"JOURNAL OF RADIO ELECTRONICS"  N 5, 2014

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INVESTIGATION OF COMPLEX TRANSMISSION ZEROES ARISING AFTER INTRODUCTION OF ADDITIONAL NON-ADJACENT COUPLINGS BETWEEN CAVITIES OF A FILTER

┼. I. Lavretski
╬└╬
ź╠╩B Kompas╗

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.