Unless some really tight accuracy is required for the impedance measurem= ent, which I doubt, just use a TDR and be done with it. Is all this fre= quency domain tap dancing driven by only having a VNA to work with? My = condolences if that is the case. Sal Hi Cody, I don't think your fundamental question, "Can anyone recommend a = different method for measuring characteristic impedance over frequency?"= = was ever answered, but first I think it would be instructive to = understand the limitations of the method you are using. The basis for the equation: |Zo| =3D sqrt(|Zopen|*|Zshort|) is that for the special case of an open load, the impedance at the = source or generator end of the transmission line is: Zopen =3D {g(el)*[Go(w) + jwCo]}^-1 Similarly, for a shorted load, the impedance is: Zshort =3D g(el)*[Ro(w) + jwLo] where j is the sqrt(-1), w is radian frequency, Ro(w) represents ohmic = losses (both dc and ac) in the conductor, Go(w) represents dielectric = losses, Lo & Co are respectively the intrinsic inductance of the = transmission line in units of H/m and the intrinsic capacitance in F/m, = = and g(el) is a function, the argument of which is the electrical length = = (el) of the line. Note that el is itself a function of w, as well as = the relative permittivity of the transmission dielectric. The above = assumes the absence of parasitics that often result with a short or = open, and an el less than a quarter wavelength. It also ignores the = interconnect between the VNA and the line under test. The most severe limitation of this method comes about from the fact that= = as el increases and approaches a quarter wavelength, g(el) approaches = infinity. This happens for all odd multiples of 1/4 lamda with = increasing el. At 1/4 lamda, g(el) has a discontinuity as it changes = sign. With el increasing and approaching an even multiple of 1/4 lamda,= = g(el) becomes less negative and approaches zero. Mathematically, in the= = limit, the above equation for Zo still holds, but achieving accurate = measurements and accurately computing the product of a very small number= = and a very large number when el is near a 1/4 lamda becomes difficult. = This leads to the "resonances" you observed. The above method will = yield accurate results as long as measurements are restricted to the = frequency ranges that are sufficiently removed from 1/4 lamda = ("sufficiently" will depend on the measurement system and the line under= = test). Obviously, this shortcoming can be avoided if the electrical = length of the line is such that it is sufficiently shorter than 1/4 = lamda at the highest measured frequency, but this may not be an option = for you. For those skilled in the use of the Smith Chart, the above is fairly = intuitive. For anyone who would like to learn about the Smith Chart, = there is an upcoming free webinar given by Les Besser. You can find out= = more about and sign up for the webinar at: http://www.besserassociates.com/webSpecials.htm Click on the "FREE tutorial webinars" link at the top of the page. As for measurement alternatives for the characteristic impedance over = frequency, it is possible to use a TDR and software like Tektronix = iConnect to derive S11 as a function of frequency from time domain = measurements. However, this method can also have limitations in that = sometimes the s-parameters the FFT algorithm produces are not causal or = = yield passive gain. Best regards, -Bill /************************************ / William C. Wurst, PE / / billw@xxxxxxxxxxx / / Advanced Electronic Concepts, LLC / / www.aec-lab.com / ************************************ =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Peter J wrote: > Hi Cody, > It is normal to get those resonaces when you measure an open stub like= that. > If you calibrate and do the measurement accurately and take the formul= a > (vektor multiplication) |Zo| =3Dsqrt(|Zopen|*|Zshort|) you get the Z0= over > the frequency VNA frequency range. When you calibrate be sure that you= > de-embedded fixture, koax/board transition, or what ever means you hav= e > connected your test object. > = > BR > Homer > = > 2007/9/5, tao xu <helen.tao.xu@xxxxxxxxx>: > = >>Hi, Cody >>my first thought is that you can make the trace short to avoide the >>resonance appearing in your intrested frequency range. But please make= >>sure >>length is till much bigger than width. >> >>thanks and regards >>Helen >> >> >>On 9/5/07, codymiller@xxxxxxxxxx <codymiller@xxxxxxxxxx> wrote: >> >>>All, >>> >>>I am trying to measure the characteristic impedance of a strip line >>>trace, using a VNA. I would like to determine the frequency at which = the >>>trace becomes very lossey. I am using a formula found in the Agilent >>>Impedance Measurement Handbook page 5-23. >>>http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf >>> >>>The method requires S11 of the transmission line open as well as S11 >>>with it shorted. >>> >>>|Zo| =3D3D sqrt(|Zopen|*|Zshort|) >>> >>>The method seems to work except I get some resonance at different >>>frequencies where the impedance increases significantly. The frequenc= ies >>>of these resonants changes with different lengths of PCB trace. >>> >>>Can anyone recommend a different method for measuring characteristic >>>impedance over frequency? >>> >>>Thanks, >>>Cody Miller >>>codymiller@xxxxxxxxxx _____________________________________________________________ Be your own boss today! Go to Technical School. 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