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  • mbenkerumass 10:33 pm on November 14, 2019 Permalink | Reply
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    IP3 Distortion and Linearity 

    RF/Photonics Lab
    November 2019
    Michael Benker

     

    IP3 Distortion & Linearity

     

    Linearity is the measure of a system’s performance as an output signal being proportional to the input signal level as characterized by Ohm’s Law, V = I*R. Not every system can be expected to perform ideally and thus linearly. Devices such as diodes and transistors are examples of non-linear systems.

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    The intercept point of the third order, IP3 is a measure of the linearity of a system. IP3 is the third order of a Taylor series expansion of the input signal’s presence in the frequency domain. Being third order, this term in a Taylor series expansion is understood as distortion since it is different from the sought output signal. In contrast to the second order harmonics, which fall outside of the frequency band of the first order signal, the third order is found in the same frequency band as the original or first order signal. Similarly, consecutive even orders (4, 6, 8, etc) are found outside of the frequency band of the first order signal. Consecutive odd orders beyond the third order such as IP5 and IP7 also cause distortion but are not of primary focus since the amplitude of these order signals are weaker after consequent exponentiation.

    The meaning of an intercept point of an nth order (IPn) on a dBm-dBm axis is the point at which the first-order and nth-order powers would be equal for a given input power. In the case of IP3, this indicates the power level needed for a third-order power to potentially drown out the first-order signal with distortion. The 1 dB compression point defines the range of linear operation for a system.

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  • mbenkerumass 5:53 pm on November 13, 2019 Permalink | Reply
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    Scattering Parameters 

    RF/Photonics Lab
    Jared Alves
    November 2019

    Scattering Parameters

    After the mid-1900s, high frequency networks became increasingly prevalent. When analyzing low frequency circuits parameters such as voltages and currents are easily realized. From these signals, Y and Z (admittance and impedance) parameters can be used to describe a network. For the Radio Frequency and Microwave range, S parameters are much more applicable when studying a network of a single port or multiple ports. Each S parameter can be placed in an NxN square matrix where N is the number of ports. For a single port network, only the parameter S11 (also known as ᴦ (gamma or voltage reflection coefficient)) can be realized. The S parameters are unitless because they are ratios of voltages. The parameters can be viewed as both reflection and transmission coefficients for multi-port networks. S parameters with subscripts of the same number are reflection coefficients, as they describe the ratio of voltage waves at a single port (reflected to incident).

    For a two-port network, only parameters S11, S12, S21, S22 exist. For a simple network like this, S11 represents return loss or reflection at port 1. S22 is the output reflection coefficient.  S12 and S21 are transmission coefficients where the first subscript is the responding port and the second the incident port. For example, S21 would be the “forward gain” at port 2 incident from port 1. The following diagram shows an abstracted view of a two-port network, where each “a” and “b” are normalized by the system’s characteristic impedance. Each S parameter can be calculated by terminating a port with a matched load equal to the characteristic impedance. For example, when calculated return loss for a two-port network, port 2 should be terminated by a matched load reducing a2 to zero.  For calculating S1,2 or S2,2 port 1 would be terminated with a matching load to reduce a1 to zero. Each “a” is an incident wave and “b” a reflected wave. Having a matched load at a port results none of the incident wave being reflected due to impedance mismatching.610

    This leads to the following voltage ratios:

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    An amplitude with a negative superscript indicates a reflected wave, and an amplitude with a positive superscript indicates a forward propagating wave.

    SParameters

     

     
  • mbenkerumass 5:29 pm on November 12, 2019 Permalink | Reply
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    Smith Chart 

    RF/Photonics Lab
    Jared Alves
    November 2019

    Smith Chart

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    The Smith Chart, named after laboratories engineer Phillip Smith, is a graphical tool for solving RF transmission line problems. There are many specific uses for a Smith Chart, but it is most commonly used to visually represent impedance matching problems. Although paper Smith Charts are outdated, RF equipment such as Network Analyzers display information using the chart as well.

    The Smith Chart is a unit circle (radius of one) plotted on the complex plane of the voltage reflection coefficient (ᴦ – gamma). As with any complex plane, the vertical axis is the imaginary and the horizontal axis the real. The Smith Chart can be used as an admittance or impedance chart or both. For a load impedance to be plotted on the chart, it must be normalized (divided by) the characteristic impedance of the system (Zo) which is the center of the chart. With this information in mind, it is apparent that a matched load condition would result in traveling to the center of the chart (where ZL=Zo). Along the circumference of the chart, there are two scales: wavelength and degrees. The degrees scale can be used to find the angle of the complex reflection coefficient. Since the plot is the polar representation of the reflection coefficient, if a line is drawn from the load impedance point to the center of the chart this would be considered the magnitude of the reflection coefficient. By extending the line to the circumference of the circle, the angle (in degrees) can be found. The wavelength scale shows distance across a transmission line in meters. A clockwise rotation represents moving towards the generator whereas a counter-clockwise rotation represents moving towards the load side.

    It is important to note that a Smith Chart can only be used at one specific frequency and one moment in time. This is because waves are functions of both space and time as shown by the equations:

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    VF is the forward propagating voltage wave and VR is the reverse propagating voltage wave. If a transmission line system is not impedance matched, a reflected wave will exist on the line which will cause partial or fully standing waves to occur on the line (the reflected wave will add to the incident wave). For the matched condition the reflected wave is zero. Because the Smith Chart can only be used at a specific instant in time and at one frequency the first exponential term in each equation drops out. Because the reflection coefficient is the ratio of the reflected wave to the forward propagating wave, the reflection coefficient becomes:

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    Where C is the ratio of the amplitudes of both waves. For a passive load, the reflection coefficient must be equal to one or less because the reflected wave cannot be greater in amplitude than the incident wave.

    Many transmission lines can be approximated as lossless and therefore have zero attenuation. This leads to:

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    The propagation constant is a complex number that describes how a wave changes as it propagates down a transmission line. The real part is attenuation constant (Nepers/meter) and the imaginary part is the phase constant or wave number (radians/meter).

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    For the lossless condition the attenuation is zero, as stated previously.

    On the Smith Chart, the wavelength λ = 720. This is because the reflected wave must travel the roundtrip distance moved (it must propagate forward and then back again). Using the piece of information, a half wavelength distance is one complete revolution on the chart. This leads to the conclusion that a transmission line that is a half wavelength long does not transform impedance.

     

    The following image shows common points on the Smith Chart.

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    The left-hand side of the chart (lying on the real axis) represents a short circuit load. This makes intuitive sense because the reflection coefficient must be real and negative for a short circuit. This is because short circuits have a voltage drop of zero across them which would require a same-amplitude wave with a 180-degree phase shift to cancel the forward propagating wave. The right-hand part of the real axis represents the open circuit load, where the reflection coefficient is purely real but has no phase shift. For an open circuit, the current wave would have to be phase shifted by 180-degrees, but since the reflection coefficient is a voltage reflection coefficient it is not necessary for it to be phase shifted. As shown in the image, the upper half plane is inductive (positive reactance) and the lower half is capacitive (negative reactance).

     

    Smith Chart

     

     
  • mbenkerumass 4:35 pm on November 9, 2019 Permalink | Reply
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    Angle Modulation 

    RF/Photonics Lab UMASS Dartmouth
    November 2019
    Michael Benker

    Angle Modulation

    In comparison to Amplitude Modulation, which varies the magnitude of the sinusoidal carrier wave, Angle Modulation varies the phase of the carrier wave. The two most common forms of angle modulation are phase modulation (PM) and frequency modulation (FM). Phase modulation varies the instantaneous angle linearly with the message signal, while frequency modulation varies the instantaneous frequency with the message signal. The signals on the right are understood (from top to bottom) as the carrier frequency,the modulating wave and the result signal of amplitude modulation, phase modulated and frequency modulation. Due to phase modulated and frequency modulated waves having constant amplitude AC, noise is expected to be lower, although the transmission bandwidth is increased.Rates of distortion are reduced with a reduced possibility of a polarity shift. The average power for angle modulated wave is Pave=(1/2)*(AC)2.The table below summarizes the relationship between phase-modulated and frequency-modulated waves. An FM wave can be seen as a PM wave with a substitution of the integral of the message signal for the message signal. Further, an FM wave can be represented as having gone through an integrator while a PM wave is represented as having gone through a differentiator.

    The benefits of conserving bandwidth lead to the development of the narrow-band frequency modulation scheme. To achieve this, several parameters are defined. The frequency deviation, or the maximum departure of the instantaneous frequency from the carrier frequency is defined as Δf = kfAm, where kf (as mentioned in Table 4.1) is the frequency sensitivity factor.The modulation index, β is the ratio of the frequency deviation to the modulation frequency: β = Δf/fm. The angle of the FM wave and the FM wave itself are described as: The following block diagram depicts a method for generating a narrow-band FM wave:Carson’s rule defines an approximate relation for the transmission bandwidth of an FM wave generated by a single-tone modulating wave. From the following expression(Carson’s rule), it is understood that large values of the modulation index β the bandwidth is slightly greater than the twice the frequency deviation Δf and for small values of the modulation index, the spectrum is limited to the carrier frequency and a pair of side-frequencies at fc± fm, in which case the bandwidth approached 2*fm.

    anglemodulation

    angle1angle212349fm

     
    • Jared 11:28 pm on November 9, 2019 Permalink | Reply

      Phase modulation is a bit tougher to understand for me than frequency modulation. Awesome post

      Like

      • mbenkerumass 5:59 am on November 10, 2019 Permalink | Reply

        For phase modulation, I think one way to understand it is to think of the effects we talk about using transmission lines. Depending on the length of the line in comparison to the wavelength, there is a phase shift on the output. This is a type of phase modulation. It would be interesting to ask Dr. Gendron about that one.

        Like

  • mbenkerumass 5:46 pm on November 8, 2019 Permalink | Reply
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    Fiber Optics (Introduction) 

    RF/Photonics Lab at UMASS Dartmouth
    November 2019
    Michael Benker

    Fiber Optics

    When the frequency of a signal is increased, so does the transfer rate. On the electromagnetic spectrum, light waves occupy frequency ranges of several hundred Terahertz. Fiber optics and photonics take advantage of the speed of light waves to allow for a different approach to data communications. When using light waves instead of electrical charges, this drastically alters the normal characteristics of electrical information transfer. A light wave being sent through glass in a fiber optic wire is no longer restricted to Ohm’s law for example, since a light wave will move through a resistor without any loss. Although light waves are susceptible to quantum noise, they are immune to noise caused by heat (in many cases, this means they are virtually noise-less). Fiber optics, due to their high data rates, flexibility and immunity to noise offer an extraordinary opportunity for scientific and engineering progress.

    external-content.duckduckgo.com

    fiberoptics

     
  • mbenkerumass 8:43 pm on October 12, 2019 Permalink | Reply
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    NanoVNA – Handheld Vector Network Analyzer 50kHz-900MHz 

    A Network Analyzer for $60 on Amazon. Looking forward to owning my own and spending more time with Network Analyzers, like the one’s in the RF/Photonics Lab.

    nanoVNA

     
  • mbenkerumass 4:58 pm on May 26, 2019 Permalink | Reply
    Tags: OrCAD, PCB Design   

    OrCAD Tutorial/Study Guide 

    This post outlines the steps needed to create a schematic using OrCAD and then prepare it for manufacture as a PCB board. Writing this post helps me to learn OrCAD better and this will serve as a guide for review later. I will be using the free version, OrCAD Lite.

     

    Opening a new project

    1. First, start a new project.

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    2. Give the project a name and create the folder that you want for the project files. Select PSpice Analog or Mixed A/D.

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    3. Select “Create a blank project” if starting from scratch.

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    Building the Schematic

    4. Select the “Place Part” button or press P to open the parts menu.

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    5. This next part requires a bit of knowledge about where which libraries the components are found under. Here, I want to place a resistor, so I typed R and selected the library “Analog”. If the libraries are not added, you can find them in the OrCAD folder on the PC and add them using the “Add libraries” button shown on the screen.

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    6. Double-click on the part in the menu to place it on the schematic page. I also added a VDC, which is found in the Source library. Finish placing parts.

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    7. In this case, I will add an LED, but I am unable to find it in the Place Part menu. To find what I am looking for, I chose “Place”, “PSpice Component…” and “Search…” to open a new menu shown below. Further components can be found here if you are unable to find what you need. Under part name and description, select one from the list to add it to the schematic.

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    8. Press G on the keyboard to add a ground. I chose “0/CAPSYM”. Now select the “Place Wire” button or W to put down the wires.

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    9. Double-click on the voltage and resistor values to change them as necessary.

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    Simulation

    10. To run a simulation, on the drop down menu, select “PSpice”, “New Simulation Profile”. Give this simulation a name.

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    11. Define the parameters for this simulation, click apply and Ok.

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    12. Select the voltage probe and add it to the circuit.

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    13. Select “Run” and open the new simulation window to view results.

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    PCB Design

    14. First, a folder will be created for the PCB. Rename the schematic folder in the main project folder., then rename the default “PAGE1” page name.

    14.0

    14.1

     

    15. Right-click on the main project folder and create a new schematic. This will be for the PCB board. Now, copy the schematic from the schematic folder and paste it to the new PCB folder. Rename the copied schematic to indicate it is for PCB and not the schematic.

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    15.2

     

    16. Make the PCB folder the root folder. Click and open the PCB schematic file.

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    17. For the PCB board, the DC voltage needs to be replaced with connectors. Select the VDC and delete it. Select “Place Part” and choose to add a new library. The connectors are found in the library folder shown below.

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    18. Select CON1 from the part list and place the parts where the VDC was connected. Remember to save.

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    19. Select all the components and go to “Edit”, “Properties”.  Select the “Parts” tab on the lower left.

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    20. Scroll to the right to view the PCB Footprint tab. The footprint names here are then changed to the footprint names found in the libraries. Save.

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    21. Now, open the OrCAD PCB Designer program and create a new drawing in the main project folder. I put it in a separate folder inside the project folder. Selecting the board wizard will take you through a series of prompts.

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    22. Continue through the wizard (in this case using only default settings until Spacing Constraints). At Spacing Constraints, change the Minimum line width from the default 0 (this default setting can be problematic). Then select the default via padstack. I chose “Via”. Select ok. Continue through the wizard. In this case, choose a rectangular board. After finishing the wizard, you will see an empty square. Now, save and close the PCB designer.

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    23. Go back to Capture CIS, open the project tab and select the PCB schematic file. Select “Tools”, “Create Netlist…” to begin transferring the schematic to a printed circuit board. Select Create of Update PCB Editor Board and choose the file created using OrCAD PCB Designer. For the output file, I chose to output the board to the same file, since I won’t be needing the empty board file. Now, select Open Board in OrCAD PCB Editor and select OK.

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    24. Now, go to “Place”, “Components Manually…” to add the parts from the schematic to the PCB. Select the components you need to place (or select all if you will place all of them) and hide the menu.

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    25. Place the parts on the board. Parts may need to be rearranged to fit nicely. When satisfied,, right-click and select “Done”. Save and “overwrite”.

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    26. Now select in the menu, “Route”, “Connect” to place wires connecting the components. When finished, right-click and select “Done”. Once again, save and “overwrite”.

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    26.1

     

    Generating Gerber and Drill files and Online Check

     

     
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