Microwave Devices

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Microwave Devices

• Waveguides
• Microwave solid state devices
• Microwave tubes
• Microwave antennas

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Preamble

• As frequency increases beyond the lower boundary
  that has been assigned as the microwave threshold
  (1 Ghz) conventional R.F. techniques become less
  effective.
• Lead inductance and capacitiance as well as
  connecting traces on substrates become issues
  affecting circuit performance.
• An example is TRANSIT TIME.
• Physical construction of devices must change.

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Waveguides

• Conventional transmission lines develop too much
  loss at microwave frequencies.
• Hollow waveguides present an alternative.
• Electromagnetic waves reflect from the walls of
  the wave guide as it travels its length.
• Brass, aluminum or silver plated.
• RECTANGULAR, elliptical and circular.

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Waveguides

• No radiation losses as E and H fields are
  contained.
• Dielectric losses are small. (air)
• Minimal losses in conductive walls.

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Waveguide Modes

• A waveguide operates most efficiently within
  modal boundaries.
• A given waveguide cross section will have a
  cutoff frequency where a signal below it will not
  propagate.
• What type of filter does this act as?
• Not all modes strike the walls at the same angle
  therefore the distance traveled varies.
• This is called multimode propagation.
• Effective velocity reduces.
• Pulse spreading results and subsequent pulses
  following closely will interfere. (Dispersion)

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fc cutoff frequency
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Waveguide Modes

• Singlemode operation is achieved by using the
  mode with the lowest cutoff frequency. (Dominant
  mode)
• Waveguide is used between its cutoff frequency
  and that of the mode of the next lowest cutoff
  frequency.
• TE or TM modes.

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of variations along b
of half cycles along a
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Group velocity- used to determine the length of
time a signal takes to travel the length of the
waveguide.
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Phase velocity- used to determine wavelength in a
guide. It is the rate at which a wave appears to
move along a wall of a guide based on the way the
phase angle varies along the guide.
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Impedance of a waveguide.
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The calculation for wavelength in a guide
requires phase velocity.
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Bends and Tees

• BENDS if the transition through the bend is
  gradual there will be a minimal effect.
• TEES
• gtE plane tee output is out of phase with the
  input. (series tee).
• gtH plane tee is in phase with the input.
  (shunt tee)

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Bends and Tees

• HYBRID or MAIC TEE
• If the input is to Port 3
• Output is at port 1 and 2 in phase
• Port 4 does not have an output.
• If input is to port 4
• Output is at port 1 and 2 out of phase
• Port 3 does not have an output
• Note a termination component develops 3 dB loss.

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Cavity Resonator

• ½ wavelength section
• Waves reflect end to end and in phase with the
  incident signal.
• Field strength will build
• Resonant cavity Q is on the order of several
  thousand.
• Can be made tunable with an adjustable short
  circuiting plate.
• Ex. Wavemeter.

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Attenuators and Loads

• Carbon can be used.
• As a field becomes present in the carbon a
  current flows developing a power loss.
• A carbon flap is inserted and presented over a
  varying degree.

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Circulators and Isolators

• ISOLATOR permits a signal to pass in only one
  direction. Other direction is attenuated.Ex.
  Protects a source from a mismatched load.
  Reflected power is dissipated.
• CIRCULATOR Separates signals. Signal applied to
  one port will only emerge at the next port.
• Ex. Transmit/receive switch.

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Circulators and Isolators

• PRINCIPLE OF OPERATION
• Fields interact with a ferrite. (magnetically)
• Ferrites can be comprised of ceramic compounds of
  iron oxide with other metals.
• Ferromagnetic and non conductive. Permeability
  greater than air.

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Circulators and Isolators

• PRECESSION
• When axis of a rotating electron also spins.
• A magnetic field can be adjusted to control the
  frequency at which precession occurs in a
  ferrite.
• If a magnetic field propagates through the
  ferrite and signal (wave) frequency equals
  precession frequency the magnetic field can then
  adjust precession frequency up nor down.
• For isolator
• If precession increases signal energy is removed.
• If precession decreases minimal loss.

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Circulators and Isolators

• For Circulator
• A magnetic field acting upon a ferrite also
  develops a phase shift as the wave propagates
  through the material.
• Faraday rotation.
• Similar to polarization shift for some
  frequencies paasing through the ionosphere.
• Quantity of phase shift is determined by the
  length of ferrite and the strength of the dc
  magnetic field.
• Correct selection of phase shifts will add fields
  at some ports while canceling at others.

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