Balanced Filters Wireless Communications

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Abstract

This paper deals with the profound study of 'Balanced filters in wireless communications', paper gives brief description of balanced filters, types of balanced filters, its applications and the problems related with it.

Balanced Circuits are mainly used for electronic communications, it plays significant role in constituting a standard communications system. Immunity to device electronic noise and environmental noise makes balanced circuits most essential element in setting up modern communication system. The resistance to noises by balanced circuits results in more and more usage of it in front end communication structures for example balanced mixers , push-pull amplifiers etc.

The paper also deals with different types of balanced filters used in wireless communications and their comparison. The comparison between different types of balanced filters available is very intersecting and important in improving the quality of output.

The comparison between different options available helps in finding the problems faced by balanced filter which ultimately helps in constituting a flawless communication structure. Applications of balanced filters in communication system are briefly explained in the paper.

Introduction

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Balanced filter is a circuit which uses balanced line in it. A balanced line is used in electronic communications by transmitting signals from a point to other on two wires. In Balanced filters the impedance of the two signal lines are kept similar in order make sure that the interference is common mode and removable with common-mode rejection at the receiving end of the circuit. The balanced filters give good performances, because the noise from different sources is induced equally in blocks or line in the balanced circuit, which minimises the effect of noise. Inequality in distribution of interference noise between two signal line results in incomplete removal of noise from the circuit, which effects the proper functioning of the filter. One the most important aspect of balancing the filter is to maintain same distance between noise source and two wires.

Normally a balanced circuit shows symmetry in its elements about midway between two conductors. Examples of Symmetrical and unsymmetrical balance circuit are given below.

Balanced circuits are playing significant role in constituting standardised communication system. The growing interest of scientific world in using balanced filters is due to its immunity to various noises, resistance to noises is much higher compared to single- ended circuits and unbalanced topology. This resistivity makes it first choice in the structural architecture of fist- end communications for example balanced mixers, push pull amplifiers etc.

With rapid growth in wireless communication engineering need for more accurate and result oriented balanced circuit have increased rapidly. This high demand for more compact, cheaper and accurate balanced filter have forced scientists in making essential changes and developing the new types of balanced circuits.

Types of Balanced filters used in Wireless communication

  1. Balanced to Unbalanced band pass filters.
  2. Novel Balanced coupled line Band pass filters with common mode noise suppression.

Balanced to Unbalanced band pass filters (BALUN)

The circuit used specifically for converting balanced design to unbalanced design is known as Balun. There are many different types of Balun available, even a transformer can also be used for converting balanced to unbalanced circuit. Balun are normally used for connecting lines which does not have same impedance value. Balun works on the principle of Electromagnet coupling.

Purpose of using Balanced to Unbalanced filter in Wireless Communications

In modern world wireless communication technologies are developing swiftly, and to continuing it same fashion there is a huge demand for cheaper and smaller sized passive elements. To full fill these requirements scientists are focusing on integrating the passive components, that is combining all the passive components which are essential and need more space because of their cascade connections. In this filter band pass filter is used for eliminating unnecessary signals and balun is used for conversion of balanced signal to unbalanced signal.

Design and Implementation of Balanced to Unbalanced band pass filter

In order to get the desired result with more accuracy combination of balun and band pass filter is essential this combination forms a three port balun band pass filter.

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In the given figure Port 1 act as unbalanced input with impedance Zs and other two ports Port 3 & Port 4 have Zl impedance each to balance the output with phases 0 degree & 180 degree respectively. This balanced to unbalanced band pass filter is designed in such a way that at frequency f0 the bandwidth Bw is 3 deciBel.

Basic Concept

In previous studies a 4 port balanced filter is reduced and converted into 3 port balanced filter by release its one port out of four identical. In order to attain balanced output signal the S-matrix is calculated using common/differential mode study and transmission coefficients.

The impedance of half circuit i.e the input impedance should be kept as low as 0 to get accurate differential signalling having same amplitude but differ in phase (angel or direction). And in order to match the dissimilar inputs the impedance of half circuit should be kept twice of the impedance of input terminal.

As explained earlier 3-port balanced to unbalanced band pass filter is obtained from 4 port balun by keeping 1 of the port open. It constitutes of various parts like trisection & bisection resonators, half wavelengths.

Novel Balanced coupled line Band pass filters with common mode noise suppression

Novel balanced coupled-line band pass filters are widely used in standardised Communication systems. This type of balanced filter avoids the noise interference by using balanced coupled line structure and band pass filter. The types of noises that are eliminated by balanced coupled line band pass filters with common mode suppression are of two types mainly environment noises and noises due electronics. The environment noises are produced due to using disturbances that occurs in circuits like flickering noise, noise due to thermal, noise due to disturbance in dc power supply and due to ground lines etc. In previous topic we have learned how the balanced circuits are used to convert balanced circuit to unbalanced circuit in order to reduce the effect noises. But here balanced coupled line and the resonators having ¼ wave length are used. Common mode half circuit and equivalent differential circuits are used in order to implement better balanced architecture so that desired result is obtained with reduced noise. Selection of balanced coupled section plays very significant role in obtaining the desired output and when it comes to designing, a low level signal at common mode can be achieved affecting insertion losses.

In novel balanced coupled line band pass filter types of 2nd order balanced filters are prepared by using micro strip structure and by realising two 4th order filters using same principle used in novel dual metal plate structure improves the structure.

Balanced Coupled Line Sections

Desired result with minimum noise can only be achieved by realizing the whole structure as shown in figure given above. Two types of balanced coupled are shown in figure which are mainly responsible for getting the desired response. In the first part of figure the l2 & l3 are elements through which differential input ports are passing and l1 & l4 are elements through which differential output port are passing. But in the 2nd part of fig its vice- versa.

2nd Order Filters

Reducing common mode noise and providing desire frequency response are main characteristics of a good band pass filter. Differential and common mode responses must considered for getting a perfect common mode rejection.

If balanced coupled line structures shown in figure are considered for constructing a band filter then these two can also be developed to give desired result. In this filter the software like MWO and Sonnet are used simulation.

The simulation of the filters is done using Sonnet and method used in measuring is 4 to 2 port conversion method.

Problems related to using of Balanced filters in Wireless Communications

  • The balanced circuits are normally huge in sizes and are normally connected in cascade format. In modern communication technologies it is very important for balanced circuit to be more compact in size.
  • The balanced circuits are normally costlier when compared with unbalanced or single ended circuits, some time higher cost of balancing passive elements discourages its frequent use in industry.
  • Because of the balancing passive elements in balanced filter it is not always possible to use it in some particular applications for example monopole antenna. And therefore balanced circuits are converted to unbalanced ones, which require extra effort and money.
  • In order to resist maximum of noises, a simple balanced circuit is not efficient enough. Therefore there is always a need of an extra element like band pass filter or coupling with lines is required to be done.
  • In the modern world of chip technology, the balance filters are still lagging behind there is need of more research to be done in the field.
  • Antenna efficiency is still less and need to be improved.

Applications

  • One of the important applications of balanced filters in wireless communications is Antenna applications. The filters are used in monopole antenna are first converted with the help of balun and it also includes lumped, distributed elements.
  • One of the usages of balanced filter is that it is used in electronic communications. In order to transmit information from one signal line to another line signal.
  • The balanced filters are used in telecommunication networks.
  • One of the most used applications of balanced filters is that it plays important role in broadband services.
  • The balanced filters are also used in radio transmission technologies.
  • Television broad casting also daily used application of balanced filters.

References

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