>
_________________________________________________
Dr. Howard Johnson, Signal Consulting, Inc.
tel 425.556.0800 fax 425.881.6149 email howiej@sigcon.com

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