Chaotic microwave systems based on traveling-wave tube (TWT)

31 Pages·2008·1.05 MB·English

IREAP
Chaotic microwave systems based on
traveling-wave tube (TWT) amplifiers
John Rodgers, Mike Holloway, Kristy Gaff, Michelle Adan
Institute for Research in Electronics and Applied Physics
University of Maryland
College Park, MD 20742
[email protected]
Concept: Networks of coupled chaotic time-delayed
oscillators (TDO)
High-power Output
Directional
Traveling-wave
Coupler
Device
Dynamic Control
Delay Line
Variable Atten.
•The radiation scattered off objects will couple in varying
strengths to the various nodes of the network and modulate
its dynamics.
Basic Research Questions
• How robust would synchronization in the network be in the presence of:
– Noise
– Mismatch in the dynamical parameters of the nodes
– Interference or jamming
– Complexity of the environment
Changing
Environment
Traveling-wave Traveling-wave
Device Directional Directional Device
Coupler Coupler
Dynamic Control
Dynamic Control
Variable Atten.
Variable Atten. Delay Line
Delay Line
IREAP
Outline
• Basic characteristics of the traveling-wave tube
(TWT)
• TWT nonlinearity
• Saturation models
• Results of previous simulations
• Results of experiments on TWT’s with time-
delayed feedback
• Digital control of the time-delayed system
• Future research
TWT Basics
Slow space charge wave
ω= k v
z
Slow EM wave
TWT Dispersion Diagram
•Properties:
High gain, wideband, good linearity and efficiency
•Applications:
Space communications, secure LOS, cell phones, radar, EW
Typical characteristics of the TWTA
Depressed
Pierce Gun
Collectors
SWS
20 cm
PPM Stack
•High gain 40-60 dB
•Wideband >2 octaves
•Compact m~ kg, L< 12 cm
•Efficient >80% (Dep. Coll.)
•High Frequency >50 GHz
•Modest Voltage ~few kV
Nonlinearity in TWT’s (1-D Model)
RF field on helix
Modulated electron beam
∂v ∂v e ∂(V +V ) Over-modulation of
Equation of Motion = −v − W SC
the beam increases
∂t ∂z m ∂z
∂ρ ∂(ρv) space-charge forces
Continuity = −
which saturate the
∂t ∂z
amplifier.
∂2V ρ
Gauss’s Law SC = −
∂z2 ε
0
Wave Equation ∂2V ∂2V ∂2ρ
W −c2 W = cZ A
∂t2 ∂z2 0 ∂t2
TWT Gain Saturation
Gain-Bandwidth
Gain and Output Phase vs. Input Power
Small signal
2 -40 dBm
-30 dBm
45
0 -20 dBm
Gain -15 dBm
50 30 B] -2 -10 dBm
d
g] [ -7 dBm
n [dB] 015 Shift [de ed Gain --64 -5 dBm
Gai40 Phase se aliz -8
Fund. Gain -15 ha m
P r
2nd Harm. Gain o-10
-30 N Highly Saturated
Phase
2nd Harm. -12
30 -45
-50 -30 -10 -14
Drive Power [dBm] -1.2 -0.8 -0.4 0 0.4 0.8 1.2
Offset Frequency [GHz]
In some TWT’s, saturation is well described as a quadratic function the of
input amplitude and the small-signal bandwidth as a first-order band pass
filter (BPF).
Comparison of measured TWT drive curve
with best fit to Saleh1 model
30
y = A(r)e− jΦ(r)
Measurements
|
y
|
α r e
A(r) = a d 20
u
1+ β r2 t Saleh Model
i
l
a p
m
α r2 A

φ t 10
Φ(r) = u
p
(1+ β r2)2 t
u
φ
O
0
0 0.01 0.02 0.03 0.04
Input Amplitude (r)
0 i 05
1. A. M. Saleh, IEEE Trans. Comm., Vol. 29, No. 11, (1981), pp. 1715-1720.
Model of a Time-delayed Feedback Oscillator Using Linear
and Nonlinear Blocks to Describe the TWT
(see poster presented by Wai-Shing Lee)
Linear Gain (G ) +
L
First-Order BPF
Saturation Nonlinearity
2. V. Dronov, M. Hendry, T. M. Antonsen, Jr., and E. Ott, Chaos, Vol. 14, No. 1, pp. 30-35, 2004.