New-Tech Europe Magazine | Oct 2017 | Digital Edition

making them suitable candidates for volume production. Models are available in package sizes as small as 2x2mm and in bare die format for chip and wire integration. With these advantages in mind, this article explores possibilities for use of reflectionless filters in UWB filter design. Five case studies are presented using standard reflectionless filter models available off the shelf from Mini-Circuits. Simulation results and measured data are presented to illustrate the various advantages of reflectionless filters in UWB applications. Finally, a design is presented that meets UWB bandwidth requirements and conforms to the specifications of the FCC spectral mask. Case 1: General Proof of Concept To illustrate the technique, two reflectionless filters were combined to create a bandpass response. In this case, high pass model XHF-292M+ and low pass model XLF-73+ were used. The preliminary simulation shown in figure 1 exhibits a 3 dB passband from 2.3 to 9.7 GHz (4.2:1 or 123% bandwidth). To validate these results, the filters were mounted on a test board as shown in figure 2. Insertion loss and input / output return loss were swept from 0.1 to 40 GHz and again from 45 MHz to 2 GHz, the later with fine resolution to capture the low frequency details. The measurement was then corrected for the fixture by subtracting the measured loss of a straight thru-line. The measured data for this filter is plotted in figure 3, with insertion loss in black. The response confirms the simulation results, exhibiting a 3 dB bandwidth of roughly 2.4 to 9.7 GHz (121% or 4:1). As expected, cascading the units shows no effect on the flatness of the passband. The higher rejection on the low end is due to the two-section design of

Figure 1: Simulation of band pass response combing XHF-292M+ with XLF-73+.

Suitability or Reflectionless Filters for UWB RF Front End While UWB technology has shown much potential, many design challenges remain in bringing the technology to a stage of wider industry adoption and commercialization. One of those challenges has been developing RF filters with a wide enough passband, flat response over the whole band, and sufficient selectivity to meet FCC specifications. Several approaches have been studied to achieve the desired response utilizing microstrip technology [2] [4] [5]. While these approaches have achieved varying degrees of success, they each come with drawbacks. Microstrip UWB filter designs typically occupy greater than a square inch of board space and tend to be costlier than practical for volume production. Mini-Circuits’ reflectionless filters present an attractive alternative to existing approaches for UWB filters. Because reflectionless filters absorb and terminate stopband signals rather than reflecting them back to the source, they give designers the ability to cascade filters in multiple sections without generating standing waves due to impedance mismatch

betweenstagesandotherundesirable effects. This characteristic allows combination of low pass and high pass filters to create a bandpass response, a technique that becomes useful for the purpose of designing UWB filters. In addition to their intrinsic cascadability, reflectionless filters are uniquely suited for UWB filter designs for at least three reasons. First, reflectionless high pass filters have broad enough passbands to achieve the desired bandwidths for UWB; most other filter technologies do not. Second, the low pass filters offer cut-offs that extend high enough in frequency to achieve 3 dB bandwidths well above 100%. Finally, the good impedance match at the band edges allows multiple filters to be cascaded in series without causing distortion of the passband shape, whereas cascading conventional filters can often create standing waves between stages and introduce passband ripple and phase instability. Moreover, while competing approaches employ transmission lines, reflectionless filter topologies are based on lumped elements and produced using MMIC technology resulting in much smaller size, lower cost, and excellent repeatability,

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