This invention relates to an amplifier equipped with microwave traveling wave tubes (TWTs) which constitute the final stage of telecommunication transmitters used in ground-satellite and satellite-ground links. They may also be used in other types of transmitters intended, for example, for military, scientific, metrology, telecommunication, terrestrial-link and radio beam applications, etc.
The TWT 10 is a depressed-collector TWT. Its output is on a rectangular or coaxial waveguide, for example WR 75. A filter 16 at the output side of the TWT is imposed by the “telecommunications” standards; it prevents the transmission of undesirable frequencies that would be generated because of the nonlinearities of the TWT. A circulator 18 after the filter 16 prevents any inopportune reflection of microwave energy toward the tube.
A first coupler 20 on the output side of the circulator 18, which is followed by suitable transitions and cables, makes it possible to have available on a front face of the amplifier a specimen of the output power Ps of the amplifier for measurement, if required, by the user.
A second coupler 22 on the output side of the circulator, followed by a high-frequency (HF) detector 24 and suitable cables and transitions, makes it possible to generate a signal representative of the output power Ps, needed for correct operation of the device (not shown in the figure) for controlling and regulating the TWT.
A third coupler 26, followed by another HF detector 28, likewise makes it possible to generate a signal representative of the power Pr reflected by the user, in this case by the transmission antennae 14. This reflected power signal is often used in a threshold system, thereby making it possible to safeguard all the equipment in the event of substantial mismatching of this use. Associated with the second 22 and third 26 couplers are, of course, tables and, for example, calibration charts.
These TWT output measurement devices are expensive, firstly because of the number of couplers, RF detectors and HF transitions used and secondly because of the adjustments and calibrations needed depending on the transmission power and frequencies.
In fact, the collector 42 is raised to a potential Vc that allows the electrons to be slowed down before they strike the walls of the collector. The maximum value of the collector voltage Vs is dictated by the preclusion of the electrons being reflected back toward the helix.
In the example proposed, the electrons enter the helix at a velocity V1 corresponding to a voltage of 10 kV. After having yielded up 2 kW, they leave the helix at a lower velocity V2, which corresponds to 8 kV since the beam is a 1 A beam. The voltage Vc of the collector was chosen to be 6 kV relative to the cathode. No electron is therefore reflected. The collector voltage Vc could even have been chosen to be much lower, for example 2 kV, beyond which value the electrons would have been reflected. In fact, the electrons do not leave at the velocity V2 but with a dispersion of velocities, which is often quite large, about V2, hence the margin adopted in the choice of Vc.
The deceleration of the electrons by the collector voltage Vc reduces the energy to be thermally dissipated on the walls, and therefore the energy taken from the high-voltage supply for the electron beam. The overall yield therefore becomes higher.
In addition, this increase in the yield is greater if a multi-electrode or <<multistage>> collector is used instead of a single depressed collector.
Recent measurements carried out within the context of improving the operation of a TWT in telecommunication transmitters have shown that there exists, for example in the case of a TWT with a two-stage depressed collector, a relationship between the high-frequency output power Ps of the TWT and the collector current. This is because the current Ic1 generated by the first stage E1 of the collector is an increasing, single-valued function of the degree of modulation of the beam current and therefore of the high-frequency output power Ps.
A first object of the invention is to simplify the measurements of the output power of a microwave tube amplifier.
Another object is to reduce the cost of the amplifier by eliminating parts and adjustments needed in the prior art for measuring the power of microwave tube amplifiers.
For this purpose, the invention provides a method of measuring the RF output power of a microwave tube amplifier, the tube having an electron gun delivering an electron beam, an RF circuit for interaction between an RF signal and the electron beam, the RF circuit having an amplified RF signal output, a collector having at least two electrodes for collecting the electron beam, these electrodes being respectively separated from the gun by increasing distances, the first electrode being closest to the gun, characterized in that the RF output power as amplified RF signal output is determined from the measurement of the current Ic1 coming from the first electrode, a calculation of the RF output power being carried out through a predetermined relationship between said current Ic1 and the output power of the amplifier.
The proposed simplification therefore consists in replacing the direct measurement and/or the HF detection of the RF output power Ps with the single measurement of the current Ic1 of the first collector electrode of the tube.
This measurement of Ic1 is sufficiently accurate to satisfy the indication of the front face power of the amplifier and above all to meet the needs of controlling the overall supply for the TWT, the processing logic for the amplifier and the various signal processing operations.
The measurement of the current Ic1 may be carried out directly at low voltage, as will be seen later. This measurement therefore makes it possible, with potentially better accuracy than that of the prior art, to eliminate all the HF elements associated with the output power measurements, i.e. two couplers for measuring the output power Ps, an RF measurement diode, the connectors and the coaxial cables for connection to the frames.
The invention also relates to a microwave tube amplifier, the tube having an electron gun delivering an electron beam, an RF circuit for interaction between an RF signal and the electron beam, the RF circuit having an amplified RF signal output, a collector having at least two electrodes for collecting the electron beam, these electrodes being respectively separated from the gun by increasing distances, the first electrode being closest to the gun, characterized in that it includes first means for measuring the current Ic1 coming from the first electrode and second means for determining the RF output power from the measurement of this current Ic1.
The invention will be more clearly understood with the aid of illustrative examples according to the invention, with reference to the appended drawings in which:
a,
6
b and 6c are curves showing the variation in the output power Ps as a function of the current of the first electrode of a two-stage TWT;
a,
7
b,
7
c and 7d are curves showing the variation in the output power Ps as a function of the current of the first electrode of a four-stage TWT; and
The amplifier furthermore includes a circuit 74 for measuring the current Ic1 coming from the first electrode E1 of the TWT.
The relationship between the output power Ps and the current Ic1, in the case of a TWT having a two-stage (N=2) collector, can be likened approximately to a straight line given by Ps=k×Ic1, k being a constant. For such a type of TWT, this relationship varies or may vary with the operating frequency in the allocated band and will include a transmission frequency interpolation formula. The relationship Ps=f(Ic1) for several frequencies will therefore be input into a processing circuit 76 of the amplifier, as will an interpolation formula for all the frequencies other than the previous ones.
In the case of a two-stage collector, often encountered in TWTs for ground transmitters, the relationships, depending on the transmission frequency bands, which determine the output power Ps as a function of the current Ic1 of the first electrode E1, namely Ps=f(Ic1), are very close to straight lines.
a shows a curve 80, for the output power Ps as a function of the collector current Ic1 of the first stage of a TWT, with a nominal output power of 12 W, operating at 30 GHz. This curve 80 approximates to a straight line 82 of equation
Ps=1.1491×Ic1+2.2931 (1)
Ps being expressed in W and Ic1 in mA.
b shows a curve 64 of the output power Ps as a function of the current Ic1 of a TWT of 750 W nominal power operating in the C-band. This curve 84 can be approximated to a straight line 86 of equation:
Ps=3.148×Ic1+110.2 (2)
c shows another curve 88 for the output power Ps as a function of the current Ic1 of the 750-watt TWT of
Ps=2.9243×Ic1+60.412 (3).
Over the 10 dB power output Ps dynamic range of the TWT, the difference in measurement between the output power measured directly by conventional means and the output power Ps of the TWT measured indirectly from the current Ic1 of the first electrode, this difference remains less than 10% and depends, for low levels, on the sensitivity of the measurement of the collector current Ic1 of the electrode E1.
In the case of TWTs having more than two stages, for example four stages (N=4) such as those used on satellite TWTs, measurements, for various TWT transmission frequencies, of the output power Ps of the TWT as a function of the collector current Ic1 have been carried out in the same manner. These measurements are shown by the curves in
In the case of
Ps=6.3524×Ic1+21.916 (4).
In the case of
Ps=5.1389×Ic1+21.402 (5).
In the case of
Ps=0.0174×Ic13−0.6093×Ic12+10.281×Ic1+7.4151.
In the case of
Ps=0.0705×Ic13−2.0364×Ic12+22.106×Ic1+4.8406.
Therefore, depending on the desired accuracy, a relationship Ps=f(Ic1) of greater or lesser complexity will be input into the processing circuit 76 of the amplifier, this being done at several frequency points in the allotted band. Again, as previously, an interpolation will allow operation at other frequencies.
A current transformer TX2 of the measurement circuit 74 comprises a primary 120, in series with a wire 122 for supplying the high-voltage rectifier bridge P1 with AC current, and a secondary 124 that generates an AC voltage Uc1 proportional to the AC current in the wire 122 representative of the supply current Ic1 of the electrode E1. The voltage Uc1, after rectification by a diode D6, D7, D8, D9 bridge P2, is amplified by a conventional operational amplifier A1 which delivers, at its output Sa, a voltage Us1 proportional to the current Ic1 of the first electrode E1.
The processing circuit 76 of known type establishes the relationship, as described above, between the output voltage Us1 of the detector 74 representative of the current Ic1 and the output power Ps of the amplifier 70. This processing circuit 76 may be a computer using, for example, a microprocessor or any other calculating device.
The relationship Ps=f(Ic1), which, as mentioned above in the case of a two-stage collector, can be approximately likened to a straight line, varies or may vary according to the operation frequency in the allotted band. The relationship Ps=f(Ic1) for various frequencies will therefore be input into the calculating circuit 76 of the amplifier together with an interpolation formula for all frequencies other than the previous ones.
In this embodiment according to the invention shown in
The third coupler 26, for measuring the reflected power Pr, i.e. reflected by the user, may also be eliminated provided that the circulator 18 ensures that the TWT is protected.
In the case of TWTs in which the depressed collector has more than two stages, for example four stages such as those used in satellite TWTs, the curves giving the output power Ps of the TWT as a function of the current Ic1 of the first electrode, which are shown in
To summarize, the amplifier according to the invention has the following advantages:
Number | Date | Country | Kind |
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02/03974 | Mar 2002 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR03/00861 | 3/18/2003 | WO |