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  • mbenkerumass 10:11 am on November 9, 2019 Permalink | Reply  

    Welcome to the Students' Page for the RF/Photonics Lab at UMASS Dartmouth 

    What we do in this lab:

    • RF – Radio Frequency, Microwave Electronics
    • Photonics – Lightwave Frequency Electronics, Fiber Optics, Lasers, Optics
    • Research
    • Hands-on learning

    rfphot


    If you are a student and you want to do research in the RF/Photonics lab, focus on the following and talk to Dr. Li:

    Mathematics, Fourier Transform, Differential Equations, Electrical Theory, Electromagnetic Theory, Communication Theory, Analog Electronics, Signal Processing (ECE 321, ECE 384), English Writing (research writing) and 3.0+ GPA


    If you would like to get in contact with the RF/Photonics Lab at UMASS Dartmouth, feel free to fill out the form on the Contact page. Also, visit the official page for the RF/Photonics Lab.

     

     
  • mbenkerumass 9:00 am on January 19, 2020 Permalink | Reply
    Tags:   

    027/100 Shunt Reactance on Smith Chart 

    Example 3.5-2A: Measure the effect of susceptance on Smith Chart impedance matching.

    First, build the circuit and run the Smith Chart Matching tool.

    026-3

    A shunt capacitor moves in a clockwise direction across the smith chart tool:

    027-2

    Also note that a shunt inductor moves counter-clockwise across the smith chart tool:

    027-1

     
  • mbenkerumass 6:00 am on January 18, 2020 Permalink | Reply
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    Common Source JFET Amplifier 

    The advantages of using a Field Effect Transistor were discussed previously. The design of an amplifier using a junction field effect transistor (or another FET such as MOSFET) is generally the same procedure as that done for a BJT transistor amplifier.

    csa1csa2

     
  • jalves61 6:07 pm on January 17, 2020 Permalink | Reply
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    ARRL Examination Study (Part I) 

    The ARRL (American Radio Relay League) is an organization for amateur radio enthusiasts. In order to communicate using HAM radio, at least a technician license must be obtained. The following post is meant as a useful information guide for those wishing to obtain a license.

    The ARRL provides a complete manual as a study reference for HAMs. The book is divided into nine chapters: Basic info about ARRL, Radio and Signals, Circuit components, propagation and antennas, Amateur radio equipment, HAM communication, License regulation, operating regulation and safety. The questions come directly from each chapter (35 total, 26 to pass).

     

    For Radio and Signal fundamentals, it is important to know basic properties of waves including wavelength, speed of propagation, the relation between wavelength and frequency, identifying frequency bands, the frequency ranges of various bands used by HAMs and so forth. The fundamental equation for propagation of waves is c = fλ. Because radio waves are being transmitted by antennas through air, the speed of propagation is 300 million meters/sec. This is a constant value and therefore if frequency is increased, the wavelength decreases proportionally. This speed value is roughly equivalent to the speed of light in a vacuum. The property of radio waves used to identify different frequency bands is wavelength. HAMs tend to use the frequencies occupied by bands MF through UHF. It is important to know the frequency ranges of these bands.

    bands.PNG

    In this section, it is important to know prefixes for the SI unit system, so conversions between various values can be made. The following table should be committed to memory.

    SI.PNG

    The next section deals with modulation, which is a necessary function to transmit the correct signal to receiver. It is important not to set a transmit frequency to be at the edge of any band to allow for transmitter frequency drift, allow for calibration error, and so that modulation sidebands do not extend beyond the band edge. It is important to know about FM deviation (which is dependent on amplitude of the modulating signal) and that if the deviation is increased, the signal occupies more bandwidth. Setting a microphone gain too high could cause the FM signal to interfere with nearby stations. It is important to know the types of AM modulation (Double Sideband, Single Sideband, etc) and which modulation technique is best for various frequency bands. “Continuous wave” (Morse code-esque) modulation occupies the lowest bandwidth, followed by SSB modulation. The various advantages to certain modulation techniques should be understood. For example, SSB is preferential to FM because it occupies less bandwidth and has longer range. The bandwidth for each modulation technique is shown below.

    bandwidths.PNG

    The final section of Chapter two deals with radio equipment basics. A repeater should be understood to be a station that retransmits a signal onto another channel. The following is an image of a transceiver, which transmits and receives RF signals using a TR switch to switch between each function. A repeater uses a duplexer in place of this switch to transmit and receive simultaneously.

    transciever

     
  • mbenkerumass 2:35 pm on January 16, 2020 Permalink | Reply
    Tags: , Wireless Communications   

    Feed Lines (HAM Radio) 

    The following are questions that are used for the HAM Radio Technician level license. The title of this section of questions is: “Feel Lines: types of feed lines; attenuation vs. frequency; SWR concepts; matching; weather protections; choosing RF connectors and feed lines.”

    Question 1.

    Why is it important to have a low SWR in an antenna system that uses coaxial cable feed line?
    A. To reduce television interference
    B. To allow the efficient transfer of power and reduce losses
    C. To prolong antenna life
    D. All of these choices are correct

    SWR or Standing Wave Ratio refers to the efficiency of an antenna. A low SWR means that the loss will be reduced in the system. Therefore, the correct answer is B.

     

    Question 2.

    What is the impedance of the most commonly used coaxial cable in typical amateur radio installations?
    A. 8 ohms
    B. 50 ohms
    C. 600 ohms
    D. 12 ohms

    The correct answer is B. 50 Ohm transmission lines are very common in most RF systems. Television systems use 75 Ohm lines.

     

    Question 3.

    Why is coaxial cable used more often than any other feed line for amateur radio antenna systems?
    A. It is easy to use and requires few special installation considerations
    B. It has less loss than any other type of feed line
    C. It can handle more power than any other type of feed line
    D. It is less expensive than any other types of feed line

    The correct answer is A, considering it’s ease of use.

     

    Question 4.

    What does an antenna tuner do?
    A. It matches the antenna system impedance to the transceiver’s output impedance
    B. It helps a receiver automatically tune in weak stations
    C. It allows an antenna to be used on both transmit and receive
    D. It automatically selects the proper antenna for the frequency band being use

    An antenna tuners are used if the SWR is too high for a radio to operate properly. It matches the antenna’s impedance to the impedance of a transmitter. Automatic tuners also exist. Correct answer: A.

     

    Question 5.

    What generally happens as the frequency of a signal passing through coaxial cable is increased?
    A. The apparent SWR increases
    B. The reflected power increases
    C. The characteristic impedance increases
    D. The loss increases

    Coaxial cables work well within certain frequency ranges, however most are not rated to go above certain ranges. You’ll need to pay more for coaxial cable that can handle higher frequencies. The reason for them not working well at high frequencies is due to loss. Correct answer: D.

     

    Question 6.

    Which of the following connectors is most suitable for frequencies above 400 MHz?
    A. A UHF (PL-259/SO-239) connector
    B. A Type N connector
    C. An RS-213 connector
    D. A DB-25 connector

    Type N connectors are used above 400 MHz. Correct answer: B.
    N

     

    Question 7.

    Which of the following is true of PL-259 type coax connectors?
    A. They are preferred for microwave operation
    B. They are water tight
    C. They are commonly used at HF frequencies
    D. They are a bayonet type connector

    PL-259 type coax connectors are used in UHF, HF applications. Correct answer: C.pl259

     

    Question 8.

    Why should coax connectors exposed to the weather be sealed against water intrusion? A. To prevent an increase in feed line loss
    B. To prevent interference to telephones
    C. To keep the jacket from becoming loose
    D. All of these choices are correct

    The correct answer is A. Water intrusion can cause an increase in loss. Correct answer: A.

     

    Question 9.

    What might cause erratic changes in SWR readings?
    A. The transmitter is being modulated
    B. A loose connection in an antenna or a feed line
    C. The transmitter is being over-modulated
    D. Interference from other stations is distorting your signal

    Standing wave ratio is important for the measuring the efficiency of equipment. Modulation or interference therefore will not have an effect on the SWR. Loose connections may however cause an issue with SWR. Correct answer: B.

     

    Question 10.

    What electrical difference exists between the smaller RG-58 and larger RG-8 coaxial cables?
    A. There is no significant difference between the two types
    B. RG-58 cable has less loss at a given frequency
    C. RG-8 cable has less loss at a given frequency
    D. RG-58 cable can handle higher power levels

    RG-58 and RG-8, although similar are different in that the RG-8 coax cable has less loss per length. Correct answer: C. The following table lists losses per feet:

    coax1

     

     

    Question 11.

    Which of the following types of feed line has the lowest loss at VHF and UHF?
    A. 50-ohm flexible coax
    B. Multi-conductor unbalanced cable
    C. Air-insulated hard line
    D. 75-ohm flexible coax

    Air-insulated hard line coax has the lowest loss with added insulation. Correct answer: C.

     
  • mbenkerumass 6:00 am on January 15, 2020 Permalink | Reply
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    Microstrip Double-Stub Load Matching 

    The following matches a 50 Ohm line to a 100 Ohm load at 10 GHz using a double-stub component. This was designed using the ADS passive circuit DesignGuide tool. This method is a great alternative to using the Smith Chart matching tool for lumped elements if you need a microstrip line for matching.

    Circuit:

    dsmat1

    Substrate:

    dsmat2

     

    Momentum simulation result (can be tuned to center better at 10 GHz):

    dsmat3

    Layout component:

    dsmat4

     

     

     
  • mbenkerumass 9:00 am on January 14, 2020 Permalink | Reply
    Tags: ,   

    Ray Optics – Graded-Index Fibers, Matrix Optics 

    Graded-Index Fibers

    Guiding light rays with multiple lenses or mirrors is possible, however this may result in great loss of optical power due to refraction in a system if there are many lenses or mirrors. Using total internal reflection however, rays may be transmitted over long distances without these losses. Glass fibers are the primary choice for guiding light in this manner using total internal reflection. Glass fibers consist of a glass wire with a cladding. The refractive index of the outer cladding will be smaller than the glass core. This allows for a consistent total internal reflection throughout the wire.

    fiber

    A graded-index material (GRIN) has a refractive index that varies throughout the material. When a ray moves through a graded-index material, the variance in refractive index causes the ray to bend and curve according to how the graded index is laid out.

    grinparax

    The path of an optical ray in graded-index material is determined by Fermat’s principle, which states that the path of a ray is the integral of the refractive index (a function of position on the material) between two points, equated to zero. The ray equation can solve this problem, however for simplification, a paraxial approach is taken to give the paraxial ray equation.

    Ray Equation:

    rayeq

    Paraxial Ray Equation:

    rayeqpar

    A graded index glass fiber is modeled below:

    gribfiber


     

    Matrix Optics

    A paraxial ray is described by a coordinate and angle. Using this approximation, the output paraxial ray going through a system can be written in matrix form:

    abcd          ,            abcd1

    An optical system can be modeled using the 2×2 ABCD matrix. Matrices of systems may also be cascaded to describe the effect of many systems on a ray.

    abcdmatricies

     
  • jalves61 12:00 am on January 13, 2020 Permalink | Reply
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    The Human Ear 

    The Human ear is important to the study of acoustics because it is inborn pressure sensor. It is one of the most sensitive parts of the human body and its job is to sense pressure changes in air and convert these to electrical signals that the brain can process as “sound”. Humans can hear roughly between 20 Hz to 20 kHz but this range decreases with age. The human ear can sense sound intensities from 1 W/sqm to 1 trillionth of a W/sqm. What most people intuitively perceive as music loudness, pitch and timbre roughly corresponds to amplitude (or sound intensity, which is proportional to the square of amplitude), frequency and waveform shape. Of course, these are not one to one relationships because if a tone is too high in frequency (ultrasound) or too low (infrasound) it will effect the perceived loudness because it will not be heard at all, for example.

    The human ear consists of three main parts: inner ear, middle ear and outer ear. The outer ear consists of the pinna, auditory canal and eardrum. The pinna (the only visible part of the ear) serves as a guide to guide pressure waves into the ear canal. The ear canal is filled with air which is necessary because sound needs a medium such as air to transmit pressure waves. The waves reach the conically shaped eardrum, which vibrates and sends signals to the brain to process.

    ear.png

    The middle ear consists of several dense bones (ossicles) called the hammer, anvil and stirrup. These are elastically connected and serve to transmit and amplify sound from the outer to inner ear. These bones are necessary because the pressure waves are being transferred to a different medium (air to ear fluid called endolymph) and require an impedance matching network to transmit sound effectively. This is not unlike the soundboard of a guitar (for impedance matching to air) or an electrical impedance matching network design for maximum power transfer from a source to a load.

    The inner ear contains the cochlea and the semicircular canals. The cochlea contains thousands of tiny hair cells that are stimulated by the vibrations of sound. The semicircular canals contribute to our sense of balance, but not the sensation of hearing. The inner ear fluid causes the hairs in the cochlea to bend, which are converted to electrical pulses and sent to the brain. These are sent to the auditory nerve and are interpreted as sound.

    The following diagram depicts the human ear as a passive electrical circuit using the “impedance analogy”. The eardrum middle ear section is shown to be a transformer to match the outer ear to the middle ear. There could also be another transformer between the middle ear and the cochlea, as stated before. Without going into excruciating detail, it is important to show that the human ear is not all different from an electrical circuit in the sense that it impedance matches and transforms/transduces different forms of energy.

    analogear.png

     

     
  • mbenkerumass 9:00 am on January 12, 2020 Permalink | Reply
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    026/100 Lumped Element Smith Chart Movements: Series Inductor 

    Example 3.5-1: Measure the amount of movement caused by the reactance added to the circuit below. Measure the change from the starting point to the end point on the Smith Chart.

    026-1

    The circuit simulated gives the following result:

    026-2

    Recall that the circuit without a series inductor had the following result:

    025-3

    Through this simulation, it is shown that adding a series inductor causes the smith chart diagram plot to move in a clockwise direction.

    Note the change using the Smith Chart matching tool:

    026-3026-4

     

     
  • mbenkerumass 7:00 am on January 11, 2020 Permalink | Reply
    Tags: , Transistors   

    BJT vs. FET 

    Transistors are important components that are used in a variety of applications. Some types can be used for switching, some for amplification or both. Other transistors perform exclusive tasks, such as the phototransistor, which responds to light by producing a current.

    The main premise of a transistor is that by feeding a transistor a source voltage or current (depending on the type), the transistor allows for the passage of electrons. This process is accomplished through pnp or npn semiconductor structures. The following diagrams provide a general example of the function of a transistor:

    t4t2

    transistor

    t3

     

     

    Bipolar Junction Transistors (BJT) are controlled using a biasing current at the base pin. This means that they will also consume more current than other transistors such as the FET. One advantage of BJT transistors is that they offer greater output gain than an FET. However, BJT can be much larger in size than FET and for this reason, they are less popular, despite being easier to manufacture.

    bjt

    Field Effect Transistors (FET) are voltage-controlled. For this reason they essentially draw no current and therefore do not pose a substantial load to a circuit. FETs are not as useful for gain as BJT, however if the intent is not for amplification then this is not a problem. FETs can be manufactured very small and this is important in manufacturing integrated circuits that use many transistors. FETs and especially the MOSFET subtype are more expensive to manufacture, but remain more popular than the BJT.

    fet

    Some FET transistor types are even constructed on the nano-scale. The FinFET for example is about 10 nm, currently used by Intel, Samsung and others.

    FinFET size

    (2)

    http://www.learningaboutelectronics.com/Articles/Types-of-transistors.php

     
  • jalves61 7:36 pm on January 10, 2020 Permalink | Reply
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    Quarter Wave Transformer Matching – Using Theory of Multiple Reflections 

    There are two ways to derive an impedance value for a quarter wave transformer line. The transformer is an excellent tool to match a characteristic impedance to a purely resistive load where a large bandwidth is not required. It is much easier to find this relationship by examining it from an impedance viewpoint, however the theory of multiple reflections is an excellent topic because it illustrates the contribution of multiple impedance lines to the overall reflection coefficient.

    The following circuit with the matching transformer is shown below.

    reflection

    The addition of the matching transformer introduces discontinuity at the first port. Ideally, the addition of the transformer will match the load resistance to Zo, minimizing all reflection, as will be shown. the bottom figure provides a “step by step” analysis of each trip of the wave as it travels. When the wave first hits the Zo and Z1 junction, it sees Z1 as a “load” and does not yet see the actual load resistance. Depending on the impedance match, some of the wave will be reflected and some will be transmitted. The transmitted part of the wave then travels to the load and a portion is again reflected with reflection coefficient 3. As that portion of the wave travels back to the Z1 and Zo junction, the process repeats. This process continues infinitely and results in the following equation.

    gamme

    Using the definition of a geometric series and writing the reflection coefficient in terms of impedance, the equation reduces to

    reduce.PNG

    The reflection is seen to reduce to zero when Z1 (the impedance of the quarter wave section of transmission line) is set to q.PNG

     
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