Combining MMIC Reflectionless Filters to Create UWB Bandpass Filters
Reflectionless filters provide a novel approach to filter design and offer several practical advantages over conventional microstrip designs for ultra-wideband (UWB) applications. In addition to delivering superior electrical performance, they are smaller, lower cost and more repeatable making them suitable candidates for use in commercial applications where volume manufacturability may be a requirement. Design examples are provided.
Ultra-wideband (UWB) is defined as any RF radio technology utilizing a bandwidth of greater than 25 percent of the center frequency or a bandwidth greater than 500 MHz.1-2 While UWB has been around since the end of the 19th century, restrictions on transmission to prevent interference with narrowband continuous wave signals have limited its applications to defense and relatively few specially licensed operators.1 In 2002, the FCC opened the 3.1 to 10.6 GHz band for commercial applications of UWB technology; since then, it has been a focus of academic study and industry research for a promising variety of emerging applications. To prevent interference with neighboring spectrum allocations, such as GPS at 1.6 GHz, the FCC has imposed specific rules for indoor and outdoor transmission, limiting transmissions in the permitted frequency range to power levels of ‐41 dBm/MHz or less.
Research has explored many potentially valuable applications. For example, the wide bandwidth provides high channel capacity, allowing high speed data transfer at very low power. While the FCC power mask limits the range of UWB transmission to within roughly 10 m, its high speed, low-power characteristics make it an attractive technology for certain short-range machine-to-machine (M2M) communication applications like wireless personal area networking and low power sensor networks.1
UWB has proven viable for new applications in detection, positioning and imaging. Modulation of UWB signals using ultra-short pulses, on the order of nanoseconds, enables precise location and ranging to the cm level.1,3 This has potential for use in military surveillance systems and other high-accuracy location and detection applications. Its high resolution, high penetration properties have also attracted research in the medical field. For example, UWB systems have been used for noninvasive, precise detection of heart movements and for high fidelity imaging using safe, nonionizing radiation, as an alternative to more harmful X-ray imaging.4
UWB technology has shown much potential, but design challenges remain in bringing it to a stage of wider industry adoption and commercialization. One of those challenges is the development of RF filters with wide enough passbands, flat response and sufficient selectivity to meet FCC spectral masking specifications. Several approaches have been studied utilizing microstrip technology. 2,5-6 While achieving varying degrees of success, each have drawbacks. In general, microstrip UWB filters are large, typically occupying greater than 1 in.2 of board space, and tend to be too costly for volume production.
REFLECTIONLESS FILTERS FOR UWB RF FRONT ENDS
Reflectionless filters provide an attractive alternative to existing approaches. Because reflectionless filters absorb and terminate stopband signals, rather than reflecting them back to the source, they can be cascaded in multiple sections without generating standing waves and causing distortion of the passband shape. This facilitates the combination of low and highpass filters to create a bandpass response, a technique that is useful in designing UWB filters. Reflectionless highpass filters have broad enough passbands to achieve the desired bandwidths for UWB, while most other filter technologies do not; reflectionless lowpass filters offer cut-offs that extend high enough in frequency to achieve 3 dB bandwidths well above 100 percent.
While competing approaches employ transmission lines, reflectionless filter topologies are based on lumped elements using MMIC technology. Smaller size, lower cost and greater repeatability make them more suitable candidates for volume production. Filter models are available in package sizes as small as 2 mm × 2 mm and as bare die for chip-and-wire integration.
The remainder of this article describes the use of reflectionless filters in UWB filter design, with examples using filters available from Mini-Circuits to demonstrate their advantages. Simulated performance is compared with measured results, and a final design is shown that meets UWB bandwidth requirements and the specifications of the FCC spectral mask.
Case 1: General Proof of Concept
Two reflectionless filters, Mini-Circuits highpass (2.9 to 8.7 GHz) and lowpass (DC to 7 GHz) models, are combined to create a bandpass response. The simulation shown in Figure 1a exhibits a 3 dB passband from 2.3 to 9.7 GHz (4.2:1 or 123 percent bandwidth). To validate these results, the filters are mounted in the test fixture shown in Figure 1b. Insertion loss is swept from 0.1 to 40 GHz and again from 45 MHz to 2 GHz, the latter with fine resolution to capture the low frequency details. After correcting for fixture loss by subtracting the measured loss of a straight thru-line, the measured data for the combined filter is plotted in Figure 1c. The response exhibits a 3 dB passband from about 2.4 to 9.7 GHz (4:1 or 121 percent bandwidth). Cascading has no effect on the passband flatness. The higher rejection on the low frequency end is due to the two-section design of the highpass filter.
Case 2: Maximizing Bandwidth
Case 1 establishes the viability of combining highpass and lowpass reflectionless filters to create UWB bandpass response. The same technique can be used with different filter models to shape the response. In this case, two-section highpass and three-section lowpass models (0.58 to 3 GHz and DC to 3.53 GHz, respectively) are combined to create the widest possible passband. In addition to a wide bandwidth, because this combined filter incorporates two- and three-section designs, it also exhibits high rejection in both the upper and lower frequency stopbands.
A simulation combining these two models in series is shown in Figure 2a, exhibiting a 3 dB passband from 450 MHz to 5.7 GHz (12.7:1 or 171 percent bandwidth). A logarithmic frequency scale is used to better show the shape of the response. Note the lower frequency stopband rejection greater than 30 dB and upper frequency stopband rejection of 50 to 60 dB.
The filters are shown mounted in their test fixture in Figure 2b. Insertion loss is measured as in Case 1 and shown in Figure 2c. The filter achieves a 3 dB bandwidth from about 500 MHz to 5.2 GHz (10:1 or 165 percent bandwidth). The measured data exhibits a slightly narrower passband than the simulation, yet still achieves greater than a full decade of bandwidth. The lower stopband rejection is between 30 and 40 dB, and the upper stopband rejection ranges from 40 to greater than 60 dB, corresponding to the simulation. The passband shows excellent flatness with no distortion from adverse interactions between the filter stages.
Case 3: Confirming Stopband Rejection to 40 GHz Without Re-Entry
Cases 1 and 2 illustrate that cascaded reflectionless filters can achieve ultra-wide passbands, enabling bandwidths at least to a full decade, to support the bandwidth requirements of UWB applications. Another concern for system designers is the potential for “re-entry” out of the band at higher frequencies. Such unintentional radiation can potentially interfere with signals at neighboring frequencies and violate FCC rules. Therefore, UWB filters must exhibit good stopband rejection without re-entry to a very high frequency. In part due to their fabrication using MMIC technology, reflectionless lowpass filters provide stopband rejection extending to 40 GHz. Many conventional filter approaches would suffer re-entry over this bandwidth.
In this case, a highpass model (2.01 to 10.1 GHz) and lowpass model (DC to 7 GHz), both single-section designs, are combined. Simulation results for the combined filter are shown in Figure 3a, demonstrating a 3 dB passband from 1.6 to 10 GHz (6.25:1 or 145 percent bandwidth). Stopband rejection remains better than 15 dB up to 40 GHz without re-entry. Figure 3c plots the measured insertion loss of the test board shown in Figure 3b. The measured response shows a 3 dB passband from about 1.7 to 9.3 GHz (5.5:1 or 138 percent bandwidth), with stopband rejection well above 15 dB up to 40 GHz, confirming that this technique can be used in UWB applications without unintentional out-of-band emissions due to re-entry.
Case 4: Adding LTCC Filters to Sharpen Selectivity
We have shown that reflectionless filters can be combined to achieve ultra-wide passbands and that this approach provides excellent stopband rejection up to 40 GHz without re-entry. To come closer to real world requirements of UWB systems under FCC specifications, it may be necessary to sharpen the transition to conform to the FCC spectral mask.
The absorptive characteristic of reflectionless filters means that they are not only cascadable with other reflectionless filters, but with all manner of conventional filters. This hybrid approach enables the desired wideband response while incorporating the selectivity of another filter technology. In this case, a two-section, highpass reflectionless filter (2.9 to 8.7 GHz) is combined with a lowpass LTCC filter (DC to 10.6 GHz) to use the greater selectivity of the latter. Simulation results are shown in Figure 4a, along with the FCC spectral mask for indoor UWB transmissions. This combination exhibits a passband from 2.4 to 10.9 GHz (4.5:1 or 128 percent bandwidth). Deep rejection at the lower stopband, below 2.4 GHz, keeps transmissions at neighboring frequencies, such as GPS at 1.6 GHz, clean of emissions. While the data for the LTCC filter stops at 15 GHz, it is clearly approaching some re-entry at that point. This is a trade-off when incorporating a different filter technology.
The test board for this filter combination is shown in Figure 4b and the measured insertion loss is shown in Figure 4c. It has a measured 3 dB passband from about 2.45 to 10.9 GHz (4.5:1 or 127 percent bandwidth), consistent with the simulation. The combination with the LTCC filter introduces a few noteworthy differences from the previous cases. First, the insertion loss suffers some re-entry around 25 GHz, enough to just cross the FCC limit. Also, the return loss in the upper stopband (not shown) degrades because the LTCC filter is fully reflective in its stopband. Overall, the filter approaches the desired response for real world UWB transmission, yet is still wider than ideal. A similar approach with the right combination of filters may come closer to the ideal filter behavior.
Case 5: UWB Filter Meeting the FCC Emission Mask for Indoor UWB Transmission
To realize a filter response closer to the ideal for real world UWB transmission, careful model selection leads to the combination of a three-section, highpass reflectionless filter (5 to 11 GHz) and a lowpass LTCC filter (DC to 8.4 GHz). A simulation of this filter combination is shown in Figure 5a, including the FCC mask for indoor UWB transmission. The simulated 3 dB passband is from 3.9 to 9.4 GHz (2.4:1 or 83 percent bandwidth). Although the LTCC filter does show some re-entry in the upper stopband, it is not significant enough to become a secondary passband, remaining well below the FCC mask.
The test board is shown in Figure 5b and the measured insertion loss is in Figure 5c. The filter response exhibits a 3 dB passband from 4.25 to 9.15 GHz (2.2:1 or 73 percent) and conforms well to the FCC spectral mask. Again, the reflectionless LTCC hybrid approach comes with some tradeoffs that warrant mentioning. First, as expected, the filter exhibits reflective behavior in the upper stopband and return loss degrades above 9 GHz. Secondly, while the upper stopband achieves excellent rejection to 25 GHz, it suffers some re-entry around 30 to 35 GHz. A different lowpass filter model may be needed to suppress this re-entry at higher frequencies. Nonetheless, this example illustrates how reflectionless filters can be successfully cascaded with other filter designs to achieve the desired passband shape for UWB communications.
The examples in this article show how reflectionless filters provide a novel and highly viable approach to filter design for UWB applications. They all employ standard, catalog filters available from Mini-Circuits. Mini-Circuits offers over 50 reflectionless filter models from stock, and custom designs are available to meet exact application requirements.
The approach demonstrated provides designers several practical advantages over previously studied approaches using microstrip structures. In addition to the electrical properties that make reflectionless filters ideal for UWB, the filters are smaller, less costly and more repeatable compared to competing technologies, making them suitable candidates for use in commercial applications where volume manufacturability may be a requirement.
Mini-Circuits is currently developing new designs with lowpass and highpass filter dice cascaded within a single package to reduce size, lower cost and minimize parasitic effects.
While this article discussed the suitability of reflectionless filters for UWB applications, it should serve to broaden the reader’s understanding of these innovative products as flexible building blocks with numerous applications in RF system design, many of which still remain to be explored.
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- C. Hsu, F. Hsu and J. Kuo, “Microstrip Bandpass Filters for Ultra-Wideband (UWB) Wireless Communications,” IEEE MTT-S International Microwave Symposium Digest, October 2005.
- C. Cansever, “Design of a Microstrip Bandpass Filter for 3.1-10.6 GHz UWB Systems,” Thesis, Syracuse University College of Engineering and Computer Science, 2013.
- J. Pan, “Medical Applications of Ultra-Wideband (UWB),” Washington University, St. Louis, April 2008, www.cse.wustl.edu/~jain/cse574-08/ftp/uwb/index.html.
- L. Zhu, S. Sun and W. Menzel, “Ultra-Wideband (UWB) Bandpass Filters Using Multiple-Mode Resonator,” IEEE Microwave and Wireless Components Letters, Vol. 15, No. 11, November 2005, pp. 796–798.
- A. Sheta and I. Elshafley, “Microstrip Ultra-Wide-Band Filter,” PIERS Proceedings, March 2011, pp. 198–200.