Ground Penetrating Radar

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Ground Penetrating Radar
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Application

• Useful in Construction
• Prevent damage to underground lines
• Real-time data gives proximity to pipes
• Safety Conscious

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Challenges

• Ground is very lossy medium
• Any moisture will absorb most of signal
• Requires more power than detection in air
• Power ? Pulse Width
• Low end of microwave regime used for detection

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Pulse Width

• Pulse short enough to stop transmitting before
  reflected signal returns to avoid interference
• Pulse width Depth of Penetration / c , where c
  is the speed of light
• Depth of buried lines usually 2m
• Short pulse means less energy transmitted

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Frequency Calculations

• Calculate Skin Depth to ensure most of the power
  reaches the pipes
• No greater than 2 meters
• ds sqrt(2 / (?µs) )
• µ µ0
• ssoil 1 to 10-4 S
• ? 2pf

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Continued..

• Solve the skin depth formula for the frequency, f
  633 MHz
• Therefore, the low end of the range of
  frequencies is 600 MHz
• The high end was chosen to be 1000MHz to allow
  better resolution

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FMCW Radar

• Frequency Modulated Continuous Wave Radar (FMCW
  Radar)
• Key characteristic
• Allows a signal to simultaneously be transmitted
  and received without interference
• Benefit
• Pulse width can be increased -- no constraint on
  when it must end

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FMCW Radar
The Constant offset between the transmitted
signal (solid curve) and received signal (dashed
curve) is proportional to the distance from the
object
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General Block Diagram - FMCW Radar
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Frequency Modulator

• Modulating Signal
• Saw tooth from 600MHz to 1000MHz
• Carrier Signal
• Sine wave
• Frequency Modulated Signal
• Sine Wave with frequency varying linearly from
  600MHz to 1000MHz

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Voltage Controlled Oscillator

• Benefits
• Mobile, can be placed on machinery
• Vary frequency with Voltage input
• Issues
• High Frequency needed
• Tuning Diode

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Circuit Design
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Building Modulator

• Ordered Parts
• VCO (MN100EL1648 ON Semiconductors)
• Tuning Diode (MMBV609 ON Semiconductors)
• Create Printed Circuit Board
• Surface Mount Parts
• Solder Surface Mount Parts
• Build Complete Circuit

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Providing Continuous Wave Capability
Power Divider
Transmit
Bandpass Filter
Mixer
LNA
Receive
Output Signal
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Design

• Power Divider
• 3-port Device
• Output ports receive 85/15 Power Ratio
• Low Noise Amplifier
• Maxim2640 w/ frequency range 400MHz 2.5GHz
• Bandpass Filter
• Reject frequencies outside of 500MHz-1GHz
• Mixer
• Maxim2680 Down converter
• Output frequency range 10 500 MHz

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Transmit / Receive Antennas
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Antenna Design

• Bow-tie
• Operates over necessary range
• Simulation software available
• Possible to construct, test and tune within
  allotted time and budget
• Alternatives
• TEM Horn
• More control over directivity, but would be big
• Yagi Antenna
• More narrow beam, better for detection

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Balun

• Importance
• Matches Impedance of antenna to impedance of line
• Minimizes VSWR which corresponds to a wide
  bandwidth
• Length needs to be a quarter wavelength
• At 750 MHz, ?/4 100 mm
• Length can be changed to get best impedance match
  using the Smith Chart
• Final Length 128.23 mm

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Modulator Testing

• Use wire for 10 nH inductor
• Precision issue
• Tank Circuit Performance
• Tuning Diode less precise at lower Voltage input
• Use HP Spectrum Analyzer
• Detect Frequency output
• Lower frequency than desired
• 520MHz

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Continuous Wave Testing

• Power Divider provided power to each arm, however
  well below the desired levels
• Bandpass Filter worked for a tighter band than
  desired
• Lack of power from power divider would not have
  allowed mixer to operate properly

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Theory/Reality

• First time design and build of this type of
  circuit
• Designed material ahead of class instruction
• Damaged a chip
• Power Divider not working as intended
• Bandpass Filter provides filtering but in a
  tighter range than intended.

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Power Divider

• Wilkinson Design instead of T-junction
• Use Resistor to Provide Isolation
• Multiple matching sections to provide broadband
  matching

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Bandpass Filter

• Design using Q-functions
• Q ?0 / Bandwidth
• Allows greater control over the response
  characteristics
• Applied to the stubs

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Performance Verification of Antenna

• HP 8510 Network Analyzer
• Smith Chart to match antenna impedance to line
  impedance
• Change balun length to optimize VSWR
• Log Mag Plot to see that antenna resonates in the
  desired range of frequencies

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Transmit Antenna
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Receive Antenna
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Cost Analysis

• Frequency Modulator
• 15 for VCO
• 10 for Tuning Diode
• Filter, Amplifier, and Mixer
• 10 for etching solution
• Antenna
• 60 brass sheets
• 10 for Plexiglas, cables, etc.
• Salary

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Improvements

• Inductor Capacitor tank with high Q-factor
• Use techniques learned in class to improve power
  divider and bandpass filter
• Simulate various antenna structures
• Take data for many scenarios with various antennas

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References

• Online Resources
• http//www.georadar.com/howitwrk.htm
• http//www.ee.ucla.edu/students/archive/fredrick_m
  s.pdf
• http//www.tpub.com/neets/book12
• Books
• Surface Penetrating Radar by D.J. Daniels
• Microwave Engineering 2nd edition by David M.
  Pozar.
• Simon Haykins, Communication Systems. New York
  John Willey Sons, Inc. 2001, 88-182

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Thanks

• Professor Bernhard
• Professor Chew
• Professor Kudeki
• Professor Franke
• Judy Feng, EM graduate student
• Gentlemen in the Machine Shop
• Maxim Semiconductors