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Surface Acoustic Wave Filters
Published in John T. Taylor, Qiuting Huang, CRC Handbook of ELECTRICAL FILTERS, 2020
Some examples of the outstanding performance achievable using various types of commercially available SAW bandpass filters are shown in Figures 2 through 6. Figure 2 shows a low-frequency SAW bandpass filter with a low shape factor, the ratio of the filter 40-dB bandwidth to 3-dB bandwidth. In this example the shape factor is less than 1.2, passband ripple is less than ±0.2 dB, and group delay ripple is below ±15 ns. The passband of this filter covers the frequency range 33.9 to 39.65 MHz, the insertion loss in a 50-Ω system is 29.4 dB, and ultimate out-of-band rejection exceeds 55 dBc. Figure 3 illustrates a typical SAW low-loss bandpass filter which utilizes unidirectional transducers, exhibiting a loss of approximately 7.6 dB and a shape factor of slightly more than 2. Figure 4 shows an example of the sculptured amplitude responses possible using SAW devices. Figure 5 illustrates the capability of producing bandpass filters with outstanding amplitude characteristics and independently sculptured group delay. This device was developed for use as a vestigial sideband filter in CATV applications. Figure 6 illustrates the wide bandwidths possible using dispersive SAW transducers. This device consists of dispersive transducers in a structure called a slanted array compressor2,11,13 and has a center frequency of 1.3 GHz and a fractional bandwidth of 50%.
A simple frequency-tunable integrated microwave photonic filter based on sideband selective amplification effect
Published in Gin Jose, Mário Ferreira, Advances in Optoelectronic Technology and Industry Development, 2019
X. Zhang, J. Zheng, T. Pu, Y. Li, J. Li, X. Meng, W. Mou, G. Su, Y. Tan, H. Shi, Y. Chen, T. Dai, S. Ju
In this paper, we propose a novel approach to realize a microwave photonic filter which is ultra-sample compared with previously proposed MPFs because the bulky optical injection system is replaced by the integrated mutual injection laser. The tunable MPF is obtained based on optical injection and sideband selective amplification effect which is demonstrated in the experiment. In our previous wok, we found that the MPF is quite an important mechanism which is used as an insert filter in the proposed optoelectronic oscillator (Xin Zhang, 2019). So, we experimentally research the characteristics of the MPF in detail for further applications. The MPF has a wide tunable range from 16 GHz to 36 GHz which is controlled by adjusting the bias currents of the integrated laser. The out-of-band rejection ratio is 30.7 dB when the center frequency of the MPF is 24.8 GHz, and the 3-dB bandwidth is 10 MHz. The corresponding Q factor is 2480.
A bandwidth-reconfigurable optical filter based on a stimulated Brillouin scattering effect using an optical frequency comb
Published in Khaled Habib, Elfed Lewis, Frontier Research and Innovation in Optoelectronics Technology and Industry, 2018
Jingwen Gong, Xiaojun Li, Qinggui Tan, Wei Jiang, Dong Liang
We demonstrated a bandwidth-tunable microwave photonic filter based on stimulated Brillouin scattering in fiber with a bandwidth from 11.8 MHz to 202 MHz by using a 25 line comb with a spacing of 20 MHz obtained by two cascaded DDMZMs. Experiments have shown that the 3 dB and 20 dB bandwidth of the filter is 202 MHz and 314 MHz, respectively and its 20 dB shape factor is 1.56. The passband ripple is ~2.5 dBm with a stop-band rejection of 30 dBm. The experimental result was very close to the simulation result, and the difference was due to the imperfection of the devices we used. Although the ability of the optical comb to spread the spectrum was proven experimentally, the disadvantages of combs such as high flatness and extreme instability caused by bias point drifting severely restrict its application. Therefore, a pseudo-random bit sequence pulse modulation is proposed to extend the pump bandwidth in the future, then the SBS gain spectrum width would be extended to spread the SBS filter bandwidth.
Liquid crystals for signal processing applications in the microwave and millimeter wave frequency ranges
Published in Liquid Crystals Reviews, 2018
Robert Camley, Zbigniew Celinski, Yuriy Garbovskiy, Anatoliy Glushchenko
Electrically tunable filters represent another important category of liquid crystal-based microwave devices. The most important types of liquid crystal-based filters include band-stop filters [170-172], sometimes also called notch filters [128,173-175], and band-pass filters [127,176-185] (see Tables 8 and 9). According to a standard definition [186], band-stop filters (or band-rejection filters) are two-port devices, which are designed to reject a range of frequencies known as the stopband, while passing through all other frequencies making up the passband. There are a number of technical parameters that are used to characterize the performance of band-stop filters. If one plots S21 as a function of frequency for a band-stop (or notch) filter, ideally one would see a horizontal line with a downward notch at the band-stop frequency. The parameters that are used to describe this include: The central stopband frequency, i.e. the frequency at which the maximum attenuation occurs;The Q-factor, essentially, the narrowness of the notch. This is given by the inverse width of the stop band in frequency multiplied by the central frequency of the stop band. On a linear scale, one usually measures width by something like full-width at half-maximum. Since transmission S21 is normally given in dB the width of the stop-band region, the width, in this case, is measured at a position 3 dB away from the maximum loss. In practice, both narrow and wide band-stop filters are desired, depending on the application;Maximum stop-band insertion loss S21. One generally wants values of at least -20 dB at the notch position;The stop-band return loss, S11 i.e. how much signal is reflected at the frequency where very little signal is transmitted. Ideally one does not want reflections, so S11 should be small, at least on the level of -20 dB.Tuning range, [186]. In practice, a range of about 5 GHz is often sufficient for applications with central frequencies near 30 GHz.