[SI-LIST] Re: Finding Er (Permitivity)

  • From: V S <for_si2003@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 18 Jul 2003 14:08:07 -0700 (PDT)

Hi,
 
Venu does not have any board detail. 
 
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.
 
the propogation delay  in ps of microstrip is roughly
 
 t = L * 85 * sqrt ( 0.475 * Er +0.67 )    ps  , L is in inch
 
 
For example, if      L =  4 inch
                    skew   t  =  570 ps
 
Then ,   Er =     [   ( t/ 85*L)^2 - 0.67 ] / 0.475
 
                  =  4.3
 
TDR can also be used.
      
 
Thanks 
 
Vikas Shukla
 
 
 

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) = (epsilon(r)+1)/2 + [(epsilon(r)-1)/2]*f(W/h) - 
[(epsilon(r)-1)/4.6]*(t/h)/sqrt(W/h)

where,

epsilon(eff) = effective dielectric constant
epsilon(r) = dielectric constant
f = frequency
W = microstrip line width
h = height (of dielectric between ground and line)
t = 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 
effective dielectric constant, not the actual dielectric constant. If 
you can use stripline instead, this won't be a problem, otherewise your 
going to need to try to recover the dielectric constant from the 
effective dielectric constant. This will be a little bit approximate 
since the effective dielectric constant is frequency dependant. However, 
the dependence is week, and there are effective dielectric calculators 
floating around on the web, so you should be OK.

Regards,
Tim


Clewell, Craig wrote:

>Jim,
> 
>I've got a big bat . 
> 
>You are correct...sorry about that.
> 
>Thanks for the clarification.
> 
>Craig
> 
>-----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, 
>
>it might be a pretty big ballpark!! 
>you would have to square the equation to get Er. 
>
>Vp(m/s)= 3e8(m/s)/sqrt(Er) 
>so 
>Er = (3e8(m/s)/Vp(m/s))^2 
>
>regards, 
>Jim 
>
>-----Original Message----- 
>From: Clewell, Craig [ mailto:cclewell@xxxxxxxxxxxxxx
> ] 
>Sent: Friday, July 18, 2003 8:40 AM 
>To: 'kamal_elx@xxxxxxxxxxx' 
>Cc: si-list@xxxxxxxxxxxxx 
>Subject: [SI-LIST] Re: Finding Er (Permitivity) 
>
>
>Venu, 
>
>If the board is a Microstrip you can do a quick measurement of the delay and
>
>use this value to calculate the E'r. 
>
>Use E'r = 3e8(m/s) / vp(m/s) 
>
>where vp = propagation velocity = inverse of the delay. This should get you
>
>in the ballpark 
>
>Regards, 
>
>Craig 
>
>-----Original Message----- 
>From: Kamal EC [ mailto:kamal_elx@xxxxxxxxxxx 
>] 
>Sent: Friday, July 18, 2003 7:25 AM 
>To: si-list@xxxxxxxxxxxxx 
>Subject: [SI-LIST] Finding Er (Permitivity) 
>
>
>Hello Everybody, 
>
>I am having a PCB Board for which I want to calculate the Permitivity. I 
>don't know what material is used for the pcb. 
>I don't have any details of the board, except the fabricated board. 
>
>Is there any method by which i can calculate. 
>
>Regards, 
>Venu 
>
>_________________________________________________________________ 
>It's new, it's here! It's full of fun! 
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