Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-10-21
2001-11-27
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S094000, C361S097000
Reexamination Certificate
active
06324041
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a protection circuit for a traveling wave tube (TWT), and more particularly to a protection circuit for a traveling wave tube driven by multiple input tones.
BACKGROUND ART
An exemplary traveling wave tube (TWT)
100
is shown in FIG.
1
. The elements of the TWT
100
are generally coaxially-arranged on along a TWT axis
102
. The elements include an electron gun
104
, a slow wave structure
106
, a beam focusing arrangement
108
which surrounds the slow wave structure
106
, a microwave signal input port
110
and a microwave signal output port
112
which are coupled to opposite ends of the slow wave structure
106
, and a collector
114
. A housing
116
is typically provided to protect the TWT elements.
In operation, the electron gun
104
injects a beam of electrons into the slow wave structure
106
. The electron beam has a given power level. The beam focusing arrangement
108
guides the electron beam through the slow wave structure
106
. A microwave input signal
118
is inserted at the input port
110
and moves along the slow wave structure
106
to the output port
112
. The slow wave structure
106
causes the phase velocity (i.e. the axial velocity of the phase front of the signal) of the microwave signal to approximate the velocity of the electron beam.
As a result, the electrons of the beam are velocity modulated into bunches which interact with the slower microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal causing the microwave signal to be amplified. The amplified signal is coupled from the output port
112
as a microwave output signal
120
. After their passage through the slow wave detector
106
, the electrons are collected in the collector.
Typically, an individual power supply (not shown) is associated with each TWT and the power supply delivers the necessary bias voltages and currents. Standard protection circuits for TWT's are included in an external power conditioning unit (EPC). The protect ion circuit detects excessive helix current and either rapidly turns off the voltages to the TWT, or shuts down the inverters in a switching power supply to remove the voltages from the TWT. The standard protection circuit is typically a simple voltage-level detector with a short time-constant integrator for monitoring the helix current across a sense state resistor with an integrating operational amplifier, a Schmidt trigger circuit, or both. The detector is designed to detect an arc, fault, or overdrive situation and limit the total energy (in joules) that can be delivered to the TWT in order to avoid damage to the internal electrodes.
Standard protection circuits are typically designed for a TWT driven by a single tone. However, in modern communication systems, it is more common to drive the TWT with multiple tones to increase utilization of the available bandwidth. Prior art protection circuits that use a second sensor, called line sensors, use the second sensor to measure input power to the power supply. The line sensor provides a very slow response and are only occasionally used.
Multiple tones cause large transient power fluctuations in the TWT, which results in complex helix current waveforms compared to single or two-tone operation. When a TWT is driven by multiple tones, the helix current fluctuates with the varying phase of the different tones. Even with a constant average output power, the varying phase of the tones causes large body currents which interfere with the standard protection circuit. The line sensor is not practical for helix applications because it is too slow to protect the helix from excessive helix current.
In order to use a standard protection circuit for a multiple-tone driven application, the sensitivity of the detector circuit must be reduced by raising the peak threshold level, increasing the time constant, or both. The de-sensitization avoids spurious shut down from the protection circuit mistaking multi-tone phase-up with fault situations. However, desensitizing the circuit can lead to excessive energy being delivered to the tube in the event of a fault and excessive average power in the TWT when it is operated with only a single tone.
SUMMARY OF THE INVENTION
The present invention is a protection circuit that protects a traveling wave tube from excessive helix current when the TWT is driven with multiple tones. The present invention independently detects the peak and average power levels associated with helix current interception in the TWT by a two-stage sense circuit. This allows the protection circuit to turn off the TWT in the appropriate situations, even in the presence of multiple tones and large helix current spikes.
The protection circuit of the present invention has two detectors. A first detector is designed for the signature of arcs or high voltage faults which have fast rise times and high peak values of the helix current. The second detector is designed to limit the average power loading on the electrodes associated with the normal operation of a TWT. Average power loading has long time constants and lower helix currents.
The dual sensor circuit eliminates spurious shutdowns and circuit damage that is caused by desensitized single stage detector. Safe operation with a plurality of tones and continuous-random-variation of the phase signals can be accomplished with the protection circuit of the present invention.
It is an object of the present invention to protect a TWT from excessive helix current when the TWT is driven by multiple tones. It is another object of the present invention to independently detect peak and average power levels associated with TWT helix current.
It is a further object of the present invention to have a dual sensor protection circuit. It is yet a further object of the present invention to have a first detector designed to limit the peak power loading, and a second detector to limit average power loading.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.
REFERENCES:
patent: 3890545 (1975-06-01), Rosen
patent: 4323853 (1982-04-01), Kurokawa
patent: 4745369 (1988-05-01), Wanninger
patent: 5932971 (1999-08-01), Goebel et al.
patent: 5942852 (1999-08-01), Goebel et al.
Goebel Dan M.
Lewis David E.
Liou Ronglin
Gudmestad Terje
Hughes Electronics Corporation
Sherry Michael J.
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