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Traveling-Wave Tube Amplifier Model to Predict High-Order Modulation Intersymbol InterferenceDemands for increased data rates in satellite communications necessitate higher order modulation schemes, larger system bandwidth, and minimum distortion of the modulated signal as it is passed through the traveling wave tube amplifier (TWTA). One type of distortion that the TWTA contributes to is intersymbol interference (ISI), and this becomes particularly disruptive with wide-band, complex modulation schemes. It is suspected that in addition to the dispersion of the TWT, frequency dependent reflections due to mismatches within the TWT are a significant contributor to ISI. To experimentally investigate the effect of these mismatches within the physical TWT on ISI would be prohibitively expensive, as it would require manufacturing numerous amplifiers in addition to the acquisition of the required digital hardware. In an attempt to develop a more accurate model to correlate IS1 with the TWTA and the operational signal, a fully three-dimensional (3D), time-dependent, TWT interaction model has been developed using the electromagnetic particle-in-cell (PIC) code MAFIA (solution of Maxwell's equations by the Finite-Integration-Algorithm). The model includes a user defined slow-wave circuit with a spatially tapered region of loss to implement a sever, and spatially varied geometry (such as helical pitch) to implement a phase velocity taper. The model also includes user defined input/output coupling and an electron beam contained by solenoidal, electrostatic, or periodic permanent magnet (PPM) focusing allowing standard or novel TWTs to be investigated. This model comprehensively takes into account the effects of frequency dependent nonlinear distortions (MAM and AMPM); gain ripple due to frequency dependent reflections at the input/output coupling, severs, and mismatches from dynamic pitch variations; drive induced oscillations; harmonic generation; intermodulation products; and backward waves.
Document ID
20020027139
Acquisition Source
Glenn Research Center
Document Type
Preprint (Draft being sent to journal)
Authors
Kory, Carol L. (Analex Corp. Cleveland, OH United States)
Andro, Monty (NASA Glenn Research Center Cleveland, OH United States)
Williams, W. D.
Date Acquired
September 7, 2013
Publication Date
January 1, 2001
Subject Category
Avionics And Aircraft Instrumentation
Meeting Information
Meeting: IEEE International Vacuum Electronics Conference
Location: Monterey, CA
Country: United States
Start Date: April 23, 2002
End Date: April 25, 2002
Sponsors: Institute of Electrical and Electronics Engineers
Funding Number(s)
PROJECT: RTOP 755-1B-00
CONTRACT_GRANT: NAS3-00145
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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