New-Tech Europe Magazine | Oct 2017 | Digital Edition

Figure 5: Test board for XHF- 581M+ and XLF-312H+

Figure 6: Measurement plots of S21 (black), S11 (red), and S22 (blue) for combined XHF-581M+ and XLF-312H+, exhibiting a bandpass response with roughly 165% bandwidth.

frequencies. Such unintentional radiation can potentially interfere with signals at neighboring frequencies and violate the FCC rules. Therefore, UWB filters must exhibit good stopband rejection without re-entry up to a very high frequency. Reflectionless filters offer advantages in this regard. In part due to their fabrication using MMIC technology, reflectionless low-pass filters provide stopband rejection extending up to 40 GHz. Many conventional filter approaches would suffer re-entry over this bandwidth. This characteristic allows us to create a UWB filter response that meets the FCC power mask up to 40 GHz without re-entry. In this case, we combine high pass model XHF-23+ and XLF-73+, both single section designs. S-parameter simulation for these models is shown in figure 7, exhibiting a 3 dB passband from 1.6 GHz to 10 GHz (6.25:1 or 145% bandwidth). Stopband rejection remains better than 15 dB up to 40 GHz without re-entry. Figure 9 shows measured insertion

Figure 4: Simulation of band pass response combining XHF-23+ and XLF-73+

Case 3: Confirming Stopband Rejection up to 40 GHz without Re-Entry It’s clear from Cases 1 and 2 that combining reflectionless filters can achieve ultra-wide passbands, allowing bandwidth at least up to a full decade, amply supporting the bandwidth requirements of UWB applications. Another concern for system designers building UWB transmitters is the potential for “re-entry” out of band at higher

The filter achieves a 3 dB bandwidth from roughly 500 MHz to 5.2 GHz (10:1 or 165% bandwidth). The measured data exhibits a slightly narrower passband than the simulation, but still achieves full decade of bandwidth. As expected lower stopband rejection is between 30 and 40 dB, and upper stopband rejection ranges from 40 to greater than 60 dB. The passband shows excellent flatness with no distortion from adverse interactions between the filter stages.

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