In the previous section, we have inctoduced a new circuit scheme for tuning the center frequency by using varactors. Where in the CT and LT determine the values of reactance X.
Moreover, the positive sign of X makes the center frequency shift toward higher end; on the contrary, the center frequency shifts toward lower frequency as the sign of X is negative.
As a result, if we can changed the reactance X from positive to negative value, we could have a wider tunable band-pass filter than the one described previously. Beside, the bandwidth of tuning depends not only on the range of reactance X, it is also dependent on the value of CT
(when LT is fixed). However, the usage of a varactor having wide range of CT is not cost-effective in such a type of circuit design.
Now we will use the similar theory but the dissimilar way to design a wideband tunable band-pass filter such as Figure 5.14, where N is the diode numbers on each branch of the ring (namely, when N=1, the total diode numbers in this tunable filter are four). The diode in this circuit is regarded as a switch. In order to design conveniently, the equivalent circuit in Figure 5.2 (b) can substitute for the switch OFF, and the short circuit can substitute for the switch ON.
Now we consider the circuit in Figure 5.14 (N=1). When the switch ON, the X=ωLT >0 and the frequency can be increased. On the contrary, when the switch OFF, the
T
T) L
C / 1 (
X= − ω +ω . The X<0 can be achieved by controlling the voltage of CT. Therefore, this filter can have two states of center frequency.
In the design we use the substrate (RO4003C), ON SEMICONDUCTOR (BAV70LT1) diode serves as a switch, the resonant frequency of the ring resonator without varactor is 3.3 GHz(λ0/2=28.7mm), BW=150MHz, t=2mm, S=0.5mm, g=0.5mm and the varactor tapping position d1=5mm. Similarly we find the LT by Figure 5.4 circuit first. When CT=0.5pF (diode OFF), by measuring S21 in Figure 5.4, the series LC resonant frequency f=6.2GHz, and from
equation (5.1) we can find the LT=1.3nH. Figure 5.15 and Figure 5.16 are the results obtained from simulation and the measurement, respectively. Similarly, in order to obtain the relations among f0, CT, X, VR and S21, we arrange the results of the simulation and measurement as listed in table 5-3. By the Figure 5.15 and Figure 5.16, the filter has two frequency states controlled by the switch ON and OFF. We may infer that, the filter will have 2N states of frequency, if N switches are tapped on each branch of the ring of the filter. Beside, in table 5-3 we can find that, because the frequency can be shift up and down, the filter has wide tunable frequency range. However, because the Q of the switches (diode) is not good, the insertion loss in practice is greater than that of the simulation, which again confirms the previous experiments.
Figure 5.14 The circuit of the band-pass filter having wide tunable bandwidth with N switches of each branch of the ring
1.5 2 2.5 3 3.5 4 4.5 5 5.5
Frequency (GHz)
-80 -70 -60 -50 -40 -30 -20 -10 0
M a g n it u d e S 2 1 ( d B )
switch-on switch-off
Figure 5.15 The simulation of the band-pass filter having wide tunable bandwidth for one switch of each branch of the ring
1.5 2 2.5 3 3.5 4 4.5 5 5.5
Frequency (GHz)
-60 -50 -40 -30 -20 -10 0
M a g n it u d e S 2 1 ( d B )
switch-on switch-off
Figure 5.16 The measurement of the band-pass filter having wide tunable bandwidth for one switch of each branch of the ring
Table 5-3 The simulation and measurement results of the tunable band-pass filter for one switch of each branch of the ring
Switch-state CT(pF) X(Ω) f0(GHz) S21 (dB)
Simulation 3.3 -2.0
measurement
NONE NONE NONE
3.27 -1.5
Simulation 5.05 -1.36
measurement
On 0 26.95
4.8 -15.1
Simulation 2.65 -3.723
measurement
Off 0.5 -69.5
2.73 -5.94
In this chapter we proposed a new method that can tune the resonant frequency of the open loop ring resonator. We applied this method to design and fabricate some tunable band-pass filters. They include a filter whose frequency can be tuned downward, a filter whose frequency can be tuned upward, and a wide tunable frequency filter. The advantage of the new filter is that the size of the circuit is compact and the tunable frequency range is wide and, because the filters use the asymmetric feed lines, the filters have a higher selectivity.
Beside, by the tapping position of the varactor or switch, the filters have another advantage is that the filters can compensate that the values of CT and LT, thus, it provides many possibilities in choosing devices for the filter design. However, in our proposed filters, because the Q factor of the devices is limited, the insertion losses of the filters are obvious.
Some methods of the compensation for insertion loss are referred in [16][9]. These methods can also be applied to the tunable filters for our design.
Chapter 6
Conclusion
In this thesis, a new scheme of tunable band-pass filter was proposed and implemented. The varactor was employed to adjust the reactance and what follows is the resonant frequency of the band-pass filter. In addition to the reactance provided by the varactor, the position where the varactor is added is also a parameter in conjunction with the overall performance of the filter. However, since the quality factor of this diode is not considerable, the insertion loss becomes obvious and then degrades its performance. This drawback could be overcome by introducing negative resistor or etc, and may be a good research topic in the future.
Reference
[1] G. L. Matthaei, L. Young, and E. M. T. Jones, “Microwae Filters, Impedance-Mathing Networks, and Coupling Structures.” New York: McGraw-Hill, 1980, ch. 11.
[2] Jia-Sheng Hong, Michael J. Lancaster, “Couplings of Microstrip Square Open-Loop Resonators for Cross-Coupled Planar Microwave Filters,” IEEE Trans Microwave Theory and Techniques, vol. 44, NO. 12, December 1996.
[3] K. T. jokela, “Narrow-band stripline or microstrip filters with transmission zeros at real and imaginary frequencies,“ IEEE Trans. Microwave Theory Tech., vol. MTT-28, pp.
542-547, June 1980.
[4] S. Y. Lee and C. M. Tsai, “New cross-coupled filter design using improved hairpin resonators,” IEEE Trans. Microwave Theory Tech., vol.48, pp. 2482-2490,Dec. 2000.
[5] L. H. Hsien and K. Chang, “Tunable Microstrip Bandpass Filters With Two Transmission Zeros” IEEE Trans. Microwave Theory Tech., vol. 51, pp. 520-525, February 2003.
[6] J. Uher, J. R. Hoelfer, “Tunable Microwave and Millimeter-Wave Band-Pass Filters,”
IEEE MTTs, vol. 39, no. 4 April 1991
[7] S. Kumar, D. Klymyshyn and A. Mohammadi, “broadband electronically tunable microstrip ring resonator filter with negative resistance coupling” IEEE Letters Vol. 32, no 9, pp. 809-810, 25th April 1996
[8] M. Makimoto and M. Sagawa, “Varactor Tuned Bandpass Filters Using Microstrip-Line Ring Resonators,” IEEE MTT-s, pp. 411-414, 1986
[9] A. Cenac, L. Nenert, L. Billonnet, B. Jarry, P. Guillon, “Broadband Monolithic Analog Phase Shifter and Gain Circuit For Fequency Tunable Microwave Active Filters,” IEEE MTT-S Dig., Volume: 2 , 7-12, Pages:869 - 872 vol.2, June 1998
[10] X. P. Liang, Y Zhu, “Hybrid Resonator Microstrip Line Electrically Tunable Filter,”
IEEE MTT-S D, Volume: 3, 20-25 Pages: 1457 - 1460, May 2001
[11] M. Makimoto, M. Sagawa, “Varactor Tuned Bandpass Filters Using Microstrip-Line Ring Resonators,” IEEE MTT-S, Volume: 86, Issue: 1, 2 Pages: 411 – 414, Jun 1986 [12] David M. Pozar, “Microwave Engineering,” John Wiley & Sons, Inc., 1998
[13] Kai Chang, Jia-Sheng Hong, M. J. Lancaster, “Microstrip Filters for RF/Microwave Applications,” John Wiley & Sons, Inc. 2001
[14] R. Garg and I. J. Bahl, “Microstrip discontinuities,” Int. j. Electron., vol. 45, pp. 81-87, July 1978.
[15] Kai, Chang, “Microwave Ring Circuits and Antennas,” John Wiley & Sons, Inc. 1996 [16] D. K. Paul, M. Michael and K. Konstantinou, “MMIC Tunable Bandpass Filter Using A
Ring Resonator With Loss Compensation,” IEEE MTT-s Digest, Volume: 2, 8-13 Pages:
941 - 944 vol.2 June 1997
Appendix A
Element values for Butterworth low-pass filter prototypes (g0=1, LAr=3.01dB at Ωc=1, n=1 to 10)
n g1 g2 g3 g4 g5 g6 g7 g8 g9 g10 g11
1 2.0000 1.0000
2 1.4142 1.4142 1.0000
3 1.0000 2.0000 1.0000 1.0000
4 0.7654 1.8478 1.8478 0.7654 1.0000
5 0.6180 1.6180 2.0000 1.6180 0.6180 1.0000
6 0.5176 1.4142 1.9318 1.9318 1.4142 0.5176 1.0000
7 0.4450 1.2470 1.8019 2.0000 1.8019 1.2470 0.4450 1.0000
8 0.3902 1.1111 1.6629 1.9615 1.9615 1.6629 1.1111 0.3902 1.0000 9 0.3473 1.0000 1.5321 1.8794 2.0000 1.8794 1.5321 1.0000 0.3473 1.0000
10 0.3129 0.9080 1.4142 1.7820 1.9754 1.9754 1.7820 1.4142 0.9080 0.3129 1.0000