NonlinearTransmissionLines
A nonlinear transmission line (NLTL) is comprised of a transmission line
periodically loaded with varactors, where the capacitance nonlinearity
arises from the variable depletion layer width, which depends both on the
DC bias voltage and on the AC voltage of the propagating wave. An equivalent
circuit model of one section of an NLTL is shown below, followed by photos
of an actual NLTL circuit fabricated at UC Davis.
Equivalent circuit model of an NLTL.
Photo of an actual NLTL.
Close up view of a single element in the NLTL.
NLTLs can be used as broadband delay lines at
low signal levels by varying the bias voltage and hence the propagation
velocity, which varies as the inverse square root of the C(V) curve. At
large signal levels, waveform steepening occurs for the proper choice of
input waveform and soliton generation can be achieved by balancing steepening
and dispersion. Superlattice Schottky
Quantum Barrier Varactors (SSQBVs) have been employed as the nonlinear
elements in our nonlinear transmission line circuits. Short pulses with
< 27 ps fall time have been detected in initial proof-of-principle experiments
in good agreement with theoretical predictions. Current NLTL designs have
been designed and fabricated to generate >15 V pulses with ~10 ps fall
times.
The NLTL can be utilized as a broadband delay line for phased
antenna array applications. A hybrid NLTL has been built in a proof-of-principle
experimental concept test where a 1.1 ns true time delay with <6dB maximum
insertion loss has been measured. A 2x4 antenna array and the NLTL have
been utilized to demonstrate broadband (1-4GHz) beam steering. A second
generation,
1 x 8, hybrid
NDL-based PAA system has been developed, demonstrating up to 18°
of electronically controlled beam steering from 4-5 GHz.
Also, NLTLs can be employed to generate free propagating broadband pulses
of electromagnetic radiation, which can then be utilized in millimeter-wave
reflectometry applications for tokamak plasma diagnostics.
Currently, the NLTL is also being considered for use as the pulse compressor
in both ultrawideband and collision avoidance radar systems. A full collision
avoidance radar system proof-of-principle implementation is currently
being developed at 24 GHz, with the intent to eventually build a system
at 94 GHz.
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