  Coupled-resonator microstrip filters are among the most versatile filter
topologies. A known design approach uses full-wave electromagnetic simulations
to determine the coupling coefficient between resonators as a function of their
relative position. This could be done using time-domain simulations using a
fast Fourier transform (FFT) to extract the couplings from the S parameters
obtained from time-domain signals. However, this approach has a poor
performance in terms of resolution and specially for weak couplings, leading to
unreasonably long simulation times. To overcome this, we introduce a technique
to obtain the couplings directly from time signals, without moving to the
frequency domain. This procedure works for strong and weak couplings, with much
shorter simulation times and a reduced simulation domain over the FFT approach.
This technique is used to design coupled resonator filter efficiently from time
domain simulations. We implement this procedure using the finite-difference
time-domain framework using an open source solver and discuss our
implementation. We show that its results are very similar to previously
published ones obtained from frequency domain simulations, even in the case of
very weak couplings. Finally, we design and measure a filter to show the good
performance of the proposed approach.
