Using Linecalc

Linecalc is a useful utility that is bundled with Keysight ADS, and is used to estimate the performance of pcb transmission lines.  To demonstrate what it can do, let’s look at an example using an FR4 substrate. For this example we use a typical layer stack of four layers with 0.22mm between layer 1 (signal trace) and layer 2 (ground plane).  We shall define the input parameters as follows:

  • Er = substrate relative dielectric constant = 4.5
  • Mur = relative permeability = 1
  • H = substrate thickness = 0.22mm
  • Hu = upper ground plane to substrate spacing = a big number
  • T = metal thickness = 35um
  • Cond = metal conductance = 4.1e7
  • TanD = dielectric loss tangent = 0.022
  • Rough = rms surface roughness = 0 (but can use 0.7um for microwave sims)
  • Freq = frequency for calculation = 1.6GHz
  • W = track width = 0.42mm
  • L = track length = 10mm

Clicking on “Analyze” reveals the following estimated characteristics of the microstrip

  • Z0 = characteristic impedance = 47.6 Ohms
  • E_Eff = effective electrical length of line = 34.7 degrees
  • K_Eff = effective dielectric constant = 3.264
  • A_DB = total attenuation in dB = 0.076
  • SkinDepth = 0.077

As a first approximation the design looks reasonably OK.  To find the exact track width for 50 Ohms, we can enter Z0 = 50 and click “Synthesize”