Sunday, April 22, 2018

Smartphone Antennas

A smartphone built on a mobile operating system, with more advanced computing capability connectivity than a feature phone. The smartphones combined the functions of a mobile phone, media players, low-end compact digital cameras, pocket video cameras, and GPS navigation, high-resolution touchscreens, and web browsers. With more operators announcing their LTE network coverage and plans, many smartphone manufacturers are switching to LTE based phones. Long Term Evolution (LTE) is a radio platform technology that will allow operators to achieve even higher peak throughputs than HSPA+ in higher spectrum bandwidth. The prime objective of LTE is to provide high data speed. A typical smartphone along with important electronics component is represented in Figure.

Sunday, December 3, 2017

DESIGNING WAVEGUIDE FILTERS FOR COMMUNICATION DEVICES

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.
8-Pole Interdigital Bandpass Filter
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.
8-Pole Interdigital Bandpass Filter Result
8-Pole Interdigital Bandpass Filter meshThe 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.
8-Pole Interdigital Bandpass Filter filed

2. Dielectric-filled Co-axial line Filter

Coaxial Dielectric filter filedThis 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.
coaxial line filter resultThis 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.
Coaxial Dielectric filter meshCoaxial Dielectric filter

3. Four-Resonator Comb Line Bandpass Filter

EM simulation is the key design tool for filter design and has reduced experimental comb Line Filterdesign 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.
combline filter resultcombline filter filed
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.

Sunday, September 10, 2017

CAPACITIVE RF MEMS SWITCH DESIGN AND SIMULATION

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.
sw1
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
  1. Wafer: This MEMS is fabricated on substrate as silicon, GaAs as active substrates
  2. Bridge: Gold (Au) is used as the membrane material.
  3. Dielectric: Silicon Dioxide(Sio2) is used as insulator
  4. Silicon as substrate
sw2
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.
off2.jpgThis 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.
on2It 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.
mesh

Saturday, August 12, 2017

QFN Package Simulation

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.
qfn1The 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.
qfn24×4 [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, qfn3.jpga 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 qfn4.jpg15GHz 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.

Sunday, May 14, 2017

PILLBOX ANTENNA DESIGN AND SIMULATION

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.
pil1.jpgThe 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 pil2.jpgwaveguide 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.
pil3.jpg
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.

Sunday, March 26, 2017

RFID Tag and Reader Antenna Design Techniques

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.