Two different bandpass filter structures were evaluated for achieving good harmonic suppression at microwave frequencies.
Harmonic filters are invaluable for removing unwanted higher-order harmonic signals from microwave multipliers and mixers in receiver designs, among other applications. Although a variety of filter configurations have been developed to reduce the level of harmonic signals, the authors investigated two varieties of bandpass filter designed for good harmonic suppression. Parallel coupled line sections were employed in both filter approaches, with phase equalization of coupled resonators employed to improve harmonic suppression. The filters are designed for center frequencies around 1.25 GHz.
Traditionally, parallel-coupled microstrip filters suffer from spurious responses located at twice the fundamental frequency. These spurious responses cause response asymmetry in the upper and lower stop bands. Another major limitation of parallel-coupled microstrip filters comes from the weak lateral coupling between lines in a conventional structure. This mandates small values of strip width and strip spacing in order to achieve tight coupling and desired filter response. Such small dimensions are difficult to fabricate accurately and, hence, suffer from repeatability. These limitations are due to the inhomogeneous nature of microstrip lines. The inhomogeneity of microstrip lines results in unequal even-and odd-mode phase velocities, a condition often rendered worse by manufacturing irregularities in the fabrication process.
Fortunately, these limitations can be overcome by either providing different line lengths for even and odd modes or equalizing the modal phase velocities. It is well known that the addition of a short uncoupled line section at either end of a coupled line section can result in improved filter characteristics. 1 Structures so far reported in the literature based on this filter design approach include wiggly-line filters, uniplanar coupled bandgap filters, corrugated coupled microstrip-line filters, meandered parallel coupled-line filters, cascaded band-reject filters, and split-ring filters. 2- 7 Unfortunately, these structures either increase the circuit complexity by recalculating the design parameters or require unrealistically tight fabrication tolerances to be effective.
The authors worked with the concept of phase equalization using a topology that is a combination of an edge-coupled and hairpin-line filter. For this design, the electromagnetic (EM) energy for the odd mode gathers around the outer metallic edges' conductors. 2 To achieve equalization, the odd-mode path can be extended, or the even-mode path can be shortened. The filter topology presented in Fig. 1 employs the concept of phase equalization. There are two coupling mechanism involved in this configuration, namely electric and magnetic coupling. The controllable transmission zeros are tuned by means of controlling these coupling mechanisms. By properly adjusting these coupling mechanisms, first-harmonic suppression of better than 30 dB is possible (Fig. 2). Unfortunately, higher-order harmonic signals are prominent and cannot be eliminated with this topology.
Figure 3 shows a second filter topology designed for wideband suppression of harmonic signals. In this topology, the concept of coupling between the resonators has been modified with the incorporation of the ground plane. This results in controllable transmission zeros at the harmonic frequencies, yielding high wideband suppression. Also, the undesired evanescent mode has been eliminated at the discontinuity by means of grounding pads. This filter topology can be further tuned by varying the substrate thickness. Figure 4 shows simulated and measured results for this filter topology.
Advanced Design System (ADS) simulation software from Agilent Technologies (www.agilent.com) was used to simulate harmonic bandpass filters designed for center frequency of 1.25 GHz, using EM simulation tools to predict the performance of the filter structure. 8 The simulated results are further verified using EM simulation tools from Linmic (www.linmic.com). Actual filters were fabricated on RF Duroid 6010 polytetrafluoroethylene (PTFE) circuit-board material from Rogers Corp. (www.rogers-corp.com). The dielectric substrate material has a thickness of 50 mils and dielectric constant of 10.2. Fabricated filters were measured with a model ZVK vector network analyzer (10 MHz to 20 GHz) from Rohde & Schwarz (www.rohdeschwarz.com). The measured return loss is better than 10 dB with the bandwidth of around 10 percent. The skirt rate is controlled by reallocation of transmission zeros.
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