Hello all! I am trying to use the LMG tool to model a simple 100ohm, 30cm FR4 PCB microstrip line, but I fear am obtaining strange results. For example, using the tool with (exactly) the following entries... Model type: Lossy, wideband Transmission line type: SubLossLine No. Layers: 2, No. Lines: 1, No. GndPlates: 1 Dielectric constant (er): 4.1 1 Dielectric thickness (d): 800 um Dielectric loss type: tan=sigma/(w*ep0) Dielectric loss tangent: 0.018 0.0 Conductor width: 843 um Conductor thickness: 35 um Conductor height (h): 800 um Ground plate thickness: 20 um Ground plate conductivity: 5.6e7 S/m Signal line conductivity: 5.6e7 S/m Conductor length: 30000 um Fmax: 20 GHz ....I generate a file "my_model.scs" which I later link, in a testbench schematic, to an instance of the "mtline" component in analoglib (using the option "use lmg subckt" on the componet's CDF). The testbench contains just an "vsin" generator feeding the mtline at the near-end, a termination resistor of 100ohm at the far-end, and a ground pin. When I perform an AC sweep of this testbench up to 20GHz and plot the effective impedance seen at the input port of the transmission line and the attenuation at the far end, I seem to get strange results: 1. The effective input impedance of the line increases greatly with frequency (almost linearly after 1GHz), and rises to very high values. For instance, it reads 763ohm at 10GHz and 1531ohm at 20GHz! 2. The attenuation seems to be too high at high frequencies. For instances, the -3dB point is 1.318GHz and at 10GHz it is -17.6dB! It approximately falls with -20dB/dec after 2GHz. What further makes me unsure about these results is the fact that when press the button "Generate Macromodel" in LMG, it outputs a message in the console that reads: "For substrate loss case, widebandwidth model generation may not be accurate, you can use mtline to check it". I really need a wideband model, because I intend to model the transmission of pulses along this line and not just pure sinusoids. According to the documentation, the model should be valid from DC to the specified frequency, 20GHz in this example. I am also clueless about how to use the .dat LRGC file that is also generated. What are the differences between using the .scs model and the LRGC data file? When should I use each one? The examples included in the documentation seem outdated and talk about components which doesn''t seem to exist anymore in the analoglib, like "tline3" and "mline" :S I would be really grateful if anybody could comment on the correctness of these results, indicate the correct usage of the .scs models and/or LRGC datafiles, or point out any other resource for transmission line modeling that I could use for cross-checking the models produced by the LMG tool. Thanks in advance for any help! ...and sorry for the long post! :) Regards, Jorge.
