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S  M  T  W  T  F  S 





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From: Stefan Jahn <stefan@gr...>  20070314 20:18:21

Am Mi, 14.03.2007, 21:04, schrieb walter steffè: > Hi Stefan Hello Walter, >> The Sparameter port is different from what you used (you used a >> succircuit >> port). You need the "power source" from components/sources. In the >> properties there is a Z to define. > > Ok now I have used the power source > >> >> When you press F2 (simulate) then you will get the Sparameters of >> the circuit normalized to the ones which you specified. >> >> BTW: When you press in your schematic F2, then the simulator complains >> about a missing "Pac" which is the power source I mentioned. >> > > And finally I got the S parameter plot. > > But there are some things I would like to change. > > 1) First of all the S parameter is a complex number and I should be able > to > select between its amplitude and phase while it seems that qucs plots only > the amplitude. > > 2) Usually the amplitudes of S parameters are plotted in dB > (S_dB=20 log10(S). > The plot properties let me to specify a logaritmic scale which is not > exactly > the same as a dB scale and it seems that it does'nt work. Add equations: dB_S21=dB(mag(S[2,1])) ph_S21=phase(S[2,1]) Then you can plot dB_S21 and ph_S21... >> You don't need to renormalize, since Sparameter simulation can do >> it one its own. > > Great . Then the only thing I would like to have are two additional > parameters > in the power source. These parameter are the cut off frequency fc and a > flag that selects between TE and TM modes. The port impedance (used in > the > normalization) has to be redefined as: > > Zc(f) = Zinf f / sqrt(f^2fc^2) for TE ports > Zc(f) = Zinf sqrt(f^2fc^2) / f for TM ports > > You can easely see that Zinf is the limit of both expression when the freq > f > goes to the infinite. > > If you put fc=0 you can see that both expressions reduce to Zc(f)=Zinf > =cost > and this condition corresponds with the TEM case. Hm. Ok for this we would need 'frequency' in equations. I put it on the TODO list. With the new release you will be able to write: freq=1e9 fc=2e8 Zinf=75 Zc=Zinf * freq / sqrt(abs(freq^2  fc^2)) Then you can put Zc into the "Z" property of the power source. Also you will be able to define a subcircuit with freq, fc and Zinf as parameters... Cheers, Stefan. 
From: Stefan Jahn <stefan@gr...>  20070314 17:20:59

Am Mi, 14.03.2007, 08:34, schrieb walter steffè: > Hello Stefan Hi Walter, >> Have you already looked at the QucsTranscalc tool? There is synthesis >> and analysis for rectangular waveguide available. It also tells if >> there >> occur beside the TEM mode any TE or TM modes. > > Not yet. I will look at it Ok, good luck. >> When creating a waveguide component we can probably easily make a good >> model for the TEM mode. When higher order TE or TM modes occur it gets >> perhaps difficult because each mode has different Zl and gamma. > >> > The only problem I see is that the S parameters I have spoken about >> are >> > those >> > that are normalized versus the characteristic impedance. This can also >> be >> > written as: Zc_TE=beta/beta_wg, Zc_TM=beta_wg/beta. >> > The rules for the computation of the S matrix normalized versus Zc are >> > similar >> > to those explained in your paper but you must take care that Zc is a >> > function >> > of frequency and that it can be different on the different ports. >> >> I don't yet understand it fully... :( >> >> But the Sparameter simulation results are normalized to the impedance >> given in the Sparameter port (e.g. 50 Ohm). When Zl is different you >> get appropriate reflections, etc., also depending on the frequency. > > To better clarify what I am asking for look on the annexed schematic. > It is a very simple two port circuit. > First thing I am not able to specify the impedance (Zp=50 ohm or anything > else) associated with the ports. I think there is the need of an > additional > type of port (waveguide port) which gives this possibility. The Sparameter port is different from what you used (you used a succircuit port). You need the "power source" from components/sources. In the properties there is a Z to define. When you press F2 (simulate) then you will get the Sparameters of the circuit normalized to the ones which you specified. BTW: When you press in your schematic F2, then the simulator complains about a missing "Pac" which is the power source I mentioned. > When this is > done > a way to proceed is the following: > > 1) Compute the admittance Y (which is a 2x2 matrix) of the 2 port circuit. When you have the simulated Sparameters, then you can add an equation on the schematic: Y=stoy(S) resimulate and then you have the admittance parameters. > 2) Renormalize Y with respect to the ports characteristic impedance Zp > > y = sqrt(Zp) Y sqrt(Zp) > > here Zp and sqrt(Zp) are diagonal matrices > the diag entries of sqrt(Zp) are the square root of Zp entries. > NB the y matrix is dimensionless You don't need to renormalize, since Sparameter simulation can do it one its own. If, whyever you just use e.g. the default 50 Ohm S parameters, you can renormalize then using an equation: S_new=stos(S,100,50) which renormalizes from 50 to 100 Ohms. > 3) Compute the scattering matrix S as > S = (Iy)/(I+y) I is the identical 2x2 matrix This won't be necessary... Hope this helps, Stefan. 