978-1-4799-0036-7/13/$31.00 2013 IEEE 237 36th Int. Spring Seminar on Electronics Technology

Design of Microstrip Band Pass Filter Based on LTCC for UWB Sensor System

Kornel Ruman1), Alena Pietrikova1), Igor Vehec1), Pavol Galajda2) 1) Department of Technologies in Electronics, Technical University of Kosice, Slovakia

2) Department of Electronics and Multimedia Communications, Technical University of Kosice, Slovakia kornel.ruman@tuke.sk

Abstract: This paper deals with design, simulation, manufacturing and experimental testing of microstrip bend pass (BP) filter for I Q (In-phase Quadrature) demodulator that is a part of Ultra Wide-Band (UWB) sensor system. Paper refers to the needs that should be focused on the design and manufacturing of microstrip filters based on LTCC (Low Temperature Co-fired Ceramic) with emphasis on issues of quality of transmitted signals in the high frequency (HF) area in the terms of actual production. There are presented simulated and measured results of insertion loss (S21) and return loss (S11) of microstrip BP filter for I Q demodulator made from material system Green Tape 951 XP. This paper demonstrates the design and full-wave electromagnetic simulation of microstrip BP filter using the HyperLynx 3D EM Designer (from Mentor Graphics). It assesses the suitability of LTCC material system Green Tape 951 XP as well as conductor paste DuPont 6145 for the production of microstrip BP filters for HF area. The presented filters should be used as a BP filter mean for I Q demodulator presented in [1] which is a part of laboratory UWB sensor system.

1. INTRODUCTION

The UWB system refers to a technology which uses the signals that occupy an ultra large bandwidth of frequency spectrum [2]. The Federal Communications Commission (FCC) defines a UWB device as any device where the fractional bandwidth is greater than 0.2 or which occupies the absolute bandwidth greater than 0.5 GHz. The rapidly growing field of UWB applications in various areas pushes the requests for the new enhanced UWB radar systems. One of the very promising solutions for the UWB device realization is based on so called M-sequence approach, where the operation is based on a special type of the M-Sequence presented in [3]. This UWB sensor system is simply expandable with I Q demodulator on side of receiver [4]. I Q demodulation is very useful if we need to obtain both the magnitude and phase of the received signal. The remarkable approach with an I Q demodulator is an easy and quick collection of all needed information by just measuring two voltages. Therefore, I Q demodulator is an important building block in RF (Radio Frequency) receiver with digital modulated baseband signals [1].

I Q demodulator consist from several parts: input BP filters; a pair of single chip, that contains wide-band mixer; delay line for 90 phase shift and output low-pass filters. The lack of relatively cheap, long-term stable and accurate filters represents frequently problem. The main purpose of the filter is to attenuate the unwanted frequency components which appear in the I Q demodulator spectrum. Sensor system is designed in order to match the ECC (Electronic Communications Committee) frequency bandpass (from 6 to 8.5 GHz). Therefore, received signals on I/Q demodulator inputs are necessary to take out by using BP filters. In present, for this purpose, I/Q demodulator is mounted by commercially fabricated band-pass LTCC filters BFCN-7900+ from Mini-Circuits Company [1]. Because the bandpass of this filter is only 0.3 GHz (from 7.8 GHz to 8.1 GHz) the operating bandwidth (2.5 GHz) set by ECC is not fully utilized.

For increasing of this operating bandwidth, design, simulation, manufacturing and experimental testing of microstrip BP filters with a bandwidth from 6 to 8 GHz and a minimum attenuation 40 dB in stop band, are the main challenging problem.

978-1-4799-0036-7/13/$31.00 2013 IEEE 238 36th Int. Spring Seminar on Electronics Technology

2. FILTER DESIGN

2.1. Substrate

In the HF (High Frequency) area, the dielectric properties of substrate have a major impact on the quality, stability and dimensions of the filter. Each substrate at the market has various dielectric parameters. This is the reason why not every substrate is proper for UWB area and choosing the appropriate substrates should pay close attention. Significant parameter in HF area is the value of loss tangent and dielectric constant and their stability in HF environment.

The progressive trends in the development of new materials for HF areas initiated the usage of LTCC ceramic for production accurate filters, thanks to its excellent stability and mechanical and dielectric capabilities. As substrate we chose LTCC material system Green Tape 951 PX that comprises a complete cofireable family of Au and Ag metallization, buried passives and encapsulants. Green Tape 951 is available in multiple thicknesses and is designed for use as an insulating layer in multichip modules, single chip packages, ceramic printed wiring boards and RF modules [6]. For correct design and simulation is necessary to know the physical and dielectric parameters of the substrate (Table 1).

Table 1. Specification of material system Green Tape 951 PX [6].

Property Units Typical Value

Unfired Thickness m 254 3

X, Y Shrinkage % 12.7 0.3

Z Shrinkage % 15 0.5

Surface Roughness m < 0.34

TCE (25 to 300 0C) ppm/0C 5.8

Density g/cm3 3.1

Camber (m/25 mm) 25

Thermal Conductivity (W/m.K) 3.3

Flexural Strength (MPa) 320

Dielectric Constant (3 GHz) 7.8

Loss Tangent (3 GHz) 0.006

Loss Tangent (10 GHz) 0.014

Low dielectric loss as typical property of LTCC material system Green Tape 951 allows using them in many applications where at high operating frequency are conventional laminates circuit boards limited. The

reason is that value of substrates relative permittivity influences the capacity of microstrip line and thereby its impedance and scattering parameters.

2.2. Metallization

The line metallization as well as width design is necessary to be considered because improper design of transmission line material and width can cause reflection. For correct design and simulation is necessary to know also the type and parameters of used metallization (Table 2). For simulations of microstrip BP filters we use parameters of conductor paste DuPont 6145. DuPont 6145 is an external solderable cofireable silver conductor compatible with LTCC system Green Tape 951 [7] that is distinguished with high conductivity (quality of signal transmission).

Table 2. Specification of DuPont 6145 [7].

Property 6145

Viscosity (Pa.S) 120 - 200

Dried Line Resolution (m) line/space 125 / 125

Fired Thickness (m) 18 - 25

Fired Resistivity (m/sq) < 3

Dupont 6145 is ideally suited for applications requiring excellent conductivity.

2.3. Simulation

Study correlation between measurements and simulations of microstripe BP filter play important role. Advances in CAD (Computer-Aided Design), such as full-wave EM (Electromagnetic) simulators did coup in design of filters. For design of BP filters was used HyperLynx 3D EM Designer software, which facilitated the mentioned filters based on LTCC [5].

This filter is designed as a microstrip BP filter which role is to take out the required frequency band from the whole spectrum of signals propagating in a free space. The selected frequency band must meet the requirements of the organization ECC, e.g the microstrip BP filter will be designed to take out only the spectrum from 6 to 8 GHz. Very important parameters in simulation are microstripe line dimension, directions, angles, width of lines, etc. that are dependent on dielectric characteristics of substrate. Based on result of comparison of various

978-1-4799-0036-7/13/$31.00 2013 IEEE 239 36th Int. Spring Seminar on Electronics Technology

possibilities for the shape of microstripe BP filter (Microstrip Gap-Coupled Bandpass filter, Microstrip Parallel-Coupled Bandpass filter, Microstrip Hairpin Bandpass filter, etc.) that offer CAD software we decided for hairpin type of structure.

Fig. 1. 3D layout of a six-pole hairpin microstrip BP filter.

The Fig. 1 shows the structure of six-pole microstrip BP filter, which is designed as a cascade of parallel resonant circuits, among which is also capacitive coupling. Resonant circuits are realized using half wave U-resonators (also called hairpin resonators) and capacitive coupling through their mutual distance [8].

The simulation must calculate with shrinkage of LTCC ceramics and conductive paste because material system Green Tape 951 doesnt have zero shrinkage after firing process in axis x, y and z (Table 3). Shrinkage of ceramics plays a major role since results of simulations are calculated for dimension of BP filter after firing. The hairpin lines width and spacing was set at 100 m and width between the hairpin legs (hairpin gaps width) was set to 749 m. The filter design dimensions were optimized to meet the specifications in the pass band.

Fig. 2. Simulated results of insertion loss and return loss of

microstrip hairpin BP filter.

The Fig. 2 shows simulated results of Scattering parameters (S parameters) after planar EM analysis of

microstrip BP filter. In this simulation, we verified the suitability of the ceramic material Green Tape 951 for the realization of this filter. Simulated transmission characteristic of BP filter satisfied initial condition.

The corner frequency shifted from 6.0 to 6.3 GHz and from 8.0 to 7.8 GHz is shown in Fig. 2. This figure also shows that designed hairpin band pass filter meets the minimum attenuation of 40 dB with the corner frequency shift of 1 GHz in suppress band.

The BP filter input and output are matched to 50 ohm characteristic impedance. The 50 ohms choice is a compromise between power handling capability and signal loss per unit length, for air dielectric.

3. RESULTS

3.1. Filter Construction

We manufactured three kinds of BP filters each with different scaling factor (13, 16 and 20 %) for prediction of shrinkage effect of material system Green Tape 951 (Table 3) and conductor paste DuPont 6145 (Table 4).

Table 3. Scaling factors of microstrip BP filters and shrinkage of LTCC system Green Tape 951.

Samples BP filter 1 BP filter 2 BP filter 3

Number of samples 32 16 16

Scaling factor [%] 13 16 20

Axis x y z x y z x y z

Dimension before firing [mm] 13.55 22 0.25 13.7 22.2 0.25 14.2 23 0.25

Dimension after firing (average) [mm]

11.5 18.9 0.22 11.6 19.1 0.22 12.1 19.8 0.22

Difference [mm] 2.05 3.1 0.03 2.11 3.08 0.03 2.12 3.18 0.03

Shrinkage (average) [%] 15.13 14.1 13 15 13.9 13 15 13.8 13

From the Table 3 we can see that shrinkage of material system Green Tape 951 is bigger in axis x, y and smaller in axis z as DuPont specified in technical data sheet (Table 2). These differences between specified and measured values of shrinkage affected result of measured S parameters (S11 and S22).

The shrinkage of conductor paste DuPont 6145 was tested on two straight lines with different width (0.125 and 0.130 mm).

978-1-4799-0036-7/13/$31.00 2013 IEEE 240 36th Int. Spring Seminar on Electronics Technology

Table 4. Shrinkage of conductor paste DuPont 6145.

Samples 1 2 Number of samples 30 30 Line width before firing [m] 125 130 Line width after firing (average) [m] 99.8 103 Difference [m] 25.2 27 Shrinkage [%] 20.16 20.76

We can see that conductor paste DuPont 6145 has in combination with material system Green Tape 951 shrinkage around 20 % (Table 4).

Quality and thickness of the print depends on many screnn printing factors (printing pressure, printing speed, thickness of the emulsion, characteristic of the cloth, etc.). Very important role plays firing conditions as well. The roughness on boundary of transmission lines influences quality of transmitted signals in HF area. Thats because the roughness can cause reflection of transmitted signals what is leading to the signal attenuation. To achieve fine line of BP structure characterized by homogeneity of thickness, without roughness of the surface and line edges we apply 400 mesh cloth screens.

The area of microstrip hairpin BP filter itself is approximately 4.25 by 6 mm at the thickness of LTCC 0.25 mm. The final prototype of BP filter including pads for SMA (Sub-Miniature version A) connectors (Fig. 3) has dimensions (W/L/H) 11.6 x 19.1 x 0.25 mm3.

Fig. 3. Final prototype of microstrip hairpin BP filter with

SMA connectors.

3.2. Measurement of Scattering Parameters

The insertion and return losses were measured using the Rohde & Schwarz vector network analyzer. Comparisons of simulated and measured insertion loss of three different miscrostrip BP filters (Table 3) are showed in the Fig. 4.

Fig. 4. Comparison of simulated and measured insertion

loss of different microstrip hairpin BP filter.

As we can see the transmission characteristic in pass band of all microstrip hairpin BP filters move down to value 10 dB. The transmission characteristics oscillated up and down between values 10 and 19 dB. These oscillations are caused by difference between specified and measured value of shrinkage in axis z (Table 3). This difference caused that dielectric (substrate) height is bigger as we use for calculation lines width for matching input and output to 50 ohm characteristic impedance.

Microstrip BP filter 1 and 2 doesnt meet with simulated results. The corner frequency of Microstrip BP filter 1 shifted from 6.3 to 6.5 GHz and from 7.8 to 8.3 GHz. The corner frequency of Microstrip BP filter 2 shifted from 6.3 to 6.7 GHz and from 7.8 to 8.5 GHz. These filters also do not meet the minimum attenuation of 40 dB with the corner frequency shift of 1 GHz in suppress band.

Microstrip BP filter 3 achieved the best results from the compared filters (Fig. 4). The insertion loss meets with simulated results (bandwidth from 6.3 to 7.8 GHz). Filter fit to the minimum attenuation of 40 dB with the corner frequency shift of 1 GHz in suppress band. It is possible to use this BP filter in UWB devices such as IQ demodulator presented in [1] or part of system for Through Wall Moving Target Tracking by M-sequence UWB Radar presented in [9].

Comparisons of simulated and measured return loss of three different miscrostrip BP filters are in the Fig. 5.

978-1-4799-0036-7/13/$31.00 2013 IEEE 241 36th Int. Spring Seminar on Electronics Technology

Fig. 5. Comparison of simulated and measured return loss

of different microstrip hairpin BP filter.

The return loss of all measured microstrip hairpin BP filters doesnt match with simulated results. But this problem was also caused by difference between specified and measured value of shrinkage in axis z (Table 3).

4. CONCLUSION

The microstrip hairpin BP filters made from LTCC material system, Green Tape 951 PX, in combination with conductor paste DuPont 6145 were designed, simulated, constructed and tested. Comparison between simulated and measured results of insertion loss and return loss was made and the differences in result of comparison were caused by shrinkage of material. We have demonstrated that shrinkage of material system Green Tape 951 in axis x, y and z is different as DuPont specified in their technical data sheet. We point out that conductor paste DuPont 6145 has in combination with material system Green Tape 951 shrinkage around 20 %. We achieved the best results with Microstrip BP filter 3 (scaling factor 20 %) which confirm 20 % shrinkage factor. Transmission characteristic of Microstrip BP filter 3 is acceptable for use in I Q demodulator that should be a part of our UWB sensor system. We find out that using of material system Green Tape 951 is a bit problematic for HF application from the technological point of view.

ACKNOWLEDGEMENT

This paper was developed with support of the project "Centrum excelentnosti integrovanho vskumu a vyuitia progresvnych materilov a technolgi v oblasti automobilovej elektroniky", ITMS 26220120055, that is co-financed from Structural Funds EU ERDF within Operational

programme Research and Development OPVaV-2009/2.1/03-SORO and preferred axis 2 Support of Research and Development.

REFERENCES

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[2] FCC, Revision of part 15 of the commissions rules regarding ultrawideband transmission systems, First report and order, ET Docket 98- 153, FCC 02-48, Feb. 2002, pp. 1118.

[3] Sachs J., Peyerl P., A New Principle for Sensor-Array-Application, Proceedings of 16th IEEE Instrumentation and Measurement Technology Conference, pp. 13901295, 1999, IMTC/99 Venice, Italy.

[4] J. Sachs, M. Kmec, R. Zetik, P. Peyerl, and P. Rauschenbach, Ultra wideband radar assembly kit, Geoscience and Remote Sensing Symposium, 2005, iGARSS 05. Proceedings. 2005 IEEE International.

[5] KLMA, M.; SZENDIUCH, I. Nvrh 3D struktur realizovanch na LTCC substrtech pomoc programu HYDE. In MIKROSYN. Nov trendy v mikroelektronickch systmech a nanotechnologich. Brno: Novapress, 2011. s. 63-68. ISBN: 978-80-214-4405- 8.

[6] Dupont Microcircuit Materials, Design and Layout Guidelines, Available on the Internet: 4.1.2013; http://www2.dupont.com/MCM/en_US/assets/downloads/ prodinfo/GreenTape_Design_Layout_Guidelines.pdf.

[7] Dupont Microcircuit Materials, Technical Data Sheet Dupont 6145, Available on the Internet:,4.1.2013, http://www2.dupont.com/MCM/en_US/assets/downloads/prodinfo/6145.pdf.

[8] Jia-Sheng Hong, "Microstip Filters for RF/Microwave Aplications," John Wiley & Sons, Inc., Hoboken, 2nd edition, New Jersey, 2011, 655.

[9] D. Urdzik, D. Kocur, and J. Rovnkov, Detection of multiple targets with enhancement of weak UWB radar signals for the purposes of through wall surveillance, in Applied Machine Intelligence and Informatics (SAMI), 2012 IEEE 10th International Symposium on, jan. 2012, pp. 137 142.

[10] N. Codreanu, C. Ionescu, P. Svasta, I. Plotog, "Accurate 3D modelling and simulation of advanced packages and vertical stacked dice", 2nd Electronics System-Integration Technology Conference, Greenwich, London, UK, 1 - 4 September 2008, pp. 857-861, vol. 2, ISBN 978-1-4244-2813-7.

[11] M. Pantazic, N. Codreanu, Multi-media DFM Course for Design of Electronic Modules/Microsystems, 15th International Symposium for Design and Technology of Electronics Packages SIITME 2009, Gyula, Ungaria, 17-20 September 2009, pp. 30-31.

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