Barry, I would be careful with taking your discussion below to heart. Your equation for frequency is actually a Bandwidth equation. Granted, if your calculating the HIGHEST frequency of the bandwidth, that's fine. But, frequency does play a significant role in the "impedance" that one wants. Ask anyone dealing with board constructions with FR4 above 800 MHz. Also, your assumption is that all the frequencies required for the transition time are in sync with one another (from Fourier). Bad assumption. Group delays and shifts in phase based on frequency are only part of what will determine the actual outcome. At best, you'll get a smeared edge at fast edge rates. When board construction for controlled impedance is most important, there's a whole other side to controlled impedance that can get nasty. doug ______________________________ Reply Separator _________________________________ Subject: Re: Controlled Impedance - Dielectric Constant- Comment Author: [log in to unmask] at internet-mail Date: 5/21/96 10:39 PM Hi Luis The dielectric constant of soldermask is slightly different to that of FR4 however in practice the variation is neglibible. (-1.55*H'/H) >From IPC-D-317 e=er[1-e ] I also noticed from the discussion that it has been mentioned that the frequency of operation will determine the impedance of the substrate. That is not exactly correct as it is in fact the rise time of the leading edge of the pulse that is the influencing factor. f=0.35/Tr where f is in GHz Tr is in nS At the end of the day what I do is just specify in the PCB Spec that the trace widths and prepreq thickness may be varied to obtain the desired impedance. And really who cares what material is used in the board manufacture as long as the impedance is what we want! Cheers, Barry Olney, In-circuit Design, Australia Luis Rivera wrote: > > Bob, > > You are correct, the dielectric constant of a material does not change > between microstrip, embedded microstrip or stripline configurations. > > Avner, > > Some general words of caution: > > Dielectric constant does change depending on resin and reinforcement > type, resin content, frequency, temperature and humidity. Typically, > dielectric constant decreases as frequency increases. > > On microstrips, the fact that there is air on one side of the > transmission line lowers the effective dielectric constant. Solder > mask coatings counteract this effect to some degree. > > Please call off-line if you would like to discuss further. > > Luis Rivera > [log in to unmask] > CompuRoute, Inc. > Dallas, TX > (214) 340-0543 > > > > > > > Avner-- > > If I am wrong, I hope someone out there will make me smarter, but > > the dielectric "CONSTANT" is just that--constant for that type of material. > > IMOP, when calculating impedance of microstrip or stripline, the dielectric > > constant of a material doesn't change, less the material's unique tolerance > > range REF: IPC-D-275 Table 3-2. Therefore, unless proven wrong, the constant > > DOES NOT change, but the impedance value WILL vary depending on which > > formula you are using depending on you pwb construction. Hope that helps. > > If wrong, please correct me via E-mail, > > > > Bob Vanech > > E-Mail : [log in to unmask] > > > > > > -- ___________________________________________________ Barry Olney Managing Director In-Circuit Design Pty Ltd Ph: +61 3 9205 9595 VeriBest Solutions Centre Fax:+61 3 9205 9410 Suite 211, Princess Tower Mbl:+61 4 1117 0827 1 Princess St, Kew, VIC 3101, Australia Email:[log in to unmask] ___________________________________________________