UltraWideband Antenna Design

1
Ultra-Wideband Antenna Design

• Stanley Wang
• Prof. Robert W. Brodersen
• January 8, 2002

2
Antenna in Communication Systems

• At Transmitter
• Antenna is modeled as a circuit component real
  part in it determines the radiated power (for
  ??)
• Current in the antenna determines Erad

• At Receiver
• E-field at the Rx is translated to a voltage
  source
• By reciprocity theorem, Zant,rxZant,tx

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Frequency Response s11 of Monopole

• The smaller the s11, the larger the radiation
• Resonant at fc/(0.5?), which leads to freq. hump
• Two ways to avoid ringing flatten the freq.
  response
• Make the conductive wire more resistive
• Shorten the dipole

s11 for 2cm monopole
s11 for 6cm monopole
4
Far-zone Electric Fields of the Monopole
6cm monopole
2cm monopole

• When L is much smaller than ?/2, no ringing
  happens
• Radiated energy is decreased, but its ok
• Undetectable UWB system transmits at noise level

5
Short Monopole Input V/I Characteristics

• The input V/I behavior resembles that as driving
  a capacitor
• Radiation is too small energy stored in
  near-field
• Modeling a 2cm monopole by a 0.315pF capacitor
• Given the same Vs Rs, Is in two cases overlap
  perfectly

6
Short Monopole Radiation

• The radiated field is the time-derivative of the
  INPUT current
• The dimension is small. Phase difference between
  I(z) at each part is ignorable?Quasi-static
  condition

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Experiment on Short Antenna

• Planar Square Antenna (2cm x 2cm)
• Amplifier at the receiver to reduce the
  quantization noise
• Use AVERAGE function in the scope to enhance SNR

Pulse Generator
Digital Sampling Oscilloscope
Courtesy to Enyi Lin
8
Experiment on Short Antenna - Results

• Blue line no objects located between Tx Rx
• Red line a table put in the middle between Tx
  Rx
• An extra transmission path is added
• Two lines overlap most of the time, even after
  the extra path has arrived Rx
• The pulse width of the

Extra path arrives

• received signal is about 2.5ns with W-shape
• Short antennas get rid of ringing

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Driver Circuit Schematic

• Inverter chain sharpens the edge of the input
  signal
• Pre-driver NAND/NOR circuits skew the signals so
  that the driver NMOS PMOS wont be on at the
  same time
• Bond-wire inductance might introduce ringing

Antenna as a capacitor
Input Signal
Driver Enable
Section Enable
10
Driver Circuit Layout

• STMicroelectronics 0.13um CMOS process

• Chip area 0.49mm2
• 1.2V Vdd
• 2 drivers with enables ? Can either drive a
  monopole or dipole
• Each driver with 16 levels of driving capabilities

Driver