диафрагмированные волноводные фильтры / cf2c14ef-3bfe-42b8-b3db-f6c6040b16bc

Proc. of Int. Conf. on Control, Communication and Power Engineering

Design and Development of Band Pass Filter at 94

GHz

Sanjeev Kumar Shah1, Vinay Negi2, Sandeep Singh2, Saurabh Tomar2 and Prachi Raizada2

1Research Scholar, Shri Venkateshwar University, Gajraula, Uttar Pradesh, India

2Uttaranchal Institute of Technology, Dehradun, Uttarakhand, India

{Sanjeev Kumar Shah1, Vinay Negi2, Sandeep Singh2, Prachi Raizada2 and Saurabh Tomar2,

sanjeevkshah}@yahoo.co.in

Abstract— Most papers [3, 4, 5] published on the design of band pass filter does not clearly defined the way of calculating GAP between resonating sections and width of resonating septum . Most paper [3, 4] either use analytical formula or use graphical analysis data [1, 2] for calculating resonator length,width and gap for it, which need rigorous algorithm for optimization to get the desired centre frequency and band width. This paper describes the design and development of band pass filter (BPF) at 94 GHz through well defined way of calculating the gap from transmission line ABCD parameter to S parameter conversion technique. This novel design uses capacitively coupled series resonators in suspended stripline configuration. This design is modeled and optimized using CST (Computer Simulation Technology) microwave studio with the help of proposed S- parameter extraction technique. The 94 GHz band pass filter exhibits an insertion loss of 1 dB with 3 dB relative bandwidth at a center frequency of 94 GHz and the return loss is better than -18 dB at a center frequency. The designed and fabricated 94 GHz band pass filter shows the good performance for planar integrated millimeter-wave circuits.

Index Terms— BPF, End coupled, Suspended stripline, Transition

I. INTRODUCTION

Low cost compact filters are important aspect of millimeter wave system. Almost all receiver systems require input filter for channelization of input signal. The basic objective in any filter design is that of achieving low insertion loss and VSWR within the required bandwidth while attaining the required out of band signal rejection [1]. Due to very less channel space available at 94 GHz, filter has been designed using capacitively coupled series resonators in suspended stripline configuration as shown in fig.1.

An Nth order filter of this form uses N resonant series sections of transmission lines with N+1 capacitive gap in between them. The resonators are approximately half wavelength at the center frequency [1]. Now Fig.1

(b) is redrawn with negative length transmission line sections on either side of series capacitors, which forms the equivalent circuit of an admittance inverter as shown in Fig.1 (c). The length ‘φ’ is λ/2 at fo. Electrical length θi of the ith section is given by;

The capacitive gap coupled filter has modeled as shown in Fig.1 (d). Procedure to get electrical length of resonating section length has been carried out in an innovative way which is very accurate at 94 GHz

© Elsevier, 2013

frequency. We need to derive [ABCD] and [S] matrix for J and K inverters which is used to calculate filter parameters.

Fig.1. Equivalent Circuit of End Coupled Resonator

The end coupled suspended stripline band pass filters are most promising due to its compact size, lightweight, low cost and ease of fabrication. In E plane circuit supporting dielectric (e.g. Fin line) causes additional losses. It may therefore often be advantageous to restrict the design for high Q millimeter wave circuit to pure metal inserts placed in the E-plane of rectangular waveguide without any substrate. The design specification for the above filter is given below.

II. SSLIN FILTER DESIGN METHODOLOGY

Design of band pass filters on RT Duroid substrate is presented in this section. End coupled lines shown in Fig.2, have been used to design the filter.

Fig .2. End coupled resonator

End coupled lines has been modeled with a series of J-inverters and transmission lines [1]. Conventional text book formulae are used to determine the J-inverter values from element values of the filter [2] & [3]. Equivalent circuit of the J-inveter is shown in Fig.3 .The S21 was calculated for each J-inverter value from [S] matrix of J inverter as derived in (2), (3) & (4).

ABCD matrix of J- inverter can be written as

cos( l), jZo sin( l)

ABCD , l /2&Yo J (2)jYo sin( l),cos( l)

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Fig .3. J-inverter and its equivalent circuit

III. DESIGN STEPS

1. Get J inverter values for the given specification. Steps to calculate J inverter values are given below.

A.Steps to calculate J inverter values:

(a)Determine the order of low pass prototype from following equation.

and determine n (no. of sections) from (5) such that L(dB) is greater than the required attenuation at desired frequency. Note that (6) is frequency transformation from band pass to low pass prototype. (b) Get element values (g0, g1, g2,……… gn,) for Tchebyscheff prototype for calculated ‘n’ and given ripple from text book [2].

(c) J inverter values for end coupled transmission line are given by following equations [2].

2.Calculate S21 for each J inverter values from [S] matrix of J inverter derived earlier in (4).

3.Now gap values (G) were calculated such that S21 obtained from CST microwave studio is equal to that of obtained from J inverter values. The simulation for the S-Parameters calculation should be done at highest possible accuracy. Lengths of resonating sections have been calculated by angle of S11 (from CST generated data for end coupled lines) with the help of following equations.

Where θ1 =|angle| of S11 corresponding to G1. θ2 = |angle| of S11 corresponding to G2.

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4. J inverter values and equivalent gap values (calculated from CST) for 5 section filter which give same S21 as of J-inverter, is provided in Table 1.

TABLE I. 5 SECTION RESONATOR FILTER

5.Once the gap values have been found, lengths of resonating sections have to be calculated. From (12), (13) & (14) L1 (Physical) = 1.1620 mm = L5 and L2 (Physical) = 1.1981 mm = L4. Similarly, L3 (Physical) = 1.1998 mm

6.The equivalent gap values (calculated from CST) for 5 section filter and optimized lengths of resonating sections provided in Table 2.

TABLE II. OPTIM IZED PARAMETER TABLE

Width of 50 Ohmline = λg/4 = 0.7325 mm β (propagation constant) = 2145.14697 rad/m

Cutoff frequency for 50 ohm line ≈ 103 GHz

IV. SIMULATED RESULTS

The filter is then modeled in CST microwave studio. The CST model and simulation results of filter are shown in Fig.4 and Fig.5 respectively.

Fig.4. CST Model of 5 section band pass filter

The simulation results shows pass band insertion loss of the order of 0.4 dB and return loss more than 35 dB and this results is better than previously published paper [3,4,5] in this millimeter wave frequency range. The obtained result does not need any optimization and center frequency also does not differ from the calculated value which saves significantly time for design of band pass filter in millimeter wave frequency. The same technique can also be used to calculate the E-plane Band pass filter data [4] with the help any EM software.

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