The paper is received on
May 19, 2014
Abstract: Within the scope of an equivalent lumped
circuit of a bandpass filter in the form of synchronously tuned cavities
coupled by mutual inductances, we analytically investigated transmission zeroes
on the complex plane arising after introduction of some additional nonadjacent couplings
between cavities. We considered physically symmetrical structures of filters
with 4,6,8 and 10 cavities. In analysis we looked for the couplings producing
transmission zeroes on the real and imaginary axes of the complex plane. It was
established that a negative coupling between the first cavity and the last one
simultaneously produces the pairs of transmission zeroes on the real and
imaginary axes for the structures of the 6th and 10th orders. For
the structures with 4 and 8 cavities the positive couplings between the first
cavity and the last one produce zeroes only on the real axis of the complex
plane, whereas the negative couplings produce zeroes only on the imaginary
axis. To confirm experimentally the analytical results we designed and
manufactured a cavity filter of 6th order of combline type with central
frequency 2642.5 MHz. In this filter we obtained the elliptic transmission
characteristic and the improved group delay time in the pass-band due to the
introduction of an additional nonadjacent negative coupling between the first
and last cavities.
Key words: a bandpass filter with nonadjacent
couplings, transmission zeroes, group delay time.