This program is for designing diplexers of both the highpass/lowpass and bandpass/bandstop types.
This interactive program accepts for its inputs the crossover frequency (for the highpass/lowpass topology) or the lower and upper bandedges (for the bandpass/bandstop topology), the order (number of parts), the passband ripple and the system impedance.
On the Plot screen, you can "tune" the crossover or bandedge frequencies, the passband ripple and the order and get instant feedback regarding how those items affect the transmission to the two ports as well as the return loss looking into the common port.
You can print the data plot and the schematic using a very high resolution. Also Diplexer generates schematic files for LTspice.