It is also possible to define a function of w (meaning omega) to produce the values needed. For example, below the transfer function magnitude in dB is calculated:

tf = 20/(20 + s)

(* 20/(20+s) *)

tfw = tf /. s -> I w

(* 20/(20+\[ImaginaryI] w) *)

LogLinearPlot[20 Log10[Abs[tfw]], {w, 1, 1000}, Frame -> True, 
 GridLines -> Automatic

dBOut[w_] = 20 Log10[Abs[tfw]] // N;

outputTable = Table[{w, dBOut[w]}, {w, {1, 5, 10, 50, 100, 500}}]

(* {{1,-0.010843812922199395`},{5,-0.2632893872234916`},{10,-0.\
9691001300805643`},{50,-8.603380065709935`},{100,-14.149733479708177`}\
,{500,-27.965743332104292`}} *)

Hi Lucas,

The x axis is logarithmic, so to get the actual values:

points = Cases[bpx, Line[pts_] -> pts, Infinity] // MapAt[(10^# &), #, {All, All, 1}] &;
points // 
  Map[ListLogLinearPlot[#, ImageSize -> Medium, Joined -> True, PlotRange -> All] &] //
  Row