Electric lamp and discharge devices: systems – Cathode ray tube circuits – Combined cathode ray tube and circuit element structure
Reexamination Certificate
2001-07-24
2002-12-17
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Combined cathode ray tube and circuit element structure
C315S039300, C315S094000
Reexamination Certificate
active
06495961
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power supplies, and more particularly to power supplies for high-voltage pulse applications, such as in a radar system using a travelling-wave tube transmitter.
BACKGROUND OF THE INVENTION
Radar systems have been used for more than fifty years, and during that time many varieties have emerged, including continuous-wave varieties. Many radar systems continue to use relatively short, high-power pulses of radio-frequency electromagnetic radiation to detect, locate and track targets. Modern radars provide very sophisticated features and capabilities at long range on small targets. In order to provide such features and capabilities, the recurrently transmitted electromagnetic pulses are often required to meet stringent frequency, power, and stability criteria while executing a recurrent program involving changing frequency and power.
In general, high performance in a radar system depends upon having a large bandwidth, so that at some times very short-duration pulses can be transmitted for making fine determinations of distance and dimensions, and at other times much longer-duration pulses can be transmitted for long-range detection. At one time, radar systems used simple vacuum tubes in their transmitters, but the power and frequency limitations of such tubes made them somewhat unsatisfactory. The development of the klystron and magnetron provided increased power at high frequencies, but with limited bandwidth. Modern radar systems use broadband sources of transmitted electromagnetic radiation, which are often in the form of one or more travelling-wave tubes (TWTs), and sometimes of arrays of solid-state transistors. At the current state of technology, the highest power with wide bandwidth is available with travelling-wave tubes.
FIG. 1
is a simplified diagram in block and schematic form, illustrating a prior-art travelling-wave tube power supply
10
. In general, stability of the transmitted pulse from a travelling-wave tube, such as
12
of
FIG. 1
, can be achieved by maintaining constant the various TWT electrode voltages. In the simplest type of high-power TWT transmitter, a travelling wave tube, such as
12
, is energized by a voltage supply, such as
14
of
FIG. 1
, connected to its cathode
12
ca
and collector
12
co.
Such an arrangement sets the power dissipation of the TWT to equal the product of the tube current and the voltage of source
14
. The actual tube voltage and power are often determined by considerations including the best linearity or least distortion introduced by the tube. In an actual system, the voltage of power supply
14
may be about 35 kilovolts (kv). This type of supply can be effective, but the unavoidable internal impedance of the power supply
14
undesirably results in a decrease of the applied voltage each time an energy pulse is produced by the TWT
12
.
In order to reduce the voltage drop attributable to the internal impedance of the power supply
14
of
FIG. 1
, the prior art parallels the power supply with an energy storage capacitor, illustrated as
16
in FIG.
1
. The energy storage capacitor tends to reduce the instantaneous voltage drop at the inception of the pulse, but a voltage decrease or “droop” still occurs over the duration of the transmitted pulse. A sufficiently large energy storage capacitor
16
can, however, keep the voltage droop within acceptable limits. The presence of the energy storage capacitor
14
, in turn, results in the possibility of a large current discharge in the event of a short-circuit or flashover within the TWT. A current-limiting resistor
18
is introduced in a serial connection between the TWT cathode and the combination of the power supply
14
and the capacitor
16
, to prevent damage to the TWT and power supply.
It should be noted at this point that the terms “between” and “across,” and certain other terms, have meanings in electrical usage which are different from those commonly used. More particularly, the terms have meanings which are not related to physical placement, but rather relate to the terminals to which electrical coupling is made. Thus, signal flow “between” A and B takes place if the signal leaves one of A and B and arrives at the other, regardless of whether the path taken happens to lie on, or pass through, a straight line extending from A to B. Those skilled in the art know this so thoroughly that little though is given to the use of the terms, and they are automatically understood.
It has been found that greatest modulation sensitivity (somewhat corresponding to “gain”) of a travelling-wave tube occurs at specific values of voltage between the cathode
12
ca
and the body
12
b
of the tube
12
. The body
12
b
of the TWT
12
should be grounded, for reasons of safety and to reduce the possibility of flashover. In general, the maximum-modulation-sensitivity cathode-to-body voltage does not correspond with the optimum cathode-to-collector voltage of the TWT. In order to obtain maximum modulation sensitivity of TWT
12
of
FIG. 1
, a second voltage or power supply
20
is provided, with its negative (−) terminal coupled to the collector
12
co
of TWT
12
by way of a current-limiting resistor
22
, and with its positive (+) terminal coupled essentially to ground, by way of a control or regulator arrangement designated generally as
30
. The voltage produced by power supply
20
introduces an offset voltage which drives all the voltages associated with the TWT in a negative direction, except for the ground voltage at
12
g
applied to the TWT body connection
12
b.
The voltage of power supply
20
is selected to a value which sets the cathode-to-ground voltage to about the maximum-modulation-sensitivity voltage, which is to say that the floating voltages of power supply
14
are driven negative until the cathode
12
ca
of TWT
12
is at the desired negative voltage relative to the ground voltage at body terminal
12
b.
In one application, the voltage of power supply
20
is about 8 kilovolts, which sets the cathode-to-ground voltage at about −43 kv. The voltage offset introduced by power supply
20
does not change the power dissipation of the TWT, because power supply
14
continues to establish the voltage (about 35 kv in the example) between cathode
12
ca
and collector
12
co.
Thus, the arrangement of
FIG. 1
as so far described maintains the cathode-to-body voltage of the TWT near the optimum for modulation sensitivity, and the cathode-to-collector voltage at some value optimized for a combination of signal distortion and TWT dissipation.
In the arrangement of
FIG. 1
, power supply
20
is also associated with a collector energy storage capacitor
24
, which serves the same purpose for power supply
20
as energy storage capacitor
16
serves for power supply
14
. It will be appreciated that the internal impedances of power supplies
14
and
20
necessarily result in reduction of TWT voltage when current is drawn, whether or not the power supplies are paralleled by energy storage capacitors. For very short pulses, such energy storage capacitors can be very helpful in ameliorating voltage drop. However, as the desired transmitted pulses become longer (of longer duration or of increased duty cycle), the size of the requisite energy storage capacitance can itself become a problem. In a large, high-power radar system using an energy storage capacitor across the cathode-to-collector path of a TWT, the weight of energy storage capacitors can be hundreds of pounds, which may be undesirable for some applications.
To the extent that transmitted frequency and power do not conform to the desired pattern or program because of variation in the power supply voltages of the TWT of a radar transmitter, signal processing can in some instances be used to compensate for the resulting deficiencies. In general, reduction in the amount of signal processing is desirable, both for reducing the amount of processing power required, and therefore the costs of the system, and for increasing the processing speed, thereby allowi
Bazin Lucas John
Heinrich Richard Johann
Duane Morris LLP
Lockheed Martin Corporation
Nguyen Hoang
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