Band Pass Filter Design

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Microwave Band Pass Filter Design

Najeeb Haider Zaidi

Lecturer, Engineering Faculty,

PAF-KIET PAF Airbase Korangi Creek, Karachi, PakistanMobile: +92(0)3333126419

Email: [email protected]

Chief Executive Officer, eLink Systems,

Suite No.6 Sands Appartments, Clifton Block 2, Karachi, Pakistan.

Phone No.(+9221)5863769, (+9221)5824754.Email: [email protected]

Abstract:

Band pass filters are essential part of any signal processing or communication systems,

the integral part of superhetrodyne receivers which are currently employed in many

RF/Microwave communication systems. At Microwave Frequencies the discrete

components are replaced by transmission lines, for low power applications microstrip are

used which provide cheaper and smaller solution of Band Pass Filter. This article is an

effort to document the designing steps of a microstrip Band Pass Filter. The specific

design discussed in the article in detail is know as Parallel Coupled Filter. In this article

an example of a filter specs is given over which a filter is designed by using certain tools.

Introduction to the Microwave Band Pass Filters:

This section of the article describes about the design of the microwave Bandpass filter by

using microstrip technology. There are many possible techniques used to create

microstrip filters, some of them are listed below;

Combline Filters: For Frequencies below 10GHz

Interdigital Filter: A broadband solution for the frequencies above 8GHz
mailto:[email protected]:[email protected]


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Parallel Coupled and edge coupled Filters: small sized, Light weight and low cost

filters for narrow bandwidth applications.

In this article the design discussed as also mentioned earlier is known as Parallel

Coupled Filters, due to their easy and cheaper design (Like they doesnt require any via,

as in the case of Combline and Interdigital) and greater immunity to errors. The filter was

designed in Agilent ADS, the design was implemented on a FR4 substrate and the final

testing was done at RF VNA. The response and the steps involving the design process are

discussed in the section.

Parallel Coupled Bandpass Filters:

This technique involves the parallel coupled transmission lines to construct many types of

filters. It is quite easy to fabricate multisection Bandpass coupled line filters, for

bandwidth less than 20%. This design cannot be used for wide bandwidth because higher

bandwidth requires small spacing between the coupled lines which is difficult to fabricate

[2]. These filters are mainly known for their low cost, small size and light weight. The

general layout of Parallel Coupled Band Pass Filters is shown in Figure 1.

Designing of the Filter:

Desired Specifications:

Center Frequency: 5.25 GHz

Chebyshev response

In band ripple :< 0.01 dB

Bandwidth 250MHz

Roll off: loss>20dB at 4.04GHz


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Port Impedances: 50

Substrate: FR4

r=4.4

h=1.6mm

Connectors: SMA

The designing of the Parallel coupled Microwave Filter comprises on four different steps

which are discussed below in details.

Step 1: Finding the fractional Bandwidth,

= (f2 - f1)/fo

=47x10-3

Step 2: Examining the filter Prototype specifications for 0.01dB ripple

By using the following equation we can find out the transformation ratio,

wi/wc = 2(fi - fo)/fo=-9.66

For the value of the transformation ratio and ripple factor of 0.01dB, the vale of N or the

order of the Chebyshev filter found out is 3 (by using prototype graph can be found in

many filter handbooks but in this particular case [1] is used) but in order to make the

circuit more reliable, I preferred the fifth order filter N=5. Following are the coefficients

for the 5th order Chebyshev filter.

g0 g1 g2 g3 g4 g51.0 0.756 1.305 1.577 1.305 0.756


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Step 3: Calculations of the inverter admittances and hence coupled-line impedances for a

50 system.

Following equations were used to calculate admittance inverter parameters

Jn,n+1/Yo= sqrt(/2gngn+1)

In order to find the coupled line Impedances following formulae are used

Zoe(n)=Zo(1+ Jn,n+1+ Jn,n+12)

Zoo(n)= Zo(1- Jn,n+1+ Jn,n+12)

The calculated values are shown in the following table.

Step 4: Calculation of the widths and spacing between the coupled lines

Line Calc tool of Agilent ADS was used for this purpose the electrical length of the

line is /4 while width can be found out by the tool by simply entering the values of odd

and even impedances.

j Ji,j+1/Yo (Zoe)j,j+1 (Zoo)j,j+1

0 0.315 70.67 39.221 0.075 54.05 46.522 0.052 52.74 47.53


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Design of the System:

The values, widths are slightly changed than the calculated ones due to optimization in

the software, in order to improve the simulated response; the design schematic of the

system is shown in Figure 2. Agilent ADS is used for the system designing and

simulation, as this software includes the optimization features, so the lengths of the

microstrip are optimized slightly in order to get the simulated response right.

Simulated Response:

Simulated response of the design came up to be a good Bandpass filter response,

following all the desired specifications, Figure 3 shows the response of the filter while

simulated, the thing to note over here is that the ripple factor objective is achieved quite

well over here, there is a slight ripple of 0.01 dB between m3 and m2, the rest of the band

is ripple free. Apart from that there is a peak at about 4.88GHz which is upto -2 dB, but

the thing is that after implementation such peaks becomes too small to be considered if

the calculations are done properly, so at the designing phase such peaks can be neglected.

Response at Vector Network Analyzer:

The system was implemented after the design, on FR4 dielectric plate, conductor at one

side of the plate was left un-etched while the conductor at the other side was etched to

make the microstrip lines. Here, at this point I would like to give a slight introduction of

VNA or Vector Network Analyzer.

VNA is a measuring instrument which when is plugged to the circuit input and output,

inputs a series of frequencies under the specified range and then calculates and presents

the S-parameters of the system. Now here in this case since we are observing the S-


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Parameters of the a Vector Network Analyzer, so there are only two parameters of our

concern, we can ignore the rest of the two parameters, the forward gain (S21) and the

input reflections (S11) the rest of the two parameters the reverse gain (S12) and the

output reflections were too low to be displayed in comparison of the other values.

Figure 4 shows the response the system gave at VNA. The forward gain was supposed to

be at 0dB as in the simulated results but the problem with definitely their lies difference

between the simulated results and the results after the implementation most of the time,

so as in this case, the reasons are discussed earlier. Another thing is the peak at about

4.88GHz is still visible at the similar frequency but its intensity is decreased quite a lot,

here it does not seem much dangerous as it is in the simulated results.

Comparison between the Simulated and Obtained (VNA) Results:

The S21 shows quite reasonable narrowband Bandpass response but the with the

7dB loss, the bandwidth is quite decreased as compared to the simulated results.

The major change is the significant increase in the input reflection coefficient S11

especially at the start of the band. Upto around 5.2GHz it is showing an improper

response but after that it is showing nice difference between S21and S11.

In both of the responses the Roll off loss >20dB at 4.04 GHz.

It seems that if the S21 is slightly shifted (increased) with respect to the frequency.

Sources of Errors:


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Initially, the dielectric constant of FR4 varies a lot from batch to batch, which

luckily didnt varied a lot in our case resulting in a slight shift in the response

frequency as pointed in the 4th point of the Comparison.

Secondly, the loss tangent of FR4 is quite high, so it can produce around 4 dB of

the loss at the output.

The third major error is generated by the right angled SMA connectors, which

doesnt give better response until you pay much for them.

As far as designing process is concerned, the first mistake in the designing

process, is the absence of discontinuities in the design schematic, the

discontinuities should be included so that the system could be optimized for the

best response along with the discontinuities between the parallel coupled lines.

The values of the width were varied slightly by optimization tool, with respect to

the measured ones in order to get the layout and the response right.

The SMA connectors are needed to be tightened properly in order to get the exact

50 input and output impedances this would be helpful in order to get the desired

value reflection coefficients.

The surface of the microstrip lines was not fine and smooth that can generate

some errors as well.

As far as the attenuation in the forward gain is concerned upto 6.5dB is due to the 2nd and

the 4th point and the rest of the error we are left with is sourced by the rest of the points

discussed in the section. The Input reflections are mainly due to the improper connections

of SMA connectors which were not matched precisely.

Applications of the System:


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Due to its physical characteristics like small size, light weight and low cost, it can

be used in variety of applications especially in the field of RF/Wireless

Communications. Figure 5 shows the actual snapshot of the Bandpass filter

designed.

Since the design is quite simple, so it can be made in a batch process, decreasing

the cost even more.

It can be easily placed with other sections on the same board.

Conclusion:

From the analysis, the conclusion that can be drawn is that parallel coupled line Filters

are easy to design and implementation of them is far easier that their designing. As far as

attenuation is concerned the system is equated with the errors, the original test was of the

wideness of its band, which comes out as a very narrow, not wider as we were expecting

it to be.

References:

1. TC Edwards and M B Steer, Foundations of Interconnect and microstripdesigns ,3rd edition, Chichester: John Wiley, c2000

2: David M Pozar, Microwave Engineering, 2nd Edition, Chichester; NY: Wiley,

c1998.

3. Editors: I.Kneppo and J.Fabian Microwave Technology; Microwave Integrated

Circuits, Series 8, London, Chapman & Hall, 1994.

4. S.R Pennock and P.R Shepherd Microwave Engineering with Wireless Applications,

Basingstoke; Macmillan, 1998.


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5. Ralph Levy, Richard V.Snyder and George Matthaei Design of Microwave Filters

IEEE Trans, Microwave Theory and Techniques, Vol 50, Issue 3, Pg:783-793, March,

2002.

6. http://www.Microwave101.com


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Figure 1: Parallel Coupled Band Pass Filter [7], the length of the line here

is taken /4

Figure 3 Simulation Response of the Bandpass Filter.


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Figure 4 Band Pass Filter Response at VNA

Figure 5 Bandpass Filter Snapshot


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Figure 2 Bandpass Filter Design Schematic

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Band Pass Filter Design