[SI-LIST] : Re: attenuation calculations

Juliusz Poltz ([email protected])
Thu, 23 Apr 1998 19:31:42 +0000

Thursday, April 23, 1998

Hello Craig,

There are few open questions about your geometry. However, assuming
that the frequency is 3 GHz, and your geometry is 1/2 oz Cu (0.7 mils)
where FR-4 has 4.7 dielectric constant and a loss tangent of 0.018,
our solver calculates the dielectric thickness (for a microstrip)
equal to 3.415 mils. For this geometry, we calculate per inch:

R=2.1234 Ohms
L=7.8405 nH
C=3.136 pF
G=911.52 uS

which could be translated into 0.3827 dB/in attenuation for 3 GHz. As
you can see the resistance is twice your predicted value. However,
our solver calculates the current distribution in the ground plane
(which I assumed to be also 1/2 oz Cu) and provides the resistance of
the entire current loop (signal + return).

Sincerely yours


> ------- Forwarded Message Follows -------
> Date: Thu, 23 Apr 1998 18:11:52 +0100
> From: Craig Callen <[email protected]>
> To: "[email protected]"
> <[email protected]> Subject: [SI-LIST] : attenuation
> calculations
> 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
> [email protected]

"Juliusz Poltz" <[email protected]>
OptEM Engineering Inc. Ph: (403) 289-0499
100 Discovery Place Fax: (403) 282-1238
3553 - 31st Street N.W. Email: [email protected]
Calgary, AB Canada T2L 2K7