In this dissertation, several reconfigurable filters are presented for LTE bands or next generation communication systems, including net-type microstrip resonators and acoustic wave resonators reconfigurable filters. In first part, the net-type dual-mode constant fractional bandwidth (CFBW) reconfigurable filter with good matching by tuning external quality factor has been presented. The net-type dual-mode CFBW reconfigurable filter is designed with second-order Chebyshev response.
For achieving the CFBW response, the coupling coefficients and external quality factors are needed to be constant value with frequency. Constant coupling coefficients are obtained by exploiting the property of short grounded-stub loaded net-type dual-mode resonator. A pair of varactors are used to tune the center frequency by adding on the end of opened-stub. The variation of coupling coefficient can be below 10%
when the length of grounded-stub is shorter than 1/5 length of opened-stub. A constant fractional bandwidth response over the frequency tuning range can be achieved.
Constant external quality factors are accomplished by introducing a pair of varactors to the feeding structures. The return loss of reconfigurable filter without tunable external quality factor is 15dB in the highest band but 7dB in the lowest band.
By using varactors to tune the external quality factor, the filter can meet the desired 20dB return loss specification over the entire tuning range of frequency. The frequency tuning range of the proposed reconfigurable filter is able to cover 1.6 GHz to 1.96 GHz.
Table 6.1 Comparison of literatures on microstrip reconfigurable filters
Frequency (GHz)
CFBW /CABW
FBW /ABW
IL (dB)
RL (dB)
With /Without Tunable Qe
Num. of varactor
[28]
2.303-2.721
CABW 8%-8.5% 1.5-3.2 15-25 Without 2
2.19-2.55 6.2%-6.8% 2.8-4.9 7.5-15 Without 2
2.415-2.732 CFBW 200.6-219.1
MHz 1.7-2.7 10-20 Without 2
2.25-2.555 183.4-195
MHz 2.4-3.78 7.5-15 Without 2
[31] 1.71-1.93
-- 9.3%-9.9% <1.7 >10 Without 2
1.92-2.25 7.6%-5.7% <4.1 Without 2
[32] Band 1: 0.79-0.86
CABW 1.46 GHz 2.6-3.1 20-25 Without 3
Band 2: 1.2-1.33 0.9 GHz 1.8-2.7 Without 3
[33] 0.95-1.55
CABW 100 MHz 2.4-2.8 >20 Without 3
1.15-1.55 120 MHz Without 3
[34]
0.669-1.215 Tuning
BW 14%-64.4% 1-2 >15 With 6
0.669-1.215 CABW 300 MHz <2 >13 With 6
0.6-1.45 Tuning
BW 12%-95% 0.95-3.5 >15 With 6
0.6-1.45 CABW 300 MHz <2.8 >13 With 6
[46] 0.6-1.03 CABW 85±5 MHz <1.8 10-25 Without 3
Ch3 1.705-2.05 -- -- 2.3-4 7-15 Without 2
1.62-1.96 CFBW 4.5-5.1% 2.84-2.9 ~20 With 4
The fractional bandwidth is constantly 5% while 20 dB return loss and the insertion loss is about 2.9 dB over the entire tuning range. The total size is 24.4mm × 32.1mm (0.32 λg
× 0.42 λg).
Comparing with the literature results is shown in Table 6.1. In designing typical reconfigurable filters without tunable external quality factor, two varactors are commonly used to change the center frequency of reconfigurable filters. The number of
varactors in the present design is similar with other references. For designing reconfigurable filter with tunable external quality factor, four varactors are needed, smaller than that in available literature, say [34].
The second part of the dissertation, the constant absolutely bandwidth (CABW) reconfigurable filters using high electromechanical coupling coefficient (kt2
) film bulk acoustic resonators (FBARs) has been presented. The general coupled resonator theory of filters has been used for reconfigurable FBAR filter design. The concept has been validated by presenting four-order Chebyshev reconfigurable bandpass filters with CABW response by high kt2
FBARs using coupling capacitors. The resonators are realized by the FBAR resonator and a series inductor, which are simplified by a parallel RLC equivalent circuit model. As a result, the coupling coefficients and external quality factors are designed by lumped elements.
For the reconfigurable design, the center frequency and bandwidth of filters have been adjusted by tunable capacitors. The tuning range is decided by insertion loss of the lowest band, while insertion loss at the lowest band is dominated by quality factor of tunable capacitors, especially for tuning center frequency. When the quality factor of tunable capacitors becomes higher, the insertion loss can be improved.
The reconfigurable filters by high kt2 lead zirconate titanate (PZT) FBARs with 45 MHz bandwidth is designed for center frequency range from 2.07 GHz to 1.8 GHz. The insertion loss is 1.46 dB to 5 dB. The tuning range of reconfigurable filters with <4dB insertion loss is about 8.2% with digital capacitors banks of Q = 150. The simulated results have been shown, with the losses of resonators and lumped elements taken into consideration.
The last part of the dissertation, the constant absolutely bandwidth (CABW) reconfigurable filter using tunable film bulk acoustic resonators (TFBARs) has been
presented. The TFBAR filters are also designed by the general coupled resonator theory of filter. A four-order Chebyshev reconfigurable bandpass filter with CABW response using TFBARs and variable coupling capacitors is designed to validate the design concept. The resonators are realized by the TFBAR resonator and a series inductor, which are simplified by a parallel RLC equivalent circuit model. The coupling coefficients and external quality factors are then designed by lumped elements.
For the reconfigurable design, the center frequency can be tuned by the thickness of general piezoelectric layer and semiconducting piezoelectric layer of TFBARs. The filter bandwidth has been adjusted by tunable capacitors, while the tuning range is decided by TFBARs. The reconfigurable filters using TFBARs with 45 MHz bandwidth is designed for center frequency range from 2.12 GHz to 1.91 GHz. The insertion loss is found to be excellent, 2.14 dB to 2.24 dB. The tuning range of TFBAR filters is about 10%. The simulated results have been shown, including the losses of resonators and lumped elements.
The major difference of the second part and last part is the tuning mechanism of center frequency of reconfigurable fitters. In second part, the center frequency of reconfigurable filters is changed by extra tunable capacitors and the losses of the lower bands are decided by the quality factor of extra tunable capacitors. Then the tuning range is limited by the quality factor of extra tunable capacitors. In last part, the center frequency of reconfigurable filters is adjusted by TFBARs and the tuning range of reconfigurable filters is also dominated by tuning range of TFBARs.
Comparison of acoustic wave resonators from literatures is shown in Table 6.2. In the design of a typical ladder-type tunable FBAR or SAW filter, each resonator is designed for transmission zeros to form a filter response in each band. At least two resonators are needed but the response is not good enough. For achieving the
specification of SAW/FBAR filters, the number of resonators will be increase and the design procedure will be complex.
By the proposed reconfigurable filters with high kt2
FBARs or TFBARs, each resonator and the filter response can be designed by filter coupled resonator theory. The method provides more systematic design procedure. The loss of reconfigurable filters with high kt2 FBARs is dominated by the quality factor of resonators and tunable capacitors. By increasing the quality factor of resonators and tunable capacitors, the insertion loss and tuning range can be improved. Because of the tuning mechanism of TFBARs [64], the number of extra tunable components and loss of reconfigurable filters with TFBARs can be reduced and the insertion loss excels.