Re: Re[4]: [SI-LIST] : Decoupling capacitor selection & plac

Bob Techentin (
Thu, 30 Oct 1997 08:17:51 -0600

On Oct 30, 8:20am, wrote:
> Subject: Re[4]: [SI-LIST] : Decoupling capacitor selection & plac
> The EmCap material has a dielectric constant of ~50. I was
> wondering about the transmission speed as well. I looked at
> what the capacitance is at a given frequency based on an area
> of 1/6th wavelength away (or what area would act like a lumped
> circuit) and I found that the material acted slightly better
> than a 0.1uF or a .001uF 1206 SMT cap impedance-wise.

Hi Todd,

I believe that you will find that a 10"x10" power/ground plane
with a 4mil dielectric with a dielectric constant of 50 will
begin to resonate at about 82MHz. Above that it will be inductive
instead of capacitive.

In our experiments, we have found that we can't treat a small
area of a power/ground system as a lumped circuit element. I
don't think you would get good results analyzing the first
fraction of a wavelength of a traditional transmission line
as a lumped circuit unless the whole structure is electrically
small. The principal appears to hold for power/ground systems.


Bob Techentin                        
Mayo Foundation                                           (507) 284-2702
Rochester MN, 55905 USA