Design of RF Diplexer for Mobile Communication

Transcription

1 Volume 85 No 4, anuary 4 Desin of RF Diplexer for Mobile Communication R.Brinda Electronics and Communication Mepco Schlenk Enineerin Collee Sivakasi,India P.Anisha Parveen Electronics and Communication Mepco Schlenk Enineerin Collee Sivakasi, India ABSTRACT In this paper, a matchin circuit for RF diplexer for mobile communication is proposed. Most wanted bands in wireless technoloies like GSM and DCS is used. Both bands are desined by usin stepped impedance resonators with sinle order and matchin between this two are implemented by Y- junction method. Cutoff frequency for GSM and DCS band are 95 MHz and 795 MHz. Microstrip Technoloy is proposed because it provides a better insertion loss, better return loss and ood isolation between the passbands. The isolation between two operatin bands is better than -5 db. The overall insertion loss obtained in diplexer is -.4 db. The overall return loss is less than - db. Keywords Diplexer, Mobile Communication, Stepped Resonator (SIR).. INTRODUCTION Diplexer plays a important role in wireless communications. Industry demand us to desin a sinle wide band compact antenna which will cover all the CDMA,WCDMA,UMTS and PCS bands with ood ain. One of the major advantaes of this diplexer is non- requirement of extra jumper cable and connector. Therefore it is economical and also avoids cable insertion loss. Moreover, Diplexer is desined should ensure desired isolation between the dual pass bands. The matchin circuit desin of lowpass-bandpass [], [] may be challene in conventional desins. Coupled line bandpass topoloy is used. It requires more space []. Generally the junction circuit desin is more complex and critical part of the Diplexer because the matchin circuit may provides easy independent desin for each filter.[3]-[6]..folded coupled-line structure proposed in [4] and dual-mode Stripline rin resonators are used to produce transmission zeros to improve the selectivity of the Diplexer. Skew symmetric feed structure is used. But it does not concentrate on the matchin circuit. The Branch line resonator can also act as diplexer junction. For dual-band filter structure is for providin the required resonance at the centre frequency of each passband [8]. For better Matchin between the filter, Two BPF is combined with T junction. It can be used for several mobile applications. It consists of compact hybrid resonator. It is larer in size and complex to desin. Couplin structures with both Chebyshev and quasi-elliptic freq responses presented to achieve dual- &triple-band without a sinificant increase in circuit size.[7] some extra coupled resonator section can added for increasin the derees of freedom in extractin couplin coefficient of a multiband filter. It is Flexible desin process. But it is more complex. Many application need Hih Isolation and selectivity. Filters with asymmetric responses are very suitable for duplexer applications, because it make the use of band-width more efficient [3]. Coplanar-waveuide diplexer is usin in planar lumped-elements to realize lowpass and bandpass filter in diplexer structure. Cpw-cps cross junction is proposed. [9]. It requires MMIC components. Two bandpass filters are used with very lare dielectric constant. The filter structure is based on the stepped impedance resonator(sir). So. Spurious response is effectively reduced. The T-shaped resonator has an extremely small frequency ratio usin microstrip with practical line width. T-shaped ives better isolation.[3].but Stripline technoloy is very difficult to fabricate and desin. A compact diplexer is implemented by usin Slotline hairpin Stepped Resonator (SIR) in round plane []. Microstrip-Slotline transition is used to split the sinal without extra matchin circuits and thereby economizes the circuit size. To achieve hih isolation and compact size diplexer, the variable frequency response of stepped-impedance resonators (SIR) is applied to realize the diplexer if their fundamental and the first spurious resonant frequency are properly assined []. Microstrip diplexers, due to the low profile, liht weiht, low cost, and easy interation with other front-end circuitry, have recently become more and more popular [4][5]. It is, however, a challenin task to desin microstrip diplexers with low inband losses for both channels and ood isolation between them while keepin a small size. The matchin network and combinin circuit ensure that both filters match the antenna and have ood isolation between them. The T-junction may be the most popularly used combinin circuit. The lenth and width of its two branches must be chosen carefully [6], [5], allowin each filter to match the antenna and introduce an open circuit at the middle band of the other. T-junctions with open stubs [7] or a step impedance transformer [8] have been developed to improve the performance... PROPOSED METHODOOGY So we proposed a diplexer desin with two bandpass filter which is desined usin Stepped Resonator method and matchin between these two bandpass filter usin Y-junction matchin devices thereby we can economizes the circuit size. Diplexer desined for the base station antenna which allows combination and separation of the sinals in band and band wireless bands. To minimize band interreaction, the inputs are all isolated and have minimal insertion loss over their respective frequency bands. The desin ensures low-passive intermodulation. An efficient Diplexer needs hih isolation between two passband frequencies to avoid interference of sinal from one port to another. Each filter at two ports should act as a short circuit in its pass band and open circuit at other passband. There should be Minimum insertion loss to minimize power loss at its two ports. 3. DESIGN OF BANDPASS FITERS GSM-9 (89-96 MHz) band and DCS-8 (7-88 MHz) band are the most wanted bands in mobile communication so we can used these two bands to desin a diplexer. Chebyshev approximation is used because, In Chebyshev approximation, steep roll-off rate in its stop band frequency and tolerable ripple level in pass band by usin very less number of filter reactive elements can be achieved, and also low insertion loss and less filter size, due to usae of 6

2 Volume 85 No 4, anuary 4 less number of reactive elements..the order of the filter is more important while considerin the size of the circuit.. It is related to the number of components in the filter, because of its cost and its physical size. So here we prefer sinle order is used. Bandpass filter is desined by usin one Transmission line and two step-impedance open stubs.[9]. In this desin, a step-impedance open stub can produce two transmission zeros loaded at each side of the central frequency. The bandwidth of the filter is controlled by the positions of the transmission zeros produced by the step-impedance open stub. By introducin two step-impedance open stubs to the input and output port of the transmission line which resonates at the desired centre frequency, we can achieve filter with ood performances of low loss, ood selectivity, and sharp attenuation characteristics. In Fiure 6. shows the confiuration of a step-impedance open stub. The two sections of the resonator are quarterwavelenth (i.e. λ o /4) lon at the central frequency of the filter and have impedances of and respectively. The impedances of input and output ports in Fiure below are assumed to be 5Ω. A pair of transmission zeros can be produced at the frequency f and f -f (f is the central frequency of the filter). ). By investiatin the positions (i.e. the frequencies) of the transmission zeros usin transmission line theory, the followin equation can be obtained as follows[9]: arctan with a dielectric constant of. and a thickness of.635mm is used. In order to improve the characteristics of the filter s stop band, it is desined by introducin two similar stepimpedance open stubs to both sides of a coupled-line stae. The step-impedance open stubs produce two transmission zeros near the pass band of the filter. As a result, sharp attenuations are obtained near the pass band. The sharp attenuation also ives required isolation between two passbands.[9].a classical formalism can be used to synthesize a nth-order filter. The resonators bein defined by a proper coefficient, the desiner has just to calculate the characteristic impedances of quarter wavelenth admittance inverters defined as follows [][9] where cj, j j, j G. b. a.. j j j, j j. j b. b G. b. a n n, n. n. n λ λ /4 λ λ /4 Port Port Fi Stepped impedance open stub bandpass filter where θ represents the electrical lenth at which the transmission zero occurs. f and θ should satisfy the followin relation: f f The above formula reveals that the positions of transmission zeros produced by the step-impedance open stub are only determined by the impedance ratio, /, of the two sections of the resonator. The line-width and lenth of the transmission-line stae are calculated while the electrical lenth of the stae is λ /4. In the followin desin, a substrate In these formulas, the coefficients j are the Tchebyscheff coefficients of the equivalent low-pass filter prototype. The parameter ω is the cut-off frequency of the low-pass prototype and G a and G b the terminatin conductances of the circuit. Finally, is defined as the fractional bandwidth. As we use an admittance inverter, the coefficients can be used as a dimensionless constant. In this way, a tunin parameter is thus introduced. This parameter can be chosen arbitrarily, mostly dependin on the achievable characteristic impedance of the technoloy in use. 3. Desin of BPF for GSM-9 By usin these formulas, we can able to calculate the impedance and electrical lenth of each stubs present in the BPF. For size reduction, here we desin a BPF with first order. Calculate impedance and electrical lenth value for first BPF(89-96 MHz).After Calculation convert those electrical parameter to physical parameter by usin inecalc which is provided by Ailent Technoloies Inc., Table. Section for BPF (89-96 MHz) Sement STEP IMPED Impeda nce enth 8.3 Ω 3.33 mm 6.5 mm Ω.455 mm mm 8.3 Ω 3.33 mm 6.5 mm 7

3 Volume 85 No 4, anuary 4 ANCE Ω.455 mm mm c Ω.965 mm 3.45 mm c Ω.965 mm 3.45 mm 3. Desin of BPF for DCS-8 The same procedure which is used in the first BPF is followed for second BPF with frequency band (7-88 MHz). Also, tabulate the physical parameter for second BPF is shown in Table. Table. Section for BPF (7-88 MHz) Sement s STEP IMPED ANCE Impeda nce enth Ω 3.6 mm 8.5 mm 3.5 Ω.465 mm 6.8 mm Ω 3.6 mm 8.5 mm 3.5 Ω.465 mm 6.8 mm c Ω 3.63 mm mm c Ω 3.63 mm mm Both BPF are desined usin Microstrip structure usin the substrate RT/Duroid 6 M which is havin dielectric constant as. and substrate thickness t is.635 mm. copper is conductor with thickness 7 µm. Consider the tanent value as(tan δ) is... Here we use RT/Duroid 6Msubstrate. RT/Duroid 66 is also available. This substrate provided by ROGERS Corporation.RT/Duroid 66/6M microwave laminates feature ease of fabrication and stability in use. They have tiht dielectric constant and thickness control, low moisture absorption, and ood thermal mechanical stability. 4. DESIGN OF MATCHING CIRCUIT The Matchin network and combinin circuit ensure that both filters match the antenna and have ood isolation between them. The T-junction may be the most popularly used combinin circuit. The lenth and width of its two branches must be chosen carefully. The T-shaped resonator is composed of a three-section transmission line. Each section has an adjustable characteristic impedance and lenth. The most intuitive approach is to combine two BPFs with a T-junction /Y-junction. The method, which features the Filter A (Filter B) an open-circuitload shunted to the Filter B (Filter A) at the frequency of the latter, is adopted to Fiure the dimensions of the connectin line for reducin the mutual loadin effect between the Filters. We calculate these three impedances by usin T- Matchin networks impedance formulas. Consider this as a CC network. [3][4] Select the desired bandwidth and calculate Q = f/bw Where f is operatin frequency and BW Bandwidth Calculate = Q. R Calculate Calculate c R c R Q( QR R ( Q R R ( Q /( QR ) ) C Calculate the inductance and capacitances f C f C and lenth of each impedance is calculated by inecalc Sement Y- unction Table 3. Section for Y-junction Imped ance ) enth a Ω.34 mm 8.99 mm b Ω.6875 mm 4.5 mm c.657 Ω 3.85mm 4.86 mm 5. DESIGN OF DIPEER The first step is desinin the first Bandpass filter to et the desired output. The Bandpass Filter is desined for the frequency band of MHz. The desin should ensure that the stop band lies on the low frequency band of the diplexer. For a sinle bandpass filter for GSM Band (89-96 MHz) the desired bandwidth (89 MHz-96 MHz) has iven return loss (S) of db at 95 MHz. The correspondin insertion loss (S) obtained is -.8 db at 89-96MHz. The isolation obtained at the stop band is -4 db at.7 GHz and -55 db at.35 GHz. The next step is desinin the second Bandpass filter to et the desired output. The Bandpass Filter is desined for the frequency band of 7-88 MHz. The desired bandwidth (7 MHz-88 MHz) has iven return loss (S) of db at.795 GHz.The resultant insertion loss obtained is nearly -. db at 7-88MHz.The isolation obtained at the stop band is -37 db at.5 GHz and -45 db at. GHz 8

4 Volume 85 No 4, anuary 4 Fi. Desin of Diplexer in ayout Window For desinin the Diplexer, we need to match these two BPF. T-unction/Y-junction resonator is used to desin the Diplexer. The section from the Table 3 is usin to desin the Y- junction resonator is shown in the Fi. The Diplexer is simulated for a frequency rane of.5 GHz to. GHz. All the scatterin parameters are simulated and their response is shown in Fi 3. The Matchin between the two BPF is the final step for this diplexer. There are several methods to interate a diplexer. The most important section is the T-junction. The T-junction m serves to divide the sinal of the entire bandwidth to appropriate bands.the lenth and width of the Diplexer is iven as enth= 7.3 cm; = 6.3 mm. The entire diplexer is simulated and the result is shown in Fiure 3. The observations of the simulated results are as follows:.the overall return loss at the low frequency band (89-96 MHz) is obtained as -3.6 db. The insertion loss has been reduced to -. db for the same band. The overall return loss at the Hih frequency band (7-88 MHz) is obtained as -.49 db. The insertion loss obtained for this band is.3 db. The hiher band is perfectly isolated from the lower band. The isolation obtained is more than -5 db. The specification has been achieved from the simulated results of the diplexer. 6. CONCUSION In this paper, the stepped impedance BPF for GSM band and DCS band were desined usin Microstrip separately and analyzed usin ADS 9.The Bandwidth is obtained for both GSM9(89-96 MHz) and DCS8(7-88 MHz) effectively. In order to et required inter band isolation, the BPF are desined and open circuited step impedance stubs were placed at the input and output. Because of the structure wide stopband and excellent interband isolation of above -5 was achieved. The insertion loss obtained is less than -.4 db. The overall return loss is less than -8.5 db. The two filters are interated to form a Diplexer which can be used in hih power applications. This interated system fulfillin all the specifications. It can also be used in dual band cellular networks, common antenna sharin and in-buildin coverae systems to share two channels usin a common antenna. m - - m4 oss, db m db(s(,)) db(s(,)) db(s(,3)) -5 m Freq=96.3 MHz db(s(,))=-.65 m Freq=.795 GHz db(s(,3))=-.38 m3 Freq=96.3 MHz db(s(,))=-3.67 m4 Freq=.795 GHz db(s(,))= Freq, GHz Fi. 3 Simulated result for diplexer structure. 9

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