VIVA-VOCE QUESTIONS MICROWAVE LAB

Transcription

1 VIVA-VOCE QUESTIONS MICROWAVE LAB DAWAR PARUL EXPERIMENT NO.-2 1) How are wavelength measured?

2 2) How do you measure wavelength in a compression wave? 3) What is the units of measure for wavelength? 4) What is frequency and how is it measured? 5) How is a sound's frequency measured? 6) What is the frequency of waves measure in? A-1. By an equation. E=MC^2, or energy of a particle equals its mass times the speed of light squared. V=C/(lambda), or frequency equals the speed of light divided by wavelength. The easiest way is using Planck's constant, which allows you to find frequency and then, by the equations above, wavelength, based on energy calculated by any means. A-2. In longitudinal (compression) waves, it is measured from compression to compression or from rarefaction to rarefaction. A-3. First, the wavelength is usually measured from crest to crest or from trough to trough. Wavelength is usually represented by the Greek letter lambda, the character for length. Wavelengths vary from kilometers to Angstroms. The Angstrom is represented by the symbol (A with a small circle atop)) A-4. Frequency is number of cycles per second.measured in hertz A-5. Velocity of light is taken standard as 3X10^3 m/s A-6.Hertz EXPERIMENT NO.-3 1) Does VSWR vary with line length? 2) Can the magnitude of the reflection coefficient (ρ) be greater than 1? 3) Can VSWR be negative?

3 4) Is there a the minimum length of transmission line required on either side of the VSWR meter for valid readings? 5) Is the characteristic impedance of the sampler line section important? 6) What is an acceptable VSWR? A-1. VSWR measurements made with an instrument calibrated for Z o real (as is usually the case) will show a smooth decrease in VSWR from load to source if inserted in the line at different points. This is caused by line loss, and can be predicted with knowledge of line loss. If VSWR changes with a change in line length by more or less than accounted for by the line loss, something else is happening, most commonly feedline common mode currents are affecting the impedance at the load end of the transmission line. A-2. Yes, under conditions of quite reactive Z o (which will usually be a negative reactance) and a highly inductive load, ρ may be greater than 1. This is unlikely to happen with practical lines at HF and above. Note that an instrument calibrated for Zo real should never show ρ > 1, it is a sign of mismatched detectors in such an instrument. Reverse the instrument and see if it still happens. A-3. Considering the definition of VSWR stated above, no. The common formula for predicting VSWR based on observation of conditions at a point is VSWR=(1+ρ)/(1- ρ) which will yield negative VSWR for ρ>1 (which is a rather unusual case). VSWR is a dimensionless positive ratio so a negative value does not seem sensible, a better formula is VSWR=(1+ρ)/ 1- ρ. A-4. The VSWR meter described above samples a very small region of the transmission line within the meter, sufficiently so that it closely approximates a point source measurement for al practical purpose in well designed instruments. The instruments measurement of conditions at that point are independent of things external to the sampling section. The measurements made at that point apply at that point. With additional knowledge, the measurements may be extended to another place. For example, with additional knowledge of the loss in a length transmission line of the same Z o as the instrument calibration, the VSWR at the other end of that line can be estimated. A-5. The sampler line section behaves like any transmission line, and may transform impedance. Quality instruments will usually have a sampler line section of exactly the same Z o as the VSWR calibration base so that impedance transformation caused by the sampler line is minimised. However, the detectors may be calibrated (nulled) at some Z that is different to the sampler section line Z o. If the sampler line section is very short, the impedance transformation will be very small. For example, consider a HF sampler line section of length 20mm and Z o=75+j0 calibrated for reflected null at 50+j0Ω, the insertion VSWR in a 50Ω system is around 1.02 which is comparable to the error using a true 50+j0Ω sampler section in RG58C/U nominal 50Ω cable at low HF (ignoring cable tolerance errors). A-6. Whilst there are lots of Rules of Thumb (ROT) proposed, the answer lies in determining the loss and power handling of the transmission line in the intended scenario, and making a decision about whether they are acceptable. EXPERIMENT NO.-5 1) What is the primary purpose of a directional coupler? 2) How far apart are the two holes in a simple directional coupler? 3) What is the purpose of the absorbent material in a directional coupler? 4) In a directional coupler that is designed to sample the incident energy, what happens to the two portions of the wavefront when they arrive at the pickup probe? 5) What happens to reflected energy that enters a directional coupler that is designed to sample incident energy?

4 Directional coupler A-1 Sampling energy within a waveguide. A-2. 1/4 wavelength. A-3 Absorb the energy not directed at the pick-up probe and a portion of the overall energy. A-4 The wavefront portions add. A-5 The reflected energy adds at the absorbent material and is absorbed. EXPERIMENT NO.-6 1) What are the two basic types of T junctions? 2) Why is the H-type T junction so named? 3) The magic-t is composed of what two basic types of T junctions? 4) What are the primary disadvantages of the magic-t? 5) What type of junctions are formed where the arms of a hybrid ring meet the main ring? 6) Hybrid rings are used primarily for what purpose?

5 A-1 E-type and H-type. A-2. The junction arm extends in a direction parallel to the H lines in the main waveguide. A-3. E-type and H-type A-4 Low power-handling capability and power losses. A-5. Basic E-type junctions. A-6. High-power duplexes. 1) What is horn? EXPERIMENT NO.-7

6 2) In order to function properly, a horn antenna must be a certain minimum size.what is it? 3) Where are Horn antennas commonly used? 4) Why we use horn, rather than a dipole antenna or any other type of antenna, atthe focal point of the dish? A-1. In radio transmission, an open-ended waveguide, of increasing cross-sectional area, which radiates directly in a desired direction or feeds a reflector that forms a desired beam. Horns may have one or more expansion curves, i.e., longitudinal cross sections, such as elliptical, conical, hyperbolic, or parabolic curves, and not necessarily the same expansion curve in each (E-plane and H-plane) cross section. A-2. In order to function properly, a horn antenna must be a certain minimum size relativeto the wavelength of the incoming or outgoingelectromagnetic field. If the horn istoo small or the wavelength is too large (the frequencyis too low), the antenna will not work efficiently. A-3. Horn antennas are commonly used as the active element in a dish antenna. The horn is pointed toward the centerof the dish reflector. A-4. The use of a horn, rather than a dipole antenna or any other type of antenna, atthe focal point of the dish minimizes loss of energy (leakage) around the edges of thedish reflector. It also minimizes the response of the antenna to unwanted signalsnot in the favored direction of the dish. EXPERIMENT NO.-8 1) Why gunn diode and impatt diode are not common microwave devices? 2) Difference between gunn diode and pin diode? 3) Why diode is called diode? 4) What are regions of operation in Gunn diode? 5) Explain regions of operation in Gunn diode.

7 VI characteristics of Gunn diode A-1. These are a niche market, and the demand is not particularly high. Designing a Gunn or Impatt oscillator is not a trivial exercise, and the biasing is a pain. These designs are generally built by skilled craftsmen and are not suited to mass production. At 24 GHz or below, a GaAs MESFET or PHEMT is much more practical as an active device. A-2.Gunn diode acts as oscillator and has negative differential region which helps it to act as microwave source.pin diode acts as modulator and is used alongwith Gunn for square wave modulation. A-3. diode:= di+ iode di means two,and iode means terminal,,,, diode is two terminal device i.e anode and cathode. A-4. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. A-5. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity and reducing the gradient across it, preventing further conduction and current actually starts to fall down. In practice, this means a Gunn diode has a region of negative differential resistance. EXPERIMENT NO.-9 1)Ferrite devices are useful in microwave applications because they possess what properties? 2). Which of the two types of electron motion (orbital movement and electron spin) is more important in the explanation of magnetism? 3). The interaction between an external field and the binding force of an atom causes electrons to do what? 4) The resonant frequency of electron wobble can be changed by variation of what force? 5) Rotating the plane of polarization of a wavefront by passing it through a ferrite device is called what?

8 A-1 Magnetic properties and high resistance. A-2. Electron spin. A-3. Wobble at a natural resonant frequency. A-4. The applied magnetic field. A-5. Faraday rotation. EXPERIMENT NO.-11 Q-1.What device is used to produce a gradual change in impedance at the end of a waveguide? Q-2 When a waveguide is terminated in a resistive load, the load must be matched to what property of the waveguide? Q-3 What is the primary purpose of a dummy load? Q-4 The energy dissipated by a resistive load is most often in what form? Q-5.What is the result of an abrupt change in the size, shape, or dielectric of a waveguide? Q-6 A waveguide bend must have what minimum radius? Q-7 What is the most common type of waveguide joint? Q-8 What is the most likely cause of losses in waveguide systems?

9 A-1 Horn A-2 Characteristic impedance A-3Absorb all energy without producing standing waves A-4. Heat A-5 Reflections A-6. Greater than 2 wavelengths A-7 Choke joint A-8 Choke joint EXPERIMENT NO.-4 1)What element of the reflex klystron replaces the output cavity of a normal klystron? 2) When the repealer potential is constant, what property of the electron determines how long it will remain in the drift space of the reflex klystron? 3) The constant-speed electrons of an electron bunch in a reflex klystron must remain in the repeller field for what minimum time? 4) If the constant-speed electrons in a reflex klystron remain in the repeller field for 1 3/4 cycles, what is the mode of operation? 5)Debunching of the electron bunches in the higher modes of a reflex klystron has what effect on output power? 6) What limits the tuning range around the center frequency of a reflex klystron in a particular mode of operation?

10 Reflex Klystron A-1 The reflector or repeller. A-2 Velocity. A-3 Three-quarter cycle. A-4. Mode 2. A-5. Power is reduced. A-6. The half-power points of the mode.