Differential Filters with Tunable Devices

DIFFERENTIAL FILTERS

WITH

TUNABLE DEVICES

ABDULBASIT FARAG ALI

B.S. Electronics Engineering, The Higher Institute of Electronics, Libya, 1996

M. S. Information and Communication Technology (ICT), Universiti Utara

Malaysia(UUM), 2008

A thesis submitted to the Graduate Faculty of the

University of Colorado Colorado Springs

In partial fulfillment of the

Requirements for the degree of

Master of Science

Department of Electrical and Computer Engineering

2016

This thesis for the Master of Science degree by

Abdulbasit Farag Ali

has been approved for the

Department of Electrical and Computer Engineering

TS. Kalkur, Chair

John Lindsey

Charlie Wang

______Date

Ali, Abdulbasit (M.S.E.E., Electrical Engineering)

Differential Filters with Tunable Devices

Thesis directed by Professor TS. Kalkur

ABSTRACT

The most common parameter of filter to reconfigure is center frequency, fo. Other parameters to be reconfigured are the selectivity or bandwidth. When determining which technology is satisfactory for a particular application, the following issues must be considered by the designer: cost, size, power consumption, operating frequency and performance. This dissertation anticipates to provide a comprehensive view of the microwave tunable filters field, where diverse technologies are used for filter reconfiguration which are discussed through various sections of the dissertation. The thesis focuses on tunable differential filter for reduced noise. Finally a complete view of the field is provided in the conclusions part at the end of the dissertation.

iii

Acknowledgements

Praise be to Allah as he helped me to complete this Thesis. Thanks to Professor TS.

Kalkur for his support, valuable advices, and his patience. Also I want to thank all faculty staff of ECE department who is taught me during my stay at UCCS.

شكراً جزيا ,For everybody

TABLE OF CONTENTS

CHAPTER

1. Introduction ...... 2

Purpose of study ...... 2

Data limitations...... 6

Other limitations ...... 7

Varactor Diode ...... 10

Arrangement of thesis:...... 13

2. Literature review ...... 15

Tunable filters using active devices ...... 15

Tunable filters by using varactor diode ...... 19

Tunable filters having PIN and Varactor diodes ...... 22

Tunable filters with transistors ...... 24

Tunable filters using varactor diodes and transistors...... 24

MMIC Tunable filters ...... 25

Tunable filters by using the MEMS ...... 25

Tunable filters by using the MEMS switches ...... 25

Filters using the direct contact switches ...... 26

Filters by using capacitive switches ...... 27

Tunable filters using the ferroelectric materials ...... 30

BST ...... 31

STO ...... 31

Tunable filters with ferromagnetic devices ...... 32

Yttrium-Iron-Garnet films (YIG) ...... 32

Other devices based on ferromagnetic tuning...... 33

Tunable Filters Using Combined Technologies ...... 33

Tunable filters with MEMS switches and ferroelectric varactors ...... 34

Tunable Filters with MEMS Actuators And Dielectric Resonators ...... 34

Mechanically tuned filters ...... 34

3. Differential Filter Design ...... 36

Chebyshev low pass prototype ...... 36

Inductance and capacitance ...... 38

For Inductor ...... 40

Synthesis of w/h ...... 40

For capacitor ...... 40

For capacitor ...... 41

Inductance and capacitance ...... 41

For Inductor ...... 42

Synthesis of w/h ...... 42

For capacitor ...... 42

4. Results and Discussions ...... 43

Bandpass Schematic ...... 43

Simulation results ...... 43

Low-pass Schematic ...... 48

Simulation results ...... 48

Fabrication result ...... 49

5. Conclusion and Future work ...... 50

Bibliography ...... 51

vii

TABLES

Table

1. A Comparison between the single-ended filters and differential filters ...... 8

2. Change in Voltage and Capacitance Response ...... 13

3. Low-pass prototype elements ...... 36

4. Even and Odd mode impedances during the filter order 3 ...... 40

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FIGURES

Figure

1. Pass-band Region in Band-Pass Filter ...... 3

2. Bandwidth of the filter ...... 4

3. Single-ended and differential filters...... 5

4. Differential filter connected with Operational amplifier ...... 6

5. Types of Filters ...... 9

6. SIR Structure ...... 10

7. A reverse biased diode ...... 11

8. The equivalent circuit characteristics of SMV1233 ...... 11

9. Voltage controlled oscillator ...... 12

10. Graphical representation of capacitance and reverse voltage ...... 13

11. Switchable two state band-stop filters by using the PIN diodes ...... 17

12. Switchable band pass filter by using the PIN diodes, ...... 18

13. Band stop tunable filters by means of varactor diodes ...... 20

14. Tunable Bandstop filters measured reactions conferring to bias voltages ...... 21

15. Tunable Bandstop filters measured reactions conferring to bias voltages ...... 23

16. Inductance and capacitance...... 37

17. Schematic diagram ...... 43

18. Simulation results...... 44

19. Layout of circuit diagram ...... 45

20. Fabrication diagram ...... 46

21. Fabrication result ...... 47

22. Low-pass filter Schematic diagram ...... 48

23. Simulation results...... 49

24. Fabrication results of low-pass Filter ...... 49

CHAPTER 1

INTRODUCTION

Purpose of study

This project was dedicated to offer a comprehensive treatment of the microstrip filters for Microwave applications. This treatment was based on the structure of the microstrip. Chebyshev filters are also known as the all pole filters as the transmission zeros are located at infinity. However, in the case of the Chebyshev filters the pole lie on an ellipse. Microwaves are electromagnetic waves, which lie between the 300MHz and

300GHz range frequency. These wavelengths range from 1 m to 1 mm in free space.

Filters play a significant role as they are used to filter distinct frequency bands.

Moreover, the filters are used to confine the signals of the microwave and the radio frequency to some assigned spectral limits. There have even been many advances in the technology relevant to the design, fabrication, and characterization of filters. These techniques include monolithic microwave integrated circuit, high-temperature superconductor, and low-temperature Co-fired Ceramics and microelectromechanical system based design.

Bandpass filters are circuits that pass a band of signals between two specific frequencies

( , ) around the center frequency , and that block signals at other frequencies. Some of the bandpass filters do not use an external� source of power and consist only of passive components such as capacitors and inductors: these are passive bandpass filters. Other filters need an external source of power, and they use active components such as integrated circuits and transistors: these are active bandpass filters. Passive bandpass

filters also include bulk acoustic resonator based (a capacitor-inductor circuit), and surface acoustic resonator based and microstrip resonator based [5]. A dual bandpass filter has two pass-bands. An ideal bandpass filter would have an entirely flat pass-band region (e.g. with no attenuation/gain everywhere) and would attenuate all the frequencies outside the pass-band region. Figure 1. depicts how an ideal bandpass filter diminishes and rejects the frequencies that are not included in the pass-band region.

Figure 1. Pass-band Region in Band-Pass Filter

Also, the transfer out of the pass-band would be instantaneous in frequency. In practice, no bandpass filter is ideal. The filter does not attenuate all frequencies outside the required frequency range completely. In fact, there is a region just outside of the pass- band where frequencies are attenuated, but not rejected. This is known as the filter roll- off, and it is expressed in dB of attenuation per octave decade of frequency. In general, the design of a filter aims to make the roll-off as narrow as possible, therefore allowing the filter to perform as close as possible to its intended design. Frequently, this is realized at the detriment of a pass-band and a stop-band ripple. Figure 2 shows the bandwidth of

the filter as well as the difference between the upper and lower cutoff frequencies.

Bandpass filters are commonly used in wireless transmitters and receivers.

Figure 2. Bandwidth of the filter

Scope of study

This study shed light on the basic functions of the differential tunable band-pass filter. It was also focused on determining the ways of using various methodologies in this area. The major function of such a filter when used in a transmitter is to limit the bandwidth of the output signal to the band allocated for the transmission.

This allows the transmitter to not interfering with other stations. On the other hand, when used in a receiver, a bandpass filter allows signals within a chosen range of frequencies to be decoded or heard, while preventing signals at undesirable frequencies from getting through. Another critical role that a bandpass filter can play when used in a receiver is that it improves the signal-to-noise ratio and sensitivity of a receiver. In transmitting and receiving applications, well-designed bandpass filters, having the favorable bandwidth for the mode and speed of communication being used, maximize the number of signal

transmitters that can exist in a system. Bandpass filters reduce the competition or the interference among signals. Nowadays, bandpass filters are essentially used in many applications and fields such as the atmospheric sciences.

In its essence, a filter is a circuit that can separate or chose a band of signals depending on their frequency or some other criteria. In this study, only filters in the frequency domain were considered. Moreover, it is supposed that the required data has been encrypted in voltage signals. Voltages are measured with respect to a reference point named as the signal common or ground. If one of the signal terminals is grounded, we have a single-ended filter. If one of the signal terminals is not ground, we have a differential filter [1], as in Figures 3a and 3b.

(a) Single-ended filter

(b) Differential filter

Figure 3. Single-ended and differential filters

Data limitations

A differential filter can be seen as any filter with a differential input signals and a differential output signals. A differential input enables the user to apply the filter to differential signals. A differential output enables in setting the filter before differential points, such as the differential amplifier as in Figure 4. Also, it allows feeding the differential output for differential input to high-resolution analog-digital-converters

(ADCs). Regarding the fact that more and more receivers are being developed to use a differential signal rather than a single-ended signal, the design of differential frequency filters is still attracting a lot of the researcher’s attention. A differential signal is based on a two of signals equal in magnitude, but opposite in phases.

Figure 4. Differential filter connected with Operational amplifier

There is a virtual ground between the two signal tracks due to signal subtraction. A differential signal takes an ideal ground for reference while a single-ended signal takes the normal ground plane for reference.

The fundamentals of the structure’s design are simple differential filters with a high Common Mode Rejection Ratio (CMRR). With the help of pairing two single ended

filter structures, the resulting differential filter circuit presents a behavior that seems more complex than that of its original single-ended structure [2]. According to Pallas-Areny and Webster (1999), a circuit stage with a differential input and a differential output can be described by means of four transfer functions: is the differential mode-to- differential mode gain, is the common mode-to-common mode gain, is the common mode-to-differential mode gain, and is the differential mode-to-common mode gain, The common-mode rejection ratio (CMRR), which is defined as the quotient between the differential output due to a differential input voltage and the differential output due to a common input voltage:- . � � �� =

Other limitations

Differential filters, usually used on balanced communication lines, have several advantages when compared to the single-ended filters. For example, it has, higher noise immunity than single-ended filters, high attenuation of common mode signal, higher dynamic range of the signals, and better power supply rejection ratio. The amplitude of a differential-ended filter is two times higher than that of a single-ended filter, and has lower harmonic distortion. On the other hand, differential filters have some disadvantages such as: They need larger area than single-ended filters on the chip, which is related to more power consumption, and sometimes the design of differential filters is more complex with respect to single-ended filters. Table 1 shows the comparison between the single-ended filters and differential filters.

Filter type Differential filter Single-ended filter

Feature

Noise immunity High Low

Attenuation of common High Low

mode signal

Power Better power supply rejection ratio High Power

consumption

Amplitude 2 x Amplitude Amplitude

Harmonic distortion Low High

Area Large Small

Dynamic range of the signals High Low

Table 1. A Comparison between the single-ended filters and differential filters

Differential filters could be one of four types of filters, which are Low-pass,

High-pass, Band-pass, or Band-stop filter as in Figures 5a, 5b, 5c, 5d respectively.

Differential band pass filters (DBPFs) are one of the most necessary elements in the high speed wireless communication systems. They have high immunity to the environmental noise and low electromagnetic interference (EMI) to meet the prompt demand in the development of balanced radio frequency (RF) front-end [3]. Recently, there are various types of differential BPF configurations with both differential-mode (DM) filtering responses and common-mode (CM). In terms of noise rejection, differential filters have

been working better than single-ended ones. It is not uncommon that Classics Planar

Microstrip lines were employed for differential BPFs design.

a) Low Pass Filter b) High-Pass Filter

c) Band-Pass Filter d) Band-Stop Filter

Figure 5. Types of Filters

Stepped Impedance Resonators (SIRs)

Stepped Impedance Resonators (SIRs) can be built by the low and high impedance sections. They connect together for bandpass filter with ultra-wideband bandwidth. For narrowband bandwidth, J-inverter can be effectively used instead of series components [42]. In Stepped Impedance Resonators (SIRs), it is useful to design microstrip bandpass filters with good stopband performance. Resonant frequencies can be changed by adjusting the structural parameters, such as the impedance ratio of the low-Z and high-Z segments. As a result, the first fake harmonic can be higher than 2 . Figure 6

� 9

details the structure of SIR and shows the distributions of the associated impedances. A group of different SIR structures can be used for a bandpass filter with wide stopband.

Non-Traditionally SIRs can be used to obtain high performance bandpass filters with the control of the fake responses outside of a selected bandwidth over a large frequency range.

Figure 6. SIR Structure

Varactor Diode

In this study, variable capacitance plays a key role in the practical part. It helps produce suitable results. Variable Reactance Diode (Varactor diode), also called Variable

Capacitance Diode, is used for providing the required tuning to perform the different stages of the experiment conducted for this study. The changes in the capacitance lead to a change in the center frequency. Change in capacitance can even be obtained with the help of normal diodes but specific changes are obtained with the help of varactor diodes.

With the variations of the applied voltage in the reverse bias, the size of depletion region change. By increasing the voltage in this case, the width of the depletion region is increased, whereas the decrease in the applied voltage leads to a decrease in that region.

Figure 7 illustrates the depletion layer and how it could be increased during the reverse

bias connection. Moreover, these diodes are favorable for the reasons that they produce less noise, they are available at low costs, and produce results that are more reliable.

Figure 7. A reverse biased diode

A wide range of varactor diodes are available with different values and in this research the SMV1233 is used due to its desirable characteristics. Figure 8 depicts the characteristics of the Varactor Diode SMV1233.

Figure 8. The equivalent circuit characteristics of SMV1233

One of the most common applications of the Varactor Diode is the voltage-controlled oscillator. In such application, tuning and adjusting the bias voltage is used to control the frequency at the output. Consequently, the frequency can be controlled by the voltage controlled oscillator. Figure 9 shows how the voltage controlled oscillator works and controls the frequency with the help of the input voltage.

Figure 9. Voltage controlled oscillator

Capacitance change with applied reverse voltage can be determined by: -

C (V) = � +� Where C (V) is the varactor diode capacitance, V is the reverse bias voltage, the built in potential =0.7V for silicon, =0.5 is the slope of log (V+ ) vs. log(C)

� � �

Table 2 shows the impact of the changes in the voltage on the capacitance according to the equation above. The table shows that the capacitance decreases from 5.08pF to 0.90 pF when the voltage changes from 0.0v to 10v.

SMV1233

VR (V) CT (pF)

0.0 5.08

10.0 0.90

Table 2. Change in Voltage and Capacitance Response

Figure 10 shows a spectrum of variations in the voltage and the associated changes in capacitance for several models of SMV type of varactor diodes. From the figure it is obvious that the curve related to the varactor diode SMV1233 shows that when the voltage increase the capacitance decreased.

Figure 10. Graphical representation of capacitance and reverse voltage

Arrangement of the Thesis:

This thesis is structured as following:

A title page: the title page of research thesis includes the title of research paper, the name of university in which author is enrolled, preparation date of research paper, the name of author and the name of supervisor of the thesis.

Abstract: the abstract of thesis includes the overall summary of the research paper. It includes the goal and scope of the research, research methodology being used, the results obtained, and the influence of the study.

Table of contents page: This page lists the table of contents for this thesis report.

Introduction: The introduction of this research paper includes the details of differential tunable band-pass filter. The basic techniques used by this filter through which it passes only certain wavelengths of signal have been illustrated. The functions of these filters have been discussed briefly and different types of filters are depicted as well.

Literature review: This section provides related information about the topic. This information was extracted from previous work conducted in the same area. The references used for this work spans from seminal works up to recent published works.

Design of Differential Filter: This section contains the details about the type of research methodology employed by the author for this study.

Results/ discussions: This section shed light on the analysis and the discussion based on the result of the research methods.

CHAPTER 2

LITERATURE REVIEW

Reconfigurable of tunable microwave filters make transceivers of microwave adaptable to numerous bands of operation using a solitary filter, which is extremely necessary in today’s communications with forever rising applications for wireless communication [1]. Tunable filters substitute the requirement of switching among some filters to have more than one response of filter by introducing elements of tuning implanted into a filter topology.

Tunable Microwave filters can be classified into two groups, filters having continuous tuning and filters having discrete tuning. Topologies of Filter offering a discrete tuning usually uses of MEMS switches or PIN diodes. Alternatively, filter topologies that use of MEMS capacitors, varactor diodes, ferromagnetic materials or ferroelectric materials are often used to get a continuous tuning device. Topologies of filter mixes discrete and continuous tuning by uniting elements of tuning as well, e.g. the usage of varactors and switches on a filter topology creates part of a continuous and discrete tuned device.

Tunable filters using active devices

This section covers tunable filters that makes use of tuning elements based on semiconductor Devices making uses of diodes are attractive under 10 GHz, whereas diodes still demonstrate factors of quality above 50 with small bias voltages. Diodes typically include simple packages and are attached on boards of microwave. Numerous of

these designs are supposed as possible monolithic designs. This section discusses tunable filters that use varactor diodes, PIN diodes, and transistors.

Tunable filters by using the PIN diodes

PIN diodes are frequently used to produce tunable discrete states on a response of filter. They are much attractive for implementations due to their low cost. In this section, tunable filters by using PIN diodes are discoursed in lumped and distributed topologies as well as design by means of periodic structures.

 Distributed designs

Regarding the relationship between the reactance slope parameter and fractional bandwidth of switchable decoupling resonators has been deliberated. Distributed design method has been used to apply two band stop switchable filters. These filters alter between two states of center frequency, each possessing a distinct fractional bandwidth

[12]. These filters have been applied to deliver the identical fractional bandwidth at the different bandwidths or center frequencies distinct by the bent shape of switchable resonator extensions. Both of these topologies are displayed in Figure 2.1. Switchable two state bandstop filters can develop narrow and broad bandwidths by adjusting couplings of inter-resonator [27]. The filter in the study by Koochakzadeh et al[23], covers a tuning range of frequency from 290 to 600 MHz which is divided in four steps by using the ten PIN diodes [22].

(a) Constant Bandwidth

Figure 11. Switchable (b) two Diverse state Bandwidthband-stop filters by using the PIN diodes

originated from (Brito-Brito)[2].

An edge coupled tunable resonator filter with three frequency states at the center and two conceivable bandwidths for every state can be found in the study by Lugo et al[28]. A reconfigurable and tunable bandpass filter for UMTS and WiFi transmit standards is presented in Fig. 2.2, this filter has been intended to specifically deliver the

bandwidth and center frequency which is required for every application with low power consumption and low loss, as it only uses of two PIN diodes [11].

(a)Topology

(b) Filter Response

Figure 12. Switchable band pass filter by using the PIN diodes,

, obtained from (Brito-Brito ref)[2].

The tunable band pass filter in the study by Lacombe can get a bandpass response or a pseudo all pass response by means of PIN diodes [24]. A resonator filter with dual mode can be found in the study by Lugo et al.[29], the filter uses a triangular patch

resonator to attain a reconfigurable two state bandwidth [15]. A tunable bandwidth by means of a dual mode square resonator was proposed in that paper.

Other resonator filter with dual mode is presented in Lugo et al [28] and this filter has an irregular response, and it can tune its transmission zero position, and center frequency by means of an adjusted square resonator. The filter presented in the study by (Karim et al.)

[15] can be changed to a band pass from a band stop response for applications of ultra- wideband by means of four PIN diodes [19]. A non-uniform tunable microstrip combine filter with a tunable center frequency of above an octave in the UHF band was proposed by Koochakzadeh [23], this filter can uphold a continuous bandwidth on the tuning range of center frequency.

 Lumped element designs

Designs of the Lumped element filter by using the PIN diodes comprise a design of reconfigurable bandwidth at 10 GHz which is capable to switch amid of bandwidth of

500 MHz and a 1500 MHz (Rauscher)[36]. The filter offered in (Chen ref)[8] uses co- fired low-temperature ceramic technology and can switch amid two states of center frequency.

 Filters by using the periodic structures

The filter which is presented in Karim, Liu and Alphones switches amid a band pass and a band stop response by means of periodic structures of electromagnetic band gap on a coplanar ground surface; the filter is adjusted at 7.3 GHz. [15]

Tunable filters by using varactor diode

Varactors are typically used for filters which are continuously tuned. Varactor diodes make uses the change in the capacitance of depletion layer of a p-n junction with

an applied bias [24]. Varactors diodes can be used for high speed tuning and they do not show hysteresis.

The center frequency, bandwidth and selectivity can be reconfigured by the varactor tuned filter resulting in an adjustable band stop design. This filter photograph is shown in Fig. 2.3.

(a) Topology

(b) Filter Photography

Figure 13. Band stop tunable filters by means of varactor diodes

taken from [33]

The response of the filter while tuning these 3 factors is shown in Fig. 2.4. The center frequency or bandwidth can be tuned by the varactor filter using a compact hairpin shaped resonator [10]. The combline filters also use suspended strip line conduction paths. The 1st design is a band pass filter and the 2nd design is a band stop filter. Center frequency can be tuned by the band pass filter design by displaying good matching impedance for the diverse filter states [28]. The 3rd design shows both bandwidths as well as center frequency controller on a topology of band pass filter. The 4th design is a band pass filter along with reconfigurable center-frequency.

(a) center frequency tuning (b) bandwidth tuning

Figure 14. Tunable Bandstop filters measured reactions conferring to bias voltages , obtained from (MusollAnguiano)[33] 21

A tunable microstrip band stop filter using a varactor diode as a tunable component was designed by Makimoto & Sagawa [32]. The varactor diode as a tunable device was also used by Liang & Zhu [26] to design a hairpin band pass filter, which showed a tuning in the center frequency of the device. In addition, they have added combeline to the designed filter in order to improve the out of band rejection [29].

Tunable filters having PIN and Varactor diodes

This part covers tunable band stop filters using both varactor and PIN diodes.

Figure 2.5 shows the outcomes in topologies of filter by continuous and discrete tuning.

(a) Center Frequency Tuning

(b) Bandwidth Tuning

Figure 15. Tunable Bandstop filters measured reactions conferring to bias voltages , obtained from (MusollAnguiano)[33].

For the change of the length of resonator by frequency tuning, PIN diodes have been used for the tuning of the bandwidth at every state of center frequency in (Carey-

Smith & Warr)[4]. This effects in discrete tuning of center frequency, and also the tuning of continuous bandwidth [33].

Tunable filters with transistors

A field effect gallium arsenide transistor has been used as an element of tuning

(TorregrosaPenalva)[38], the frequency and topology response of the filter is constructed on a topology of combline and its center frequency can be tuned.

Tuning of center frequency has been attained on a two pole configuration of filter by means of two metallic field effect of semiconductor transistors [27]. The first transistor is used for the tuning of center frequency whereas the second one is used to give the circuit a negative resistance. The technique of negative resistance increases the quality factors of unloaded resonator and resulting in a better and improved response of filter [30].

Tunable filters using varactor diodes and transistors

To recompensatee the losses in filters from the use of a varactor diode, a circuit of negative resistance by means of transistors can be further added to the design. This procedure has been applied in [6] where the bandpass and bandstop filters are presented the silicon bi-polar transistor was used in the process [35]. Chandler presented a bandpass filter by using the transistor of silicon bi-polar as well. Lastly Chang & Itoh utilized field effect metallic semiconductor transistors on a topology of bandpass filter [7].

MMIC Tunable filters

The integrated silicon tunable filter can tune bandwidth, center frequency, and transmission gain. Two K-band designs of filter by using the 0.15 μm technology of gallium arsenide use negative compensation of resistance for losses [38]. The filter consist of an operating frequency range of about 120 GHz. The device has the ability to tune its bandwidth or center frequency. It uses of high-electron-mobility indium phosphide transistors. The tunable filter utilizes gallium arsenide semiconductor of metal field effect transistors and delivers tuning of frequency circuit of resistance which is also used as an element of tuning [39].

Tunable filters by using the MEMS

Radio Frequency RF MEMS tunable devices have decent compatibility with tools used in semiconductor manufacturing. They propose small size and upright integration abilities with microwave electronics. In general, RF MEMS need low currents to be functioned. Therefore, they derive low power compared to solid state devices, and they also can display linear transmission with little distortion in signal. This section covers tunable filters that use MEMS varactors or MEMS switches as elements of tuning.

Tunable filters by using the MEMS switches

This segment reviews some filter topologies by means of MEMS switches to yield discrete tuning of tunable parameters. The switches can be either a bridge type or a cantilever, and can be direct contact switches or capacitive type switches [19]. Switches can operate in two states: ‘off ’ or ‘on’. Switches of direct contact will usually make a contact of metal to metal in the on state, switches of direct contact are normally used for

applications of low frequency. Capacitive switches will show two capacitances, one in the state of ‘on’ and another one in the state of ‘off ’.

Filters using the direct contact switches

This segment reflects filters that use of direct contact switches as elements of tuning. The switches possess an ‘off ‘ state when the membrane of switch is suspended and have no contact with a metallic bottom pad. The switch in the position of ‘on’ results in a metal to metal interaction with the membrane of the switch with a metallic bottom pad. Switches of direct contact metal can be fabricated as bridge type and cantilever type

[20]. The ‘on’ and ‘off ‘ position of switch is controlled by a bias voltage applied between the membrane of the switch and actuation electrodes. This segment comprises tunable filters in lumped element designs, distributed designs and lastly a filter using periodic structures is defined.

 Distributed designs

In distributed design, the filters used are microstrip bandpass employing the hairpin resonators having switches. The switches are of the type of direct contact cantilever on the end of the resonators. They are used to increase the length of hairpin resonators while the switches are in the state of on.Therefore, the center frequency of the device, along with its center frequency response are tunned. Tunable resonators of slotline printed on a ground plane of microstrip have been castoff to make a lowpass filter by means of commercial MEMS switches in order to short-circuit the slot resonators [41].

 Lumped element designs

The lumped element bandpass tunable filter makes use of commercial switches on asubstrate of FR4; the device has the ability to reconfigure its center frequency of 25 to

75 MHz. The lumped element bandpass tunable filter can also utilize direct interaction switches for applications of local area wireless network to route the signal of microwave on two diverse paths consequential in a two state tunable center frequency filter [18].

 Filters by using periodic structures

A bandpass and a lowpass filter by means of direct interaction switches have been intended and invented, the switches can make solitary and numerous contacts on coplanar lines of transmission with periodic structures[18] [40].

Filters by using capacitive switches

This segment comprises filters that makes use of a capacitive type MEMS switches as elements of tuning. The switches can yield two capacitances, one distinct for the switch in the state of ‘off ‘, and the other one is distinct when the state of the switch is

‘on’ [40]. The two states of the switches are regualted by a bias voltage amongst the actuation electrodes and switch membrane. The capacitive switches are of two types, bridge type or a cantilever type. This segment contains designs by means of lumped anddistributed elements.

 Distributed designs

A tunable filter for applications of local area wireless network uses resonators of open loop ring loaded with capacitive switches and fixed metal-air-metal capacitors.

Capacitive switches of bridge type MEMS have been castoff to load coplanar resonators where 16 diverse states of center frequency have been attained [12].

An interdigital switchable coplanar filter possesses two states of center frequency, attained by means of a capacitive cantilever MEMS switch at the ends of the coplanar resonators. Cantilever capacitive MEMS switches have also been fabricated on a couple

of microstrip equivalent coupled line filters to attain two states of center frequency [34]

[41].

 Lumped element designs

A band pass filter having a tuning range of center frequency from 110 MHz to 2.8

GHz uses capacitive switches to create variable capacitor states to reconfigure the center frequency. The filter also encompasses metal insulator-metal capacitors in the lumped topology. Some filters uses a bank of capacitive switches to reconfigure center frequency of the filter for applications of local area wireless network [18]. A differential filter that can configure its frequency from 6.5 to 10 GHz makes use of metal air metal capacitors, and capacitive switches of MEMS to reconfigure the center frequency [39].

 Tunable filters by using MEMS varactors

The usage of MEMS varactors can be yielded by low insertion losses of filter, and are castoff to deliver a constant reconfiguration of filter parameter. MEMS varactors are appropriate for small lumped element filters because of the factor of high quality displayed by the MEMS varactors, as paralleled with conventional constituents like the metal insulator metal capacitor. MEMS varactors can similarly be castoff to load the distributed resonators to attain tunable filters. This segment deliberates filters made with cantilever type and bridge type MEMS varactors, where numerous filter topologies are defined.

 Filters using the bridge type varactors

Filters that makes use of the MEMS varactors shaped by actuators of bridge type, can reconfigure parameters of filter in a constant fashion. The actuators are fixed on both ends, and are actuated by means of a bias voltage between the actuation and bridge

electrodes, which will regulate the variable capacitance. This segment discusses filters which are made by means of bridge type MEMS varactors on lumped and distributed topologies along with a design founded on periodic structures.

 Distributed designs

As a bridge type, MEMS varactors are used to load resonators of coplanar transmission line with the purpose to achieve a reconfigurable center frequency [1]. The filters makes use of bridge varactors to regulate all parameters of filter design at millimeter waves.

 Lumped element designs

The filters employs MEMS bridge varactors in order to tune V-band band pass filters. This tunning occurs by means of compressed lumped filter designs element. Other lumped element filter by using metal air metal capacitors and spiral inductors can be originated where capacitors of bridge MEMS are castoff to tune the K-band center frequency [19].

 Filters using the periodic structures

A tunable bandstop filter which is designed by means of a periodic structure of electromagnetic bandgap can include MEMS bridge type varactors. Such varactors have been castoff in amongst the cells of bandgap to tune the response of the filter [17].

 Filters using the cantilever type varactors

This segment comprises filters which uses the cantilever type MEMS varactors as elements of tuning. These tuning elements have been castoff to produce a unceasing filter parameter reconfiguration by adjusting the capacitance amongst a flexible cantilever and a fixed metallic plate underneath it. The capacitance varies conferring to a bias voltage

which was applied amongst an actuation and the cantilever electrode. The bias voltage sources for the displacement of cantilever thus creating the variable capacitance [33].

 Distributed designs

The device which uses cantilever type MEMS varactors to attain bandwidth and center frequency tuning at Ka-band [13]. The topology of filter is based on dual performance resonators. The device consists of two poles, and the MEMS varactors were positioned at the ends of the anticipated resonators. One of the filters makes use of

MEMS cantilever varactors to adjust the frequency of the resonant of distributed resonators on a topology of bandpass filter at Ka-band. The tunable bandstop filter of slot resonator uses a thermal actuator as a tuning plate to yield a reconfigurable center frequency at about 6 GHz.

 Lumped element designs

A tunable lumped element filter at Ka-band has the ability to reconfigure its center frequency rendering to applied bias voltages to a MEMS cantilever varactor [18].

Tunable filters using the ferroelectric materials

Ferroelectric materials can alter values of permittivity proportionately to an applied DC electric field. Several ferroelectrics are appropriate for depositing thin. This section emphasizes on tunable microwave filters by means of three of the utmost shared ferroelectrics which is used to date. These three ferroelectrics are the Barium-Strontium-

Titanate oxide (BST), the Strontium-Titanate Oxide (STO), and the lead Strontium-

Titanate oxide (PST). Other ferroelectric materials explored for tunable microwave devices are the bismuth zinc niobate or the sodium potassium niobium oxide ferroelectric which are not discussed in this section. Ferroelectrics have been much attractive because

of their compatibility with planar microwave technologies and electronics to yield high speed devices which are reconfigurable.

BST

A quasi-elliptic tunable bandpass filter with BST capacitors are positioned over the open loop circle resonators [11]. The bandpass two pole filter with slow wave coplanar resonators can adjust its center frequency by means of capacitive resonators loading the throughbanks of ferroelectric varactor. The center frequency pertinent to this filter can also be tuned by the device from 11.5 GHz up to 14 GHz. The 3 pole combline filter procedures ferroelectric varactors at the one end point of resonators. This helps to generate a adjustable center frequency range from 2.44 GHz up to 2.88 GHz using good impedance corresponding for 400 MHz bandwidth and for all states.

The stopband bandwidth can be tuned by a bandstop tunable filter with slotted powdered resonators from 1.2GHz to 1.4 GHz. The tunable filter of lumped element was intended to adjust its center frequency from 31.5 MHz up to 88 MHz along with a bandwidth of 3 MHz; eight ferroelectric varactors used by the device.

STO

The STO thin film is used by the tunable filter for the purpose of tuning its center frequency, the ferroelectric film permittivity varies as the temperature changes and also changes with an amount of bias voltage applied. The range of the tuning of center frequency is from 18.3 GHz to 19.15 GHz as well as with a 4 percent fractional bandwidth [39].

PST

The silicon of high resistivity is used by the filters as a substrate along with incorporated ferroelectric capacitors. To produce a bandstop filter and tunable resonator, this structure has been used. The topology of the filter and resonator are grounded on a resonator of slotted coplanar, and its center frequency can be tune by the device from

3.65 GHz up to 4.23 GHz [9].

Tunable filters with ferromagnetic devices

Tunable filters with ferromagnetic equipments such as Yttrium-Iron-Garnet (YIG) outcomes in high unloaded factors of quality resonators along with high capabilities of power handling as well as consumption of high power. Resonators that uses YIG spheres have been used traditionally [5]. Regardless of the high unloaded quality factors attained, extremely accurate fabrication including higher expenses, as well as other disadvantages such as low speed of tuning and a difficult mechanism for tuning including coils nearby the loops.

This portion discribes tunable filters with ferromagnetic tools in planar arrangements that show the easiness of integration and fabrication with planar conduction lines of forces or bias devices. The unit is distributed in two parts, the 1st part discusses the tunable filters with YIGs whereas the other part discusses the designs built on other tuning structures of ferromagnetic materials.

Yttrium-Iron-Garnet films (YIG)

The YIG film in a two-pole topology of filter uses by the device, the design has a bandwidth of 16 MHz as well as its center frequency can be tuned ranging from 0.5 GHz up to 4 GHz. A ferromagnetic tuning of resonance on, gallium arsenide substrate is used

by the filter, bandpass and bandstop topologies are presented, the center frequency of the tunable bandpass filter can be tuned from 5.9 GHz up to 17.8 GHz. These mechanisms have an extensive ranges of tuning, great power handling as well as abilities for tuning speed.

A distinct cavity resonator having a ferromagnetic resonance makes use of the ferrite-ferroelectric coatings. The design of cavity is modified by magnetoelectric interfaces among the coatings castoff form the resonator. The tunable bandpass filter grounded on the resonance materials with piezoelectric-YIG coatings has been described, where a machine is presented with a range of tuning from 6.65 GHz up to 6.77 GHz.

Other devices based on ferromagnetic tuning

The iron layer over a gallium arsenide substrate is used by the filters. The filter has a larger range of tuning from approximately 10 GHz to 27 GHz, higher frequencies has been tuned by the devices than the ones discussed in the prior section. A coated insulator and ferromagnetic compound materials has been utilized for the tuning of resonators pairs

[37]. A stump resonator demonstrated a tuning of frequency range from 1.17 GHz to 1.71

GHz.

Tunable Filters Using Combined Technologies

This section describes the devices that pool with diverse technologies to obtain the reconfigurable purifying. It discusses the filters that pool ferroelectric resources with either active devices or MEMS, and finishes with a reconfigurable dielectric filter of resonator with MEMS elements for tuning.

Tunable filters with ferroelectric transistors and varactors

Kim & Park in 2007 stated that to compensate the circuit and ferroelectric losses, the technique of commercial trasistor with a negative resistance has been used. The topology of the filter is made out of elements of commercial loop and ferroelectric capacitors on a higher silicon substrate resistivity [19]. The bandpass machinery has 2 poles with a 110 MHz bandwidth as well as a center frequency range of tuning from 1.81

GHz to 2.04 GHz.

Tunable filters with MEMS switches and ferroelectric varactors

Cantilever straight connection MEMS switches for tuning of bandwidth and BST varactors for the tuning of center frequency can be combined [28]. A narrow and a wide configurations of bandwidth on 2 and 3 pole topologies can be provided by the filters.

The constant tunable center frequency starts from 30 GHz and goes up to 35 GHz.

Tunable Filters with MEMS Actuators And Dielectric Resonators

A bandpass tunable filter that uses higher unloaded quality aspects dielectric resonators is tuned by means of thermal MEMS actuators with great deflections to center frequency tuning [40].

Mechanically tuned filters

For the tuning of microwave filters, automatically changeable metallic or dielectric screws of tuning are generally used. To recompense fabrication tolerances, these procedures are commonly used to anywhere the screws can be rotated automatically while observing the restrained response. Programs for automatic tuning can mannualy discover an ideal filter response by redoing screw for tuning places untill a user-defined reaction is found.

The tuning screws can be strategically placed on near or top microwave resonators for the tuning of the individual resonator’s resonant frequency. The screws can be placed out between the resonators to adjust inter-resonator pairing constants. Screws can also be placed between the output/input pairing structure of the filter as well as the first or last resonator for the tuning of the output or input pairing to the filter.

Also the category of screw is significant depending on the magnetic field and electric field distribution nearby the tuned-to-be resonator. Commonly, dielectric screws for tuning are frequently used where the maximum electric field can be found around the resonator. In the same way, metal tuning bolts are frequently used where the magnetic field is found maximum around the resonator.

CHAPTER 3

DIFFERENTIAL FILTER DESIGN

This chapter covers the design and simulation of non-tunable and tunable differential filters using two types, bandpass and low-pass filters. These two types of filter are designed based on Chebyshev prototype.

Chebyshev low pass prototype

To design differential bandpass filter, it is not uncommon to start with Chebyshev low pass prototype. This prototype is basically low pass filter which conductance and source resistance are made equal to one by normalizing its element values. The

Chebyshev filter has passband or stopband ripple. Alternatively, the Butterworth filters make its roll off steeper and better than Butterworth response. In prototype filter, parameter (g) defines circuit elements such as inductive and capacitive as shown in table

3. The frequency and element transformations are made based on the principle of low pass prototype by setting the pass band ripple of the design at a value of 0.01 dB (LAr =

0.01dB), inductive/capacitive element (go) at value of 1, and the cutoff frequency of the prototype (c) at a value of 1.

Lar = 0.01dB, go= 1, c = 1

N g1 g2 g3 g4

3 0.6292 0.9703 0.6292 1

Table 3. Low-pass prototype elements 36

Table 3 represents the element values of Chebyshev low pass filter. In the design of this project, the number of elements used was 3 which defines the filter order. Then, the (g) values were extracted based on number of elements. Figure 16 illustrates the bandpass filter of order 3 which was used for the calculations related to this project.

In the calculation Z0 is the impedance of the terminals whereas Es is the voltage of the generator or voltage of the source. A low pass filter with a source impedance of 50 ohms and cutoff frequency of F0=2 GHZ is considered. Moreover, a Chebyshev low pass prototype is chosen with the structure. The Angular Frequency  varies between 0 and the cutoff whereas  varies between the values of 0 and infinity. The mapping of  and

 is done in a periodic manner.

Figure 16. Inductance and capacitance For Bandpass Filter:

F0 = 2GHZ, Z0 = 50.

FBW =   =2%, Where FBW is the fractional Bandwidth.  − o 37

10 o = 1.26 x 10 rad, Where o is the angular frequency of the center.

r = 4.34, Where r is a relative dielectric constant.

H= 1.524mm, Where H is a thickness.

The minimum stop band attenuation LAs for the pass band ripple Lar (dB) is found at  =

s. whereas s represents the equal ripple stopband starting frequency. The Chebyshev response proves a superior design over the design of the Butterworth prototype.

In this research the low pass prototype response has been transformed to a bandpass response. The passband edge angular frequency is indicated by 1 and 2.

The frequency transformation is also applied to g, which is a reactive element.

Inductance and capacitance

For g representing inductance:

 Ls 1,3 = ( ) o g1,3 = 12.48 nH,   c FBW x o Lp 2 = = 0.82nH,  o Cp Where FBW is the fractional Bandwidth. The scaling of the impedance has been taken into account in these calculations.

For g representing capacitance:

Cs 1,3= = 0.51PF,  o Ls ,  Cp 2 = = 7.7 PF,   c g For theFBW transmission x o o of the common-mode, there is the transmission zeros of the series resonators Lp 2 and Cz. Cz = =7.68 PF,  o Lp

The expressions for the inductance and the capacitance depend on the length and the impedance characteristics. Zo represents the impedance of the source. The value of the source impedance is 50 ohms for the microstrip filters. The filters compose of lossy elements as well. Thus there is a difference in the ideal response and the response actually obtained due to the power dissipation present.

Parallel coupled also known as the edge coupled microstrip bandpass filters are used.

These filters use half wavelength line resonators. Due to this arrangement there is a large coupling between the resonators for a given spacing. Thus the filter with a wider bandwidth is used. The design equation is as follows:

 J01 Zo = = 0.71 = j34 Zo, √FBW √go g  J12 Z0 = = 0.4 = J23Zo FBW √g g

Where: go ,g1… gn represent the elements of the ladder type low pass prototype.

The cutoff frequency is 1. This also makes use of the fractional bandwidth of the bandpass filter. Whereas Jj,j+1 represent the characteristic admittance of the (J) type inverters.

Whereas ZOC < ZO < ZOL

Where ZOC and ZOL represents the low and the high impedance lines, respectively.

Zoo and Zoe represent the even and the odd mode impedances of the coupled microstrip line resonators. These are represented by the following:

2 Zoe = Zo [ 1 + JZo + (JZo) ],

2 Zoo = Zo[ 1 – JZo + (JZo) ],

Table 4 shows the different values of the even and the odd mode impedances during the filter order 3.

n gn ZoJn Zoe() Zoo()

1 0.6292 0.71 110.705 39.705

2 0.9703 0.4 78 38

3 0.6292 0.4 78 38

4 1 0.71 110.705 39.705

Table 4. Even and Odd mode impedances during the filter order 3

J01, J34 = 0.0142

For Inductor

ZoL= 110.705

A =  +  (0.23+ ) =3.9   ZoL r + r – . √ r + r Synthesis of w/h

The following calculations are made by using the Wheeler and the Hammers tad.

The following is for W/h 2:

=0.16 ≤ A e � − W=L = 0.25mm

For capacitor

Zoc = 39.705

B =  =  =7.16   Zoc r Zoc r Now for W/h 2:

= [B–1–Ln(2B-1) +  [Ln(B-1) + 0.39 - ] = 2.78   r− . � r r Thus Wc = 4.24mm

The accuracy provided by these calculations is even better than 1 percent. If for further work more accuracy is needed then processes such as iterative process and optimization process can be carried out.

  reff = +   r + r – + H/W For Inductor:

reff = 2.86

For capacitor

reff = 3.39

c = = = 3.3mm  − CZocC .x x.xx ℓ reff . L = = = 20mm  − cL .x xx ℓ ZoL reff . . Lp2 and Cp2 , for ZoL= 120, Zoc = 20

A=4.23 thus WL = 0.18mm

B = 14.214 thus WC = 10.67mm

For Low-pass Filter: -

Inductance and capacitance

For g representing inductance:

 Ls 1,3 = ( ) o g1,3 = 2.5 nH,   c For g representing o capacitance:

 Cp 2 = = 1.5 pf   c g To find o out o the Length and width of microstrip we use this expression: -

For Inductor

ZoL= 110.705

A =  +  (0.23+ ) =3.9   ZoL r + r – . √ r + r Synthesis of w/h

The following calculations are made by using the Wheeler and the Hammers tad.

The following is for W/h 2:

=0.16 ≤ A e � − W=L = 0.25mm

For capacitor

Zoc = 39.705

B =  =  =7.16   Zoc r Zoc r Now for W/h 2:

= [B–1–Ln(2B-1) +  [Ln(B-1) + 0.39 - ] = 2.78   r− . � r r Thus Wc = 4.24mm

c = = = 9.7mm  − CZocC .x x.xx ℓ reff . L = = = 4mm  −  cL .x xx ℓ ZoL reff . .

CHAPTER 4

RESULTS AND DISCUSSIONS

Bandpass Schematic

The schematic used in designing the filter used in this research was conducted by means of the Advance Design Software ADS. Figure 17 depicts the initial schematic diagram used for this project.

Figure 17. Schematic diagram Simulation results

This section is devoted to discuss the results obtained in this project. Figure 18 shows the simulation results. The graph showed in this figure represents the plot of the ADS. In the graph, the x-axis represents frequency in GHz while the Y-axis shows the values in

decibels. Six plots are shown on the graph. The plot m1 has a frequency of 1.780GHz.

Frequencies of m2, m3, m4 ,m5 ,and m6 are 2.140GHz, 1.680GHz, 1.870GHz,

2.040GHz, and 2.240 GHz respectively..

At =1.78GHz and BW=1.87-1.68=0.19GHz,

At =2.14GHz and BW=2.24-2.04=2GHz

Figure 18. Simulation results

Another output of ADS is the layout of the filter which is shown in figure 19. The obtained layout shows the elements of the filter and their connections. The elements displayed are in the fabrication size.

Figure 19. Layout of circuit diagram

Figure 20 represents the physical shape of the bandpass filter. This physical shape was obtained through feeding the layout obtained from ADS to the microstrip printer. The figure also shows the varactor diode connected to the filter. The varactor diode was connected to the filter after the printout process was complete.

Figure 20. Fabrication diagram

Fabrication result

Figure 21 shows the fabrication results. From the figure one can grasp the insertion loss, return loss at the central frequency.

Differential BPF

-2 1.8E+09 1.85E+09 1.9E+09 1.95E+09 2E+09 2.05E+09 2.1E+09 2.15E+09 2.2E+09

1970000000, -4.1105056

-4 2060000000, -5.0128155

2020000000, -7.8391242 2120000000, -6.6854773 -6

-8 1910000000, -7.750331

2060000000, -7.2311683 dB

-10

2040000000, -10.327988 -12

1940000000, -11.976095

-14

-16 GB S21-0v(DB) S11-0v(DB) S21-10v(DB) S11-10v(DB)

Figure 21. Fabrication result

Low-pass Schematic

The schematic used in designing the low-pass filter used in this research was conducted by means of the Advance Design Software ADS. Figure 22depicts the initial schematic diagram used for this low pass filter.

Figure 22. Low-pass filter Schematic diagram

Simulation results

The simulation results are shown below. The graph appears in figure 23 shows the plots of the ADS. The x-axis represents frequency in GHz while the Y-axis shows the S21and

S11 values in decibels. Two plots are shown on the graph. The plot m1 has a frequency of 1.890GHz and the Frequency of m2 is 1.92GHz.

Figure 23. Simulation results Fabrication result

Figure 24 shows the fabrication results for the low-pass filter. From the figure the values of the insertion loss, return loss at the central frequency can be understood.

Differential LPF 1E+09 1.5E+09 2E+09 2.5E+09 3E+09 10 1.9, -3.9 2.1, -4.2 0

-10

-20

-30

-40

-50

-60 S21(0V) S11(0V) S21(5V) S11(5V) -70 Figure 24. Fabrication results of low-pass Filter

CHAPTER 5

CONCLUSION AND FUTURE WORK

In this study, two types of filters were designed and fabricated. These two filters were simply tunable microstrip differential filters, low-pass and bandpass filters. By using a varactor diode as a tunable element, we have got tuning in the frequency response depends on the magnitude of the applied dc bias. With the application of zero bias, the filter shows bandpass filter at center frequency 1.97 GHz with S21= -4.1 and S11= -11.9 and Bandwidth from 1.9 GHz to 2.06 GHz. By increasing the magnitude of the applied dc voltage, the center frequency of the filter shows an increasing response and reach 2.06

GHz by value of 100 MHz with S21= -5 and S11= -10.3, Bandwidth from 2.02 GHz to

2.12 GHz at 10V.

Also for the low-pass filter with the application of zero bias, the filter showed a center frequency at 1.9 GHz. By increasing the magnitude of the applied dc voltage, the center frequency of the filter showed an increasing response and reached 2.1 GHz by value of

200 MHz at 5V.

Future work

In this study a varactor diode as tunable component was used. However, there are different types of tunable component such as BST, MEMs, etc. We recommend to use them in our device to see the tenability of the device. Also, measuring the response of the designed filters in time domain to characterize the noise of the device is another promising area to study.

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