Referring to FR-4.
     
     Does Dual Stripline give you values that are similar to stripline?
     Or should the measured sample be symmetrically balanced stripline?
     With the measurement being ~ at 1 Ghz, can the copper foil topography
     of the signal effect the Er calculated values?
     (i.e., standard EDC vs VLP foils, thick black oxide vs reduced thin 
     oxide? If so, is it substantial?)
     (BTW-I know that the copper foil subject has a major impact with 
     microwave due to capacitance variance)
     
     Groovy

______________________________ Reply Separator _________________________________
Subject: Re: TDR Measured Dielectric Constant
Author:  [log in to unmask] 
Date:    2/27/96 9:15 AM


The TDR is an excellent way to measure dielectric constant.  With a known 
configuration (stripline) and a standard formula for impedance one can easily 
deduce the dielectric constant.  Or more directly, propogation velocity is 
easily measured and converted to dielectric constant.  These methods generally 
give dielectric constant values in the range of 4.2 for standard FR-4.  This 
is lower than the literature values of 4.6 to 4.8,  but the literature values 
are measured using a single frequency at 1 MHz while the TDR measures over a 
range of frequencies that are at or above 1 GHz. In fact, dielectric constant 
values provided by the TDR agree with frequency domain measurements made at 1 
GHz.  It should also be pointed out that real circuits in which impedance is 
an issue generally operate at very high frequency, and the TDR values are much 
more relevant than values at 1 MHz.
     
Nick Paulter of NIST (301-975-2405) in Gaithersburg Md has made an extensive 
study in the use of TDR to measure dielectric constants.  He is working on a 
method which can be used on un-patterned raw material.  I am sure he would be 
interested in discussing his work with interested parties.
     
Bob Holmes Phd
Lucent Technology (Formerly AT&T)
[log in to unmask]
     

------------- Begin Original Message -------------
     
     
Has anyone attempted to use a Time Domain Reflectometer (TDR) to 
     measure the "effective dielectric" constant of internal Stripline and 
     Dualstripline signals?
     
     
     
     Are the calculated dielectric constants close to what the laminate 
     supplier states or are they higher for FR4.
     
Richard Nolan
RGNolan Consulting
607-754-0079