Flat-Passband Substrate Integrated Waveguide Filter With Resistive Couplings

Abstract

Abstract-A lossy filter with resistive coupling is proposed based on substrate integrated waveguide (SIW) resonators, where nonresonating nodes are not required to simplify the realization. The sensitivity analysis of S-parameter to the resistive coupling coefficient is carried out to determine the parameters of coupling structure and mounted resistors. When resistive couplings are added to the structure, the measured 0.2-dB passband bandwidth increases from 198 to 256 MHz, compared with the case without resistive couplings. At a sacrifice on the additional insertion loss of 1.1 dB, the passband flatness and selectivity are improved significantly. The lossy SIW filter can provide a smaller in-band insertion loss than the microstrip counterparts, because the unloaded Q-factor of SIW resonators is higher than that of microstrip resonators. Moreover, a simpler topology and a less insertion loss are obtained in the proposed resistively coupled SIW filter than those of the lossy filter synthesized with lossy coupling matrix. Excellent agreement between the simulated and measured results is achieved to demonstrate our idea.

A Microstrip Lossy Diplexer With Flat Channel Passbands

“…The resistors R i (i = 1, 2, 3, 4) represent the resistance cross coupling between resonators. For each channel filter, a resistive cross coupling configuration similar to the one in [15] was used. This configuration is an effective and one of the simplest configurations to implement using microstrip resonators with uniform Q-factors.…”

“…Assuming the lower and higher channel filters are symmetrical, the resistive cross coupling coefficients are led to m 13 = m 24 , and m 57 = m 68 . The coupling matrix of the lower channel can be expressed as The resistive coupling coefficients m 13 and m 57 are obtained by using the approach from [15,20], and they are optimized to be m 13 = 0.053 and m 57 = 0.039. Accordingly, the coupling matrices of the lower and higher channels can be given as Its normalized external quality factors are identical with the reference diplexer.…”

“…The resistive cross couplings are realized by surface mounted resistors placed between two sections of thin high-impedance lines, whose initial lengths are determined to be quarter of a wavelength at the respective channel center frequencies. The resistor value was first estimated from the resistive coupling, m res , by [15]…”

“…Fortunately, this additional insertion loss can be readily compensated for by amplification in many applications. It is worth noting that 'lossy filter' is an established terminology to describe the type of filters that achieve flatter insertion loss over the passband using resonators with ununiform quality factors [12,13] or by deliberately introducing lossy elements such as coupling resistors [14,15]. This terminology has been used to distinguish from conventional filters with inherent finite quality factors (and therefore losses) without any extra resistive elements.…”