When choosing the variables to plot, we will use those with the suffix.v, as these correspond to the ac analysis that we have carried out. To be able to plot the amplification and phase shift of the circuit, we will define some additional variables, Vb mag and Vb ph, inside a second Equation block: To view the results, we will add some cartesian plots to our output tab. To plot a frequency response in Qucs we have to insert a specific source, an ac Voltage Source, and insert a specific kind of simulation, an ac Simulation. 2ģ 2 Frequency Response of a Simple Transmission Line Circuit The frequency response gives a lot of insight into a circuit s behavior. If we simulate this design, we will see that a 1 m line introduces a 5 ns delay, as corresponds to a propagation velocity of 0.66c. The equivalent length is inserted into the length property of the transmission line component. For instance, to model a lossless RG-58 coaxial line, we need a velocity factor of To achieve this, we may insert an equation item into our schematic sheet and add some equations to it: Now, if we wanted to change the physical length of the line, we would change truel. While there are other kinds of transmission line in Qucs, we will try to include an arbitrary propagation factor. It follows that Qucs is assuming that the transmission line uses air as the dielectric. After the box is inserted, we may navigate the waveform with the cursors to locate the time instant when the step is detected at the output t = Since the line length is 1 m, it follows that the propagation velocity along the line is V p = 1/ = m/s. If everything worked ok, we should see something like: The marker on the graph is inserted with or through the insert menu. Suffix V means voltage and suffix t means time-domain data. The prefix of these signals corresponds to the node name. 1Ģ We will select the traces named V1.Vt and V2.Vt. In any case, we get an empty results page where we will insert a Cartesian plot from the diagrams palette. The simulation is started with or through the menu. These will be named V1 and V2, corresponding to the input and output port of the transmission line, respectively. Finally, we will attach markers to the nodes that we will like to watch. The most important point is the final time and the time step or, as in the figure, the number of time points where the response will be computed. In this case, a transient simulation with the parameters adjusted as shown. When setting the line length do not write m for the unit of length, as m is considered mili! After placing the load resistor, we only have to insert a simulation. We choose the generic transmission line component for the transmission line. Hence, we have set the pulse width to 1 ms, which is several orders of magnitude longer than our simulation will last. In this case, we want to investigate the response to a step function.

Next, adjust the parameters of the source as depicted in the figure. This will be our first circuit, which will be built step by step: To start, go to the sources tab and select a Voltage Pulse source. 1 A Qucs Tutorial for RF Transmission Lines Pere Palà-Schönwälder February Simple Transmission Line in the Time Domain Let s start with a simple circuit composed of a voltage generator, a transmission line and a load.

Qucs frequency sweep tutorial