[SI-LIST] Re: Finding Er (Permitivity)
- From: "Juergen Flamm" <jflamm@xxxxxxxxxxx>
- To: <for_si2003@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 18 Jul 2003 14:29:45 -0700
How about a simple approach like using a capacitance meter to measure =
the capacitance between two copper structures on adjacent layers? =
Knowing the capacitance, all it takes now is to determine the =
overlapping geometry area of and distance between the two conductor =
elements to determine the dielectric constant.
If done with care this will give you the low frequency value of Er, =
which should be a good starting point.
=20
Juergen Flamm
-----Original Message-----
From: V S [mailto:for_si2003@xxxxxxxxx]=20
Sent: Friday, July 18, 2003 2:08 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Finding Er (Permitivity)
Hi,
=20
Venu does not have any board detail.=20
=20
If you have a high frequency Scope we can calculate the Er as follows. =
Find a visible PCB track on the top layer ( call it microstrip ). Feed a =
fast rising edge on one end of it and observe the rising edge at the two =
end of known length ( L) of the microstrip. Find the skew between the =
two signal.
=20
the propogation delay in ps of microstrip is roughly
=20
t =3D L * 85 * sqrt ( 0.475 * Er +0.67 ) ps , L is in inch
=20
=20
For example, if L =3D 4 inch
skew t =3D 570 ps
=20
Then , Er =3D [ ( t/ 85*L)^2 - 0.67 ] / 0.475
=20
=3D 4.3
=20
TDR can also be used.
=20
=20
Thanks=20
=20
Vikas Shukla
=20
=20
=20
jeff_latourrette@xxxxxxxxxxx wrote:
Venu:
One other tiny issue with microstrip is that in addition to frequency =
dependence, the effective dielectric constant is also very geometry =
dependent, on width and to a smaller extent the thickness:
epsilon(eff) =3D (epsilon(r)+1)/2 + [(epsilon(r)-1)/2]*f(W/h) - =
[(epsilon(r)-1)/4.6]*(t/h)/sqrt(W/h)
where,
epsilon(eff) =3D effective dielectric constant
epsilon(r) =3D dielectric constant
f =3D frequency
W =3D microstrip line width
h =3D height (of dielectric between ground and line)
t =3D microstrip line thickness
from Bahl & Bhartia "Microwave Solid State Circuit Design"
Basically the wider and thinner the microstrip line, the more of the =
field is contained in the dielectric, beneath the trace and the higher =
the delectric and slower the velocity of propagation. High impedance =
(narrow) microstrip lines have more of their field in the air, which =
also makes them more susceptible to crosstalk.
Cheers,
Jeff LaT
-----Original Message-----
From: Tim Hochberg [mailto:tim.hochberg@xxxxxxxx]
Sent: Friday, July 18, 2003 7:06 AM
To: cclewell@xxxxxxxxxxxxxx; 'Peterson, James F (FL51)';
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Finding Er (Permitivity)
Another issue is that, for microstrip, this is going to give you the=20
effective dielectric constant, not the actual dielectric constant. If=20
you can use stripline instead, this won't be a problem, otherewise your=20
going to need to try to recover the dielectric constant from the=20
effective dielectric constant. This will be a little bit approximate=20
since the effective dielectric constant is frequency dependant. However, =
the dependence is week, and there are effective dielectric calculators=20
floating around on the web, so you should be OK.
Regards,
Tim
Clewell, Craig wrote:
>Jim,
>=20
>I've got a big bat .=20
>=20
>You are correct...sorry about that.
>=20
>Thanks for the clarification.
>=20
>Craig
>=20
>-----Original Message-----
>From: Peterson, James F (FL51) [mailto:james.f.peterson@xxxxxxxxxxxxx]
>Sent: Friday, July 18, 2003 9:25 AM
>To: cclewell@xxxxxxxxxxxxxx; 'kamal_elx@xxxxxxxxxxx'
>Cc: si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: Finding Er (Permitivity)
>
>
>craig,=20
>
>it might be a pretty big ballpark!!=20
>you would have to square the equation to get Er.=20
>
>Vp(m/s)=3D 3e8(m/s)/sqrt(Er)=20
>so=20
>Er =3D (3e8(m/s)/Vp(m/s))^2=20
>
>regards,=20
>Jim=20
>
>-----Original Message-----=20
>From: Clewell, Craig [ mailto:cclewell@xxxxxxxxxxxxxx
> ]=20
>Sent: Friday, July 18, 2003 8:40 AM=20
>To: 'kamal_elx@xxxxxxxxxxx'=20
>Cc: si-list@xxxxxxxxxxxxx=20
>Subject: [SI-LIST] Re: Finding Er (Permitivity)=20
>
>
>Venu,=20
>
>If the board is a Microstrip you can do a quick measurement of the =
delay and
>
>use this value to calculate the E'r.=20
>
>Use E'r =3D 3e8(m/s) / vp(m/s)=20
>
>where vp =3D propagation velocity =3D inverse of the delay. This should =
get you
>
>in the ballpark=20
>
>Regards,=20
>
>Craig=20
>
>-----Original Message-----=20
>From: Kamal EC [ mailto:kamal_elx@xxxxxxxxxxx=20
>]=20
>Sent: Friday, July 18, 2003 7:25 AM=20
>To: si-list@xxxxxxxxxxxxx=20
>Subject: [SI-LIST] Finding Er (Permitivity)=20
>
>
>Hello Everybody,=20
>
>I am having a PCB Board for which I want to calculate the Permitivity. =
I=20
>don't know what material is used for the pcb.=20
>I don't have any details of the board, except the fabricated board.=20
>
>Is there any method by which i can calculate.=20
>
>Regards,=20
>Venu=20
>
>_________________________________________________________________=20
>It's new, it's here! It's full of fun!=20
>http://server1.msn.co.in/sp03/messengerpromo/index.asp
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