The Design and Implementation of Beidou Satellite Navigation System RF Front-End Filter

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118 International Journal of Computing Academic Research (IJCAR) ISSN 2305-9184 Volume 3, Number 5(October 2014), pp. 118-125

© MEACSE Publications

The Design and Implementation of Beidou Satellite Navigation System RF Front-End Filter

Guoping Chen,Yi Zhang,Jie Changand and Yikun Luo

Lab of Electrician theory and new technology in Chongqing University of Posts and Telecommunications, Chongqing, China


In the RF(Radio Frequency) front-end of Beidou navigation system, the high precision filter which filtering some noise of receiving signal plays an important role for the whole system. The designed Filter in this paper is based on the theory foundation of parallel coupled microstrip line band-pass filter which combine the designing method of the traditional filter with microwave circuit simulation software ADS2009(Advanced Design System) and HFSS2013(High Frequency Structure Simulator), calculated the design parameters of microstrip line size and completed the simulation of ideal model in ADS, and then set up the 3D model in the HFSS to carry on the simulation which close to the actual production condition.

Furthermore, modified and adjusted the design parameters to ensure that the design of radio frequency narrowband parallel coupled microstrip line band-pass filter circuit can conform to the project requirements of the ideal filter. At last analyzed the relevant simulation results and making simple thermal transfer microstrip line circuit board to test effect. From simulation figure it can be seen that this filter has a high precision, simple design, easy to make, small bandwidth and many other advantages, the design and implementation method possess greater project value.

Keywords:Beidou RF front-end; parallel coupled microstrip line filter; ADS; HFSS Introduction

After the US global positioning system GPS and the Russian GLONASS, China Beidou satellite navigation system is a regional active three-dimensional satellite positioning communication system designed and developed by ourself according to own technological development and the need to achieve timing, quick navigation, two-way short message communications and other functions as the goal, its superior performance plays a huge role and possess strategic significance in all areas of the national economy, rescue and military. Futhermore, the receiver and transmitter modules to achieve distortion-free transmission of information in communication systems are indispensable, since the received signal of front-end module contains a lot of noise, we must filter out unwanted interference by radio frequency bandpass filter, so the importance about the performance of the front-end filter for the whole receiver signal processing is self-evident. In this essay we study the high-performance RF filters with important significance and broad prospect in Compass ground terminal receiver[5].

In this paper, section 2 introduced the principle of parallel coupled microstrip line, in section 3, we first determined the filter parameters through the setting of the parameters of the Beidou navigation signal, and designed a parallel coupled microstrip line band-pass filter with center frequency of 2.49175GHz, then showed the result analysis of simulation experiments in HFSS and ADS. Furthurmore, section 4 illustrate a



Simple physical filter produced by FR4 to validate the performance. finally, some conclusions about presented technology are drawn in section 5.

1. Parallel Coupled Microstrip Units And Transmition Characteristics

The basic unit of the bandpass filter is parallel coupled section constituted by two closely spaced microstrip line, the odd-mode and even-mode of microstrip line can produce the generation of the odd-mode characteristic impedance (Z ) and even-mode characteristic impedance (0 o Z0e) through the coupling effects of public access floor. When the length of the microstrip line is 1/4 of the wavelength corresponding to the center frequency of filter, the microstrip lines will have a band-pass filter characteristic. As a result of a single band-pass filter unit can not get a good filter response and steep transition from passband to stopband, so we often connect the n+1 parallel coupling sections to form a parallel-coupled microstrip line bandpass filter. This geometry is shown in Figure 1, it comprises dielectric layer and microstrip lines, the thickness of dielectric layer is h and relative dielectric constant is Er(relative permittivity)[7].

Fig.1 the structure of Parallel coupled microstrip line. In the figure, W represent the width, S is the distance between two microstrip line, L is the length, ΔL is the reduced length

2. The Design Of Compass RF Front–end Filter 2.1 The parameters require of filter design

Beidou navigation system specify that transmiting navigation signals in L-band and receiving in S-band, its RF receiver module assembly works at S-band.

Therefore, we determined the center frequency of microstrip filter is 2.49175GHz, passband bandwidth is 100MHz, insertion loss is 1dB, ripple of in-band is 3dB, out-band rejection is greater than or equal to 30dB (in 2.46675GHz and 2.51675GHz), characteristic impedance is 50Ω[4]. The designed filter needs to try to meet the above parameters in order to ensure high performance of the entire system.

2.2 The design steps of RF bandpass filter (1) Calculate the normalized frequency

The transforming relationship between frequency of bandpass response and normalized frequency of low-pass prototype is under:





  0

0 0

0 1 2

' 0 1

. (1)

Where, 2 1


  is the relative width of passband, 0 is the center frequency of passband,  is a

known frequency points, ' is a frequency corresponding to the low-pass prototype.



We would obtain result by bring the parameters into Equation (1): '1.614

(2) Determine the order

Due to we have determined the ripple factor is 0.5dB before, through Inquiry the attenuation characteristics table of Chebyshev filter, we could know the order of filters: N = 5.

According to look-up table, it would shows the corresponding component parameters for the low-pass prototype filter: g0g6 1, g1g5 1.1468, g2g4 1.3712, g3 1.9750.

Meanwhile it could be obtained that equivalent circuit of low-pass prototype filter with normalized values shown in Figure 2:

Fig.2 equivalent low-pass prototype model (3) Calculate the odd-mode and even-mode characteristic impedance[1]

The normalized value of the bandwidth BW is:

4.013 10 3

u l



w w w

According to the following parameters formula(2):


0 0 1

1 2

J z g g


, 1

0 1


N N 2


J z g gBW

, 1

0 1


i i 2

i i



z g g

. (2) We can got as:

0,1 3,4 0.001258



1 , 2 2 , 3 0 . 0 0 0 0 9 5 2 8 8



. (3) Meanwhile through the formula about odd-mode and even-mode characteristic impedance shown the following (4):

0 , 1 0 , 1 2

0 1

,i  1 ii ( ii )


od Z Z J Z J

Z Zoe i,i1Z0


. (4) We brought the (3) into (4) and obtained what we need.

The odd-mode characteristic impedance is:

0 /0,1 0 /5,6 38.34

o o



0 /1,2 0 /4,5 44.52

o o



0 /2,3o 0 /3,4o 45.68



The even-mode characteristic impedance is:

0 /0,1 0 /5,6 75.35

e e



0 /1,2e 0 /4,5e 57.05





0 /2,3e 0 /3,4e 55.23



(4) Calculate the width and length of microstrip lines and distance between parallel coupled units

Enter the known odd-mode and even-mode characteristic impedance before into the ADS LineCalc to calculate the width and length of the microstrip lines and the distance between the coupling units, as shown in Fig.3. Here we need to set the parameters of microstrip line circuit board, as currently GD FR4 circuit board is used, so the board parameter is set as:

4.6mm H, 1.6mm T, 0.025mm

r   

Here, r indicates the relative permittivity of the board, H showed the thickness of PCD plate, T represents the thickness of the microstrip line.

Fig.3 using ADS to calculate the parameters of microstrip line

Through the calculation in software of figure 3, W, S and L about microstrip lines can be obtained, the values would be shown in Table 1.

Table 1 the values of W, S and L

numbers W S L

1 and 6 coupled units 2.045346mm 0.153453mm 16.0823mm 2 and 5 coupled units 2.634535mm 0.953421mm 16.4213mm 3 and 4 coupled units 2.714233mm 1.323135mm 16.0846mm 2.3 The analysis of simulation results

Because of the above calculation about the certain length, width and distance of parallel microstrip line coupling units, now we can do the model of simulation and optimization in the ADS to test its performance[2, 3].

Figure 4 is the filter simulation model in ADS software principle plate, drawing totally six sections of microstrip coupling unit to constitute a fifth-order connection filter, while the medium parameters and sweep parameters are set as required. These results of model simulation and optimization are shown as figure 5, we used S(2,1) parameter to watch and represent the filter Performance, it can be seen that the average



transmission attenuation is 2.356dB in pass-band (2.48675-2.49675GHz), the ripple is less than 3dB, rolloff outside pass-band is relatively obvious, this illustrates that the design about filter could meet our requirements.

Fig.4 model of filter simulation and optimization in ADS

m1 freq=

dB(S(2,1))=-1.9432.500GHz m2



m3 freq=


2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

2.0 3.0

-120 -100 -80 -60 -40 -20

-140 0

freq, GHz


Readout m1

Readout m2

Readout m3

m1 freq=

dB(S(2,1))=-1.9432.500GHz m2



m3 freq=


Fig.5 results of filter simulation and optimization in ADS

Because the accuracy of parameters simulated by ADS is relatively low and have a big gap with the reality in the actual territory production, so we would use another microwave simulation software HFSS to verify filter performance and optimize the values of parameters[6]. Making use of length and width about the front designed microstrip lines and distance between the coupling units as well as some other parameters to establish a three-dimensional(3D) model in HFSS as Fig.6. In the picture, under a vacuum environment, the microstrip line coupling units attached to the surface of the PCB board, we could see that the simulation model in HFSS and the actual testing project circuit board are very similar, so the results would be also more accurate.



Fig.6 model of filter simulation and optimization in HFSS

Figure 7 shows the results of simulation and optimization, we can see that through setting the length variable, width variable of microstrip lines and distance variable between them to find the most appropriate configuration parameters in different ranges, the simulation consequent has improved significantly, the center frequency is on the 2.49GHz, bandwidth is closely 100MHz, while the transmission parameter S(2,1) in passband is within 3dB and meet the design requirements of the filter.

1.50 1.75 2.00 2.25 2.50 2.75 3.00

Freq [GHz]

-100.00 -80.00 -60.00 -40.00 -20.00 0.00



XY Plot 1 m1 ANSOFT


Curve Info dB(St(p1,p1)) Setup1 : Sweep

dB(St(p2,p1)) Setup1 : Sweep Name X Y

m1 2.4900 -1.4855 m2 2.4900 -17.5157

Fig.7 results of filter simulation and optimization in HFSS

As we all know, the calculated parameters before are only approximate values, through the optimization we can get the more satisfied and more closer values shown in Table 2.

Table 2 the values of W, S and L after optimizating

numbers W S L

1 and 6 coupled unit 1.5523mm 1.1254mm 15.7214mm 2 and 5 coupled unit 2.6314mm 2.7452mm 15.8325mm 3 and 4 coupled unit 2.7223mm 3.5234mm 15.7642mm 3. Actual Circuit Board And Error Analysis

Based on the principles of microstrip filter put forward in the front, we made use of the thermal transfer technology to produce a simple Compass front-end fifth-order parallel-coupled microstrip bandpass filter, figure 8 is the actual circuit board with two SMA connectors.



Fig.8 Physical Fifth-order parallel coupled microstrip filter

Testing it on a vector network analyzer, the performance results has been shown in Figure 9, from the picture it can be seen that the actual performance and simulation performance are roughly consistent, the center frequency of filter can meet the demand(2.49GHz), the curve about S(2,1) is Precipitous and bilaterally symmetrical. However, there are also some errors, the bandwidth is slightly bigger than the simulation, and insertion loss in the passband reaches 25dB. The factors caused these errors are diverse, such as the length, width and distance about the microstrip line on circuit board are difficult to achieve a high accuracy, there may also exist many samll holes on the microstrip lines to affect transmition when the board is cauterized by sulfuric acid. Moreover, the dielectric constant of the dielectric plate is unknown, etc.

Meanwhile, we must think and reserch the most important reason—tanδ(dielectric dissipation factor), due to the tanδ of practice general FR4 is usually 0.018 in the case of high-frequency[8], as shown in figure 10, (a) displays that the in-band insertion loss is less than 1dB when the tanδ is zero, but (b) shows the in-band insertion loss becomes 20dB when the tanδ equal 0.018. By comparison, we know that tanδ will largely increase the transmission loss under the high-frequency, most of the insertion loss tested by the vector network is derived from this, so if we can use a better board to make this filter under more precise environment, its performance would be more excellent and the accuracy would be higher to be fully able to meet the experimental or industrial demands.

Fig.9 Testing Results of actual circuit board



2.2 2.4 2.6 2.8

2.0 3.0

-100 -50

-150 0

freq, GHz


Readout m1

m1 freq=

dB(S(2,1))=-2.130 2.490GHz

m1 f r e q =

d B ( S ( 2 , 1 ) ) = - 1 8 . 9 9 9 2 . 4 9 0 G H z

2 . 1 2 . 2 2 . 3 2 . 4 2 . 5 2 . 6 2 . 7 2 . 8 2 . 9

2 . 0 3 . 0

- 1 2 0 - 1 0 0 - 8 0 - 6 0 - 4 0 - 2 0

- 1 4 0 0

f r e q , G H z


R e a d o u t m1

m1 f r e q =

d B ( S ( 2 , 1 ) ) = - 1 8 . 9 9 9 2 . 4 9 0 G H z

(a) tanδ=0 (b) tanδ=0.018 Fig.10 the impact of in-band insertion loss caused by tanδ(dielectric dissipation factor) 4. Conclusions

In this paper, according to the principle of parallel-coupled microstrip line bandpass filter, applying the signals’ structure requirements of Beidou navigation system as the design conditions, and combining the traditional filter design methods and the design methods to use microwave circuit simulation tools (ADS and HFSS), we design a parallel coupled RF bandpass filter which can be used in the front-end of Compass receiver. Then the design model and simulation results are given, while being analyzed. Numerical results show that the parallel-coupled bandpass filter designed in this way not only has achieved targets, comparing to conventional filters, but also has many advantages such as the design workload and complexity can be greatly reduced, higher precision, low bandwidth, requirements of production environments are not demanding, etc. so this filter has a good prospect in Beidou navigation system.


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