RF/Microwave filters find wide application in communication systems, such as satellite links or wireless base stations. Microwave filters are passive devices employed to select a specific band of the frequency spectrum. Depending on the spectral region that is selected or rejected, they are classified in low-pass filters, high-pass filters, band-pass filters or band-stop filters. Passive devices at the output stage of the communication system must be able to deal with very high power signals. Because of that, waveguide technology is the ideal choice to implement these devices. This work presents the design of a different kind of waveguide-based filter.
Working Principle of Filter?
So, what is magic behind filter? How does it reject signals and pass others? In order to understand this, let us first go through the concept of mismatch. When there is perfect impedance match between the input impedance of system with output load impedance, maximum energy transferred from input to output otherwise there is always some energy loss. A measure of this transmission loss is the reflection coefficient and the related return loss. A frequency dependent mismatch exists in RF/uW devices, due to which signals at those frequencies where the mismatch exists will experience reflection caused by the mismatch. Extreme mismatches are caused by open and short circuits. Filters approach open or short circuit impedances in their stop bands – implying near total reflection. Passive non-resistive filters work by reflection caused by a mismatch condition introduced by the frequency dependent nature of the input impedance. In the bandpass filter (BPF), the resonators and the couplings are arranged in such a way, that the filter is transparent for passband signals. In the stop bands of the filter, the mismatch will cause reflection and thereby attenuation/rejection.
1. 8-Pole Interdigital Bandpass Filter
Interdigital filters are coupled-line structures to implement bandpass filter. The interdigital filter has compact size compared to other coupled line filter hence more popular. Below figure shows one type of 8-pole waveguide based on the center frequency at 1.5 GHz. Each resonator element is a quarter-wavelength long at the mid-band frequency and is short-circuited at one end and open-circuited at the other end. Coupling is achieved by way of the fields fringing between adjacent resonator elements.
In interdigital filter, the second passband is centered at three times the center frequent y of the first passband, and there is no possibility of spurious responses in between. The rates of cutoff and the strength of the stop bands are enhanced by multiple-order poles of attenuation at dc and at even multiples of the center frequency of the first passband.
The simulated frequency response of the filter determined using FEM solver is shows the variations of S-parameters with frequency for the L-band interdigital BPF. The unwanted harmonics are suppressed with stop band attenuation better than -14 dB everywhere. FEM mesh and simulated data is are shown in figures.
2. Dielectric-filled Co-axial line Filter
This is dielectric-filled coaxial cable low pass filter that is tuned with five annular rings (irises) that are added to the outer conductor wall in this design. To address the wideband frequency response with a fine frequency resolution, the model is simulated using fullwave 3D electromagnetic solver. The computed S-parameters show a low-pass frequency response with a cutoff frequency around 770 MHz.
This stepped-impedance low pass filter includes electrically conductive coaxial transmission line, at least one inductive element and at least one capacitive element. The capacitive elements and the inductive elements are disposed in an alternating manner along a length of the transmission line.
3. Four-Resonator Comb Line Bandpass Filter
EM simulation is the key design tool for filter design and has reduced experimental design work for distributed-element and waveguide resonator filters to a minimum or made it completely redundant. The EM simulation involves the calculation of the electromagnetic fields inside the filter structure. 3D EM simulation uses full wave analysis that is what actually exists in nature. 4-resonator combline filter fields at fc and port S-parameter response.
The design and dimensions of the model have been optimized to a point where great performance was significantly shown alongside with good matching around 535 MHz.
The switching is required in many applications at low as well as at high frequency. RF MEMS switches are the specific micro mechanical switches that are designed to operate at RF to mm-wave frequencies. MEMS switches usages some mechanical movement to achieve a closed or open circuit in the Radio Frequency transmission lines. GaAs FET switches do not have sufficient isolations to minimize cross interference and signal jamming from channels is close proximity. MEMS switches provide high isolation when open, low insertion loss when closed, and can be operated at low power consumption. Because of electromechanical isolation, RF circuit doesn’t leak or couple significantly to the actuation circuit. MEMS are small in size hence it occupies less space in circuit designs so that which was the most required device in the communicating world. Radio Frequency Micro Electro Mechanical Switches (RF MEMS) classification depends on the type of actuation, deflection axis, contact type, circuit configuration, and Structure configuration. The most used RF MEMS mechanical structures are the cantilever beam and the air bridge structures. The presented design here is electrostatically actuated capacitive fixed-to- fixed bridge base capacitive switch.
An air bridge base capacitive RF MEMS is shown here. Gold (Au) is used as a beam material, and Silicon Nitride (Si3N4) with dielectric constant 8.5 is used as a dielectric material. Silicon nitride thin film dielectrics are used in capacitive radio frequency micro-electromechanical systems (MEMS) switches since they provide a low insertion loss, good isolation, and low return loss. A capacitor is built up between the fixed electrode and movable electrode. Below are components of MEMS switch
Wafer: This MEMS is fabricated on substrate as silicon, GaAs as active substrates
Bridge: Gold (Au) is used as the membrane material.
Dielectric: Silicon Dioxide(Sio2) is used as insulator
Silicon as substrate
In air bridge type MEMS switch, the beam is fixed at both the ends and voltage is applied in the middle of the beam to note down the displacement of the beam towards the substrate. The displacement is maximum in the middle region when we go on increasing the applied voltage. The actuation voltage or applied voltage or pull in voltage is the maximum voltage at which the electrostatic force becomes superior over mechanical restoring force, causes MEMS device pull down towards the ground plane. Initially the input applied voltage is 1mv there is no deformation in the switch under this condition input is equal to output, but the second case, in addition, a 5v is added to 1mv then there is some electrostatic force is created between the electrodes then the cantilever will deform and touches to ground under this condition the output is zero. The switch closing time depends on the actuation voltage and the opening time depends on the mechanical properties of the switch.
This RF MEMS Switch is designed and simulated using Finite Element Method (FEM) tool. Finite Element Method (FEM) tools are very helpful to design and simulate the RF MEMS Switch. S- Parameter results are obtained, and examine return loss (S11) and insertion loss (S12) of switches. During the OFF state, the insertion loss is less than zero and return loss is less than 40 dB. Vice versa output graph could be obtained during the ON state simulation of MEMS switches.
It is observed from the results that the MEMS switch circuit has an insertion loss (S21) of 0.4 dB and return loss (S11) of about less than 12 dB in the OFF state up to 20 GHz. The switch has an insertion loss (S21) of -25 dB and return loss (S11) of about -0.10 dB in the up state thus exhibiting good switch characteristics.
The simulated E-filed plot and FEM mesh are shown here.
he Quad Flat No-lead (QFN) package is a CSP (plastic encapsulated package) with a copper lead frame substrate. QFN type package is one of the most cutting-edge IC packaging technologies in the electronics. The QFN is a leadless package where electrical contact to the PCB is made by soldering the leads on the bottom surface of the package to the PCB, instead of the conventional formed perimeter gull wing leads.
The QFN-type package is known for its small size, cost-effectiveness and good production yields. QFN also possess certain mechanical advantages for high-speed circuits including improved co-planarity and heat dissipation. The QFN has pins on 4 edges of the bottom surface of the package. The QFN can have either a square or rectangle body as well as symmetric or asymmetric terminal patterns. The QFN was introduced to replace the gull wing lead Quad Flat Package (QFP) because the component leads are embedded in the plastic and cannot be bent during handling to insure consistent assembly attachment.
4×4 [mm] 16 Pin QFN
Design is shown below . This QFN package has 16 pin. Metal thickness is 0.15 mm and plastic encasement is 0.15 thick. Defining a substrate stack up for QFN is pretty straightforward. The mold encapsulations are defined by dielectric bricks and the leads are defined by vias. The top side graphic shows the side view of package that is mounted on PCB. The bottom picture shows the details of leads, bondwires, and the chip, in this case, a thin film circuit.
QFN Electromagnetic simulation model and result
The input and output transmission lines on the PC board are connected to the package leads. On the top of die paddle, a thin film circuit with a thru transmission line is attached to see the package performance. Double bonding with a compensated bond pads are used to improve the frequency performance here. With this typical interconnect scheme, the simulation results show that the package can be used up to 15GHz when the required input and output return loss are around -20dB.
Package performance can be further improved by Increasing the width of input/output transmission lines to make 50ohm impedance or use two lead frames instead of single to minimize the transitional impedance profile and split the double bonding to the two lead frames.
A pillbox antenna is a linearly polarized cylindrical reflector embedded between two Parallel plates. It is usually fed by a waveguide. The pillbox is part of a family of antennas called fan beam antennas which produce a wide beam in one plane and a narrow beam in the other. The pillbox antenna can be dual polarized and is also a relatively wide bandwidth antenna.
The advantages of using a pillbox antenna for radar applications are
It is easy to design and the cost of production is low.
It is dually-polarized and it is also a wide band antenna.
It has a high power handling capability
The pillbox feed is traditionally located at the focal point of the reflector. For symmetrical antennas this is located in the middle of the aperture. Either a pin or waveguide feed can be used, depending on the system requirements. Further variations of these feeds can be found through the use of stubs which are used to obtain better impedance matching and reflector illumination, not discussed in this dissertation.
Pillbox Antenna with Waveguide Feed
Below is Pillbox antenna designed at 39 GHz. The feed is a waveguide feed. The waveguide feed is first designed separately from the system. Flares are attached to the sides and optimized for the best reflection coefficient. Once the optimal configuration is obtained, the waveguide is used as a feed in the pillbox structure. Larger flares generally give a better reflection coefficient, but effectively increase the aperture of the waveguide, lowering the beam-width. The maximum gain of antenna is 25.6 dBi. Antenna polar plot (phi=90 degree) and E-field plot is shown below in figure.
The parabolic reflector reflects rays incident on its center directly back to the feed, this together with the narrow beam-width of the waveguide causes the majority of the energy to be reflected back into the waveguide, resulting in the impedance mismatch. One solution to this problem is to design the waveguide to have a wider beam-width and to radiate less energy in the centre through the use of stubs. Enlarging the pillbox width should also decrease the amount of energy reflected back into the feed.
RFID stands for Radio-Frequency Identification. The RFID device provides a unique identifier for that object and just as a bar code or magnetic strip the RFID device must be scanned to retrieve the identifying information.
RFID System Working Principal
A RFID system has three parts:
A scanning antenna
A transceiver with a decoder to interpret the data
A transponder – the RFID tag – that has been programmed with information
In most of RFID system, tags are attached to all items that are to be tracked. These tags are made from a tiny tag-chip that is connected to an antenna. The tag chip contains memory which stores the product’s electronic product code (EPC) and other variable information so that it can be read and tracked by RFID readers anywhere. An RFID reader is a network connected device (fixed or mobile) with an antenna that sends power as well as data and commands to the tags. The RFID reader acts like an access point for RFID tagged items so that the tags’ data can be made available to business applications.
RFID Frequency Band Allocation
There are a number of RFID frequencies, or RFID frequency bands that systems may use. There are a total of four different RFID frequency bands or RFID frequencies that are used around the globe.
As part of the design of the RFID antenna, parameters such as the radiation resistance, bandwidth, efficiency, and Q all need to be considered, so that the resulting design for the RFID antenna meets the requirements and allows the required level of performance to be achieved. RFID antennas are tuned to resonate only to a narrow range of carrier frequencies that are centered on the designated RFID system frequency.
The RFID antenna propagates the wave in both vertical and horizontal dimensions. The field coverage of the wave and also its signal strength is partially controlled by the number of degrees that the wave expands as it leaves the antenna. While the higher number of degrees means a bigger wave coverage pattern it also means lower strength of the signal. Passive RFID tags utilize an induced antenna coil voltage for operation. This induced AC voltage is rectified to provide a voltage source for the device. As the DC voltage reaches a certain level, the device starts operating. By providing an energizing RF signal, a reader can communicate with a remotely located device that has no external power source such as a battery. According to the different functions in the RFID system, the RFID antennas can be divided into two classes: the tag antenna and the reader antenna.
Tag antennas collect energy and channel it to the chip to turn it on. Generally, the larger the tag antenna’s area, the more energy it will be able to collect and channel toward the tag chip, and the further read range the tag will have. Tag antennas can be made from a variety of materials; they can be printed, etched, or stamped with conductive ink, or even vapor deposited onto labels. The tag antenna not only transmits the wave carrying the information stored in the tag, but also needs to catch the wave from the reader to supply energy for the tag operation. Tag antenna should be small in size, low-cost and easy to fabricate for mass production. In most cases, the tag antenna should have omnidirectional radiation or hemispherical coverage. Generally, the impedance of the tag chip is not 50 ohm, and the antenna should realize the conjugate match with the tag chip directly, in order to supply the maximum power to the tag chip. Tag antenna may be the signal turn or multiple turns as shown here.
Reader antennas convert electrical current into electromagnetic waves that are then radiated into space where they can be received by a tag antenna and converted back to electrical current.
RFID Antenna Design
This design RFID system is used to track object placed on the storage rack. In this system, there is two component of RFID.
RFID Reader: this component is fit on the shelf and connected to data base computer system
RFID Tag: this component along with planar antenna is placed in tracking objects placed in store
When a particular object placed on the shelf or removed from the shelf, information of that object is automatically updated in database computer. The antenna is optimized to increasing the read accuracy and shortening the optimization phase. One more RFID transponder antenna designed at 13.5 MHz is shown below.
Planar Antenna for Ultra High Frequency (UHF) RFID Handheld Reader
This antenna consists of a microstrip-to-coplanar stripline transition, a meandered driven dipole, a closely-coupled parasitic element, and a folded finite-size ground plane. This Antenna is suitable for RFID handheld readers.
The helix antenna is a travelling wave antenna, which means the current travels along the antenna and the phase varies continuously. Helix antennas (also commonly called helical antennas) invented by John Kraus give a circular polarized wave. Helix antennas are referred to as axial-mode helical antennas. The benefits of this helix antenna are it has a wide bandwidth, is easily constructed, has real input impedance, and can produce circularly polarized fields. There are two mode of circular polarization in helix antenna.
Left handed helix antenna: In left handed helix antenna if you curl left hand fingers around the helix, thumb would point up. The waves emitted from this helix antenna are Left Hand Circularly Polarized.
Right handed helix antenna: In right handed helix antenna if you curl right hand fingers around the helix, thumb would point up. The waves emitted from this helix antenna are Right Hand Circularly Polarized
The minimum number of turns for a helix is between 3 and 5. There are many online tools available to calculate design parameter of helical antenna.
Design parameters for Helical Antenna
D – Diameter of a turn on the helix antenna.
C – Circumference of a turn on the helix antenna (C=pi*D).
S – Vertical separation between turns for helical antenna.
α – Pitch angle, which controls how far the helix antenna grows in the z-direction per turn
N – Number of turns on the helix antenna.
H – Total height of helix antenna, H=NS.
Broad-Band Helix Antenna Design
Below is example of one Helix antenna that work at central frequency 1.35 GHz.
Radius-20 mm Wire Radius – 2 mm Number of Turn – 12 Height – 400 mm Rotation- RHS
Helix antennas of at least 3 turns will have close to circular polarization in the +z direction when the circumference C is close to a wavelength.
The helix antenna functions well for pitch angles between 12 and 14 degrees. Typically, the pitch angle is taken as 13 degrees.
Antenna is simulated using Finite-Difference Time domain (FDTD) technique. Below is simulated return loss and field plot.
Antenna radiation pattern ( Far Field data plot) is shown below . Antenna Gain is around 15 dB .
There are many other form of Helix antenna like quadrifilar helix antenna.The Quadrifilar Helix Antenna has 4 excitations and each element driven a progressive 90 degrees in phase. Then Bifilar helix is constructed using two volutes with an equal number of turns, and their starting points positioned 180° apart. The ends of the volutes are connected with a shorting wire which adds to the structural integrity of the antenna.