A Wide Passband SAW Notch Filter

Wide passband notch filter is fulfilled by combining LC-all-pass filter with one-port-SAW resonator by means of its impedance characteristics. It possesses good frequency selectivity and low insertion loss due to good frequency selectivity of resonance-frequency resonator and low insertion loss of LC filter. Higher attenuation of specified frequency can be achieved by cascade. Compared with LC notch filter, it posses sharp cutoff frequency and narrower notch ? width. This paper first discusses the principle of this filter, and then proceeds with theoretical simulation and experimental demonstration.


Introduction
SAW notch filter is usually applied to coding and decoding system of TV (Wu Yili,1983, pp.204-207).Wide passband notch filter is fulfilled by combining LC-all-pass filter with one-port-SAW resonator by means of its impedance characteristics(O.Ikata,1992, PP.111-115).It inherited good qualities of both SAW filter and LC-circuit and thus it possesses good frequency selectivity, large working frequency and low passband insertion loss (Gopani,1990,pp.1-5).The cascade of SAW notch filter could obtain high attenuation in particular frequency band.Compared with LC notch filter, it has remarkable frequency selectivity for its sharp-cutoff frequency characteristics.Anenna duplexer used in mobile terminals, commonly takes the ladder structure as the transmitting end filter (Hikita M,1993,pp.15-23),butthis kind of structure cannot meet the request for anenna duplexer of the high demand of isolation.By coupling this notch filter with a ladder-type SAW filter, low-insertion loss in passband and high attenuation in specified frequency can be achieved (Lin Zhiqun,2006,pp.376-379).With the longitudinally coupled resonator (Moriata T,1999,pp.95-104)coupled with the SAW notch filter,its frequency response can be improved(Lin Zhiqun,2007,pp.247-249).
In this paper the theory and approaches of achieving the notch filter by combining LC-all-pass filter with one-port-SAW resonator is discussed at first, then theoretical simulation is proceeded, and ultimately experimental results are accomplished.This notch filter is characterized by small size, low insertion loss, wide passband and narrow notch frequency band.

Structure and theoretical analyses of SAW notch filter
Above-mentioned SAW notch filter is achieved by substituting capacitance in LC-all-pass network (figure 2) for one-port-SAW resonator (SAWR) (figure 1), and it is illustrated in figure 3.
As figure 1, the structure of one-port-SAW resonator (SAWR) shows, the middle part is interdigital.On the left and right are two metal gratings.And its impedance characteristics are illustrated in figure 4, the solid line represents the resistance, and the broken line represents the reactance.Impedance characteristics of one-port-SAW resonator (SAWR) shows that when frequency f is less than the resonance frequency of the resonator Fr (f<Fr), the impedance characteristics of the resonator appears capacitive; when f=Fr, the reactance of the resonator is equal to zero while the resistance tends to zero; when frequency f is near the anti-resonance frequency of the resonator Fa, the impedance characteristics of the resonator tends to the maximum.The needed notch filter is achieved by utilizing its impedance characteristics and combining it with LC-all-pass network.
The above notch filter has narrow stop band at the frequency of the anti-resonance frequency of the SAW resonator; meanwhile it has the same frequency response characteristics as the original LC circuit at other frequencies.The notch frequency is mainly determined by the anti-resonance frequency of the SAW resonator, which leads to good frequency selectivity.The notch frequency band could be controlled by adjusting the difference of resonance-frequency between the two resonators.With the increase of difference, the notch frequency band broadens while the attenuation in notch frequency decreases.So the difference should be adjusted in accordance with our needs.

Theoretical simulations of frequency response characteristics of notch filter
In figure 3, Z s1, Z s2 represent the impedance of two one-port-SAW resonators, theoretical frequency response of one-stage SAW notch filter is: In accordance with formula (1), theoretical simulation of frequency response of one-stage structure is conducted (as illustrated in figured 3).ST quartz is applied in SAW resonator.Its central frequency is 217MHz and its periphery inductance is L1=145nH, L2=40nH respectively.Simulation results are shown in figure 5 (power impedance and loaded impedance are both 50 ohm); figure A demonstrates the frequency response characteristics of narrow frequency domain and figure B demonstrates the frequency response characteristics of wide frequency domain (0~900MHz).As shown in simulation results: attenuation in notch frequency band amounts to more than 30dB, passband insertion loss less than 1.5dB and the ripple less than 2dB in the whole passband (from 0Hz to 900MHz).Deeper attenuation is obtained by multiple-stage structure when needed.

Experimental results
The experimental devices are fabricated on the piezoelectric crystal ST-X quartz in accordance with the two-stage of the structure shown in figure 3. The experimental characteristics curves are shown in figure 6.
As shown in the experimental result: attenuation in notch frequency f0 in two-stage structure amounts to more than 50dB, while at the frequency of f0±1.5MHz the insertion loss is less than 1.5dB.Compared with LC notch filter, it has much sharper cutoff frequency characteristics.The experimental results are approximately coincident with the theoretical simulation except that the loss in high frequency of the experimental results is more than that of simulation results (about 1 dB).This is somehow caused by the characteristics of inductance in high frequency.

Conclusion
Accomplishing wide passband notch filter by one-port-SAW resonator is elaborated, and theoretical simulation and experimental result are simultaneously provided in this paper.The accomplished filter inherits good qualities of both SAW resonator and LC-circuit and thus it is characterized by small size, low insertion loss, wide passband and narrow