7.13 Gradual excitation turning off
QuickWave enables feature, which allows modifying internal impedance of point sources. This feature is called Z Changing and is available in QW-Simulator in Run tab and Run main menu. Invoking Z Changing affects simultaneously all Point Sources in the circuit. This feature operates in a similar way to the disconnection stage of dynamic template generation (see also User Guide 3D: First insight template generation - low-order waveguide modes) and is very useful for generating unperturbed eigenmodes in resonators. The background theory has been explained in publication [26]. In short, it should be noted that an auxiliary point source connected to the resonator introduces local disturbance of the fields, due to the injected current. This disturbance can be physically identified as a non-propagating potential mode. In the case of sinusoidal excitation at a resonant frequency, the physical resonant mode is reconstructed by QW-Simulator, but a local disturbance is seen on the field display as a ripple that may be quite large if the source is weakly coupled to the resonator. Note that in such cases, this ripple will also strongly contaminate the integration of power and energy, and Q-factor extraction. To eliminate the disturbance we need to disconnect the lumped source from the circuit, and this can be achieved by gradually increasing its output impedance. On the other hand, by decreasing the output impedance we increase the injected current, and "pump" more energy into the circuit. Thus, these two actions are reversible. We have two ways of performing them:
automatic (Automatic Z Changing) – in this option the user can specify that the value of the output impedance should be multiplied by a factor of K, at each of the consecutive N iterations. Thus after N iterations the impedance will be scaled by a factor of X=KN. To make the source practically ineffective, we should usually increase the impedance at least by a factor of X=1000. The value of N should be big enough to avoid perturbations due to strong transients. Depending on the meshing and time step, we typically choose N=1000...10000. If we set too small K, the ripple will still be floating over the modal field pattern, and we should increase the impedance further. If we set too small N, rapid source disconnection will leave a stationary ripple (staying all the time above or below the field pattern). We should then decrease back the impedance (by invoking the same dialogue and activating the check for impedance decrease box), and then try to increase it again over a longer period. Due to the reversibility of the increase and decrease operations, proper values of K and N can be chosen by experiment.
manual – in this option the user invokes Increase Z or Decrease Z at a particular time instant, depending on what action is required. Upon this action, the output impedance will be multiplied/divided by a factor of 1.01.