Crystal filters with extended bandwidths

The properties of a lowpass prototype filter of degree 4, exhibiting equal-ripple behaviour in the passband, are discussed. Explicit solutions are given for the element values of the corresponding crystal bandpass filter, and, from these, it is seen that the attainable bandwidth depends on one parameter only of the lowpass prototype. For the case where a deliberate mismatch is introduced into the lowpass prototype design, there are two possible solutions, one of which is found to lead to designs permitting bandwidths in excess of those obtained from the conventional matched design.