This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Microstrip edge-coupled bandpass filters (BPFs) can help clean the spectrum around a desired center frequency. Fabricated on printed-circuit-board (PCB) materials, these compact filters can be integrated with other circuit functions to provide dependable filtering of communications bands and high-frequency signals for a wide range of applications. Microstrip edge-coupled BPFs can be quickly designed for a bandwidth of interest with the aid of modern computer-aided-design (CAD) circuit-design software tools, and reliably fabricated on RF/microwave PCB materials to achieve impressive performance levels. Unfortunately, even the most carefully designed edge-coupled BPFs can suffer the effects of spurious harmonic responses, resulting from even-order harmonics of the filter’s center frequency. But there is a way to control these harmonic responses, and it relies on combining a PCB laminate and a PCB bondply material to form a simple composite material structure with dramatically improved harmonic performance compared to single-PCB-material designs.

The composite PCB material is formed by combining RT/duroid® 6010.2LM laminate with the versatile 2929 bondply material, both from Rogers Corp. The dielectric layers are clad with copper metallization which is used to form the transmission lines for filters and other microstrip circuits. RT/duroid 6010.2LM circuit material has a high relative dielectric constant of 10.7 in the z-axis (thickness) of the material when measured at 10 GHz. This high-performance PCB building-block material is “fine-tuned” in terms of harmonic filter performance by adding the “glue,” 2929 bonply material. This is an unreinforced thin-film adhesive material nominally developed for bonding together the layers of multilayer PCBs. However, as found with experiments on edge-coupled BPFs, this is more than just a multilayer adhesive system: it is extremely useful in tailoring the harmonic responses of high-frequency filter circuits. The 2929 bondply material exhibits a much lower relative dielectric constant than the RT/duroid 6010.2LM circuit material, at 2.94 in the z-axis measured at 10 GHz. It features low loss, with an extremely low dissipation factor of 0.003 in the z-axis at 10 GHz. The 2929 bondply material is available in different sheet thicknesses and layers can be stacked for applications requiring increased thicknesses.

Edge-coupled BPFs provide effective high-frequency filtering functions in microstrip circuits but may exhibit spurious harmonic responses, which may interfere with an application’s desired performance. Numerous methods have been developed to suppress unwanted harmonic responses, most of which are based on equalizing the phase velocities of even- and odd-mode propagation characteristics of a PCB. Various corrective approaches have been applied, including the use of suspended microstrip circuit patterns, adding capacitors, or adding circuit patterns to make phase velocity adjustments, but all of these approaches add to the complexity of the initial filter circuit. The combination of RT/duroid® 6010.2LM  circuit material and 2929 bondply materialmakes use of a fairly straightforward suspended microstrip technique and is inexpensive and does not sacrifice reliability.

Prior to designing different filter circuit patterns for fabrication on the composite and on standard circuit material for comparison, the materials were studied to better understand the effects of combining the dielectric materials. Commercial electromagnetic (EM) simulation software from Sonnet Software was employed to model the even- and odd-mode effective dielectric constant of each of the materials used for fabricating the edge-coupled BPFs. This is a three-dimensional (3D) planar EM simulator for predicting the behavior of predominantly planar circuits, such as microstrip, stripline, and coplanar-waveguide (CPW) circuits, and the Professional version of the software, revision 14.52, was used in the filter studies. Following design and simulation, different filters were fabricated on the different material options, and fabricated filters were characterized with the help of a model E8364C programmable vector network analyzer (VNA) from Keysight Technologies. The analyzer was prepared for the measurements by means of a two-port, 12-term short-open-load-through (SOLT) calibration.

To better understand the effects of using a composite material on the harmonic responses of an edge-coupled microstrip BPF, four filters were modeled on the Sonnet software and then fabricated for analysis on the two different circuit materials, the standard material and the combination of RT/duroid® 6010.2LM  and 2929 bondply material. Two different filter circuits were used, based on a Chebyshev transfer function with center frequency at 2.5 GHz and bandwidth of either 180 or 440 MHz.

The measured results for the different filters were remarkably close to the responses predicted by the Sonnet Software simulations, with significant suppression of second-harmonic responses achieved for both filters fabricated on the composite circuit material compared to the standard circuit material.

In all cases, the fabricated filters were characterized with the model E8364C analyzer across a measurement range from 10 MHz to 6 GHz for full analysis of the different filters’ upper and lower sidebands. For both filter bandwidths, significant spurious responses appearing in the upper sidebands of the BPFs fabricated on the standard circuit material were essentially eliminated when those same filter circuits were fabricated on the composite material.

These filter circuits demonstrate one benefit of using composite circuit materials to fine-tune a circuit response. Composite circuit materials such as the combination of RT/duroid® 6010.2LM and 2929 bondply materials also exhibit differences in the coupling behavior of resonators, transmission lines, and other microstrip circuit structures, making it possible to fine-tune the harmonic responses of different circuit designs by exploring the effects of composite materials with a circuit simulation program such as Sonnet Software. In addition, many modern circuit designs employ multilayer circuit constructions, with general-purpose materials, such as FR-4, used for many standard analog and digital functions, and higher-performance materials based on polytetrafluoroethylene (PTFE) for RF/microwave and higher-speed digital circuits. In addition to providing a reliable bond between circuit layers, a versatile circuit adhesive such as 2929 bondply material can be included in the circuit modeling stage to fine-tune circuit responses, such as second-harmonic responses, as needed.

Screen shot 2014-08-08 at 1.33.54 PMAuthor’s Note: This ROG blog is based on several presentations at the 45th European Microwave (EuMW) Conference at the Palais des Congres, Paris, France, September 6-11, 2015. The presentations include a company-sponsored MicroApps session, “Composite Circuit Materials Used to Suppress Harmonic Modes in Microstrip Edge Coupled Filters,” and the EuMW Conference presentation, “Applied Methodology for Harmonic Suppression of Microstrip Edge Coupled Bandpass Filters using Composite Circuit Materials.”

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